(C) Copyright 1995-1999, Ada Core Technologies, Inc.
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This guide describes the use of GNAT, a compiler and software development toolset for the full Ada 95 programming language. It describes the features of the compiler and tools, and details how to use them to build Ada 95 applications.
This guide contains the following chapters:
gcc, describes how to compile
Ada programs with gcc, the Ada compiler.
gnatbind, describes how to
perform binding of Ada programs with gnatbind, the GNAT binding
utility.
gnatlink,
describes gnatlink, a
program that provides for linking using the GNAT run-time library to
construct a program. gnatlink can also incorporate foreign language
object units into the executable.
gnatmake, describes gnatmake, a
utility that automatically determines the set of sources
needed by an Ada compilation unit, and executes the necessary compilations
binding and link.
gnatchop, describes
gnatchop, a utility that allows you to preprocess a file that
contains Ada source code, and split it into one or more new files, one
for each compilation unit.
gnatxref and gnatfind, discusses
gnatxref and gnatfind, two tools that provide an easy
way to navigate through sources.
gnatkr, describes the gnatkr
file name krunching utility, used to handle shortened
file names on operating systems with a limit on the length of names.
gnatprep, describes gnatprep, a
preprocessor utility that allows a single source file to be used to
generate multiple or parameterized source files, by means of macro
substitution.
gnatls, describes gnatls, a
utility that displays information about compiled units, including dependences
on the corresponding sources files, and consistency of compilations.
gnatmem, describes gnatmem, a
utility that monitors dynamic allocation and deallocation activity in a
program, and displays information about incorrect deallocations and sources
of possible memory leaks.
gnatstub, discusses gnatstub,
a utility that generates empty, but compilable bodies for library units.
gnatelim, discusses
gnatelim, a tool which detects unused subprograms and produces
information that helps the compiler to create a smaller executable for a
program.
gnatpsta and gnatpsys.
This user's guide assumes that you are familiar with Ada 95 language, as described in the International Standard ANSI/ISO/IEC-8652:1995, Jan 1995.
For further information about related tools, refer to the following documents:
Following are examples of the typographical and graphic conventions used in this guide:
Functions, utility program names, standard names,
and classes.
and then shown this way.
Commands that are entered by the user are preceded in this manual by the
characters "$ " (dollar sign followed by space). If your system
uses this sequence as a prompt, then the commands will appear exactly as
you see them in the manual. If your system uses some other prompt, then
the command will appear with the $ replaced by whatever prompt
character you are using.
This chapter describes the simplest ways of using GNAT to compile Ada programs.
Three steps are needed to create an executable file from an Ada source file:
All three steps are most commonly handled by using the gnatmake
utility program that, given the name of the main program, automatically
performs the necessary compilation, binding and linking steps.
Any editor may be used to prepare an Ada program. If emacs is
used, the optional Ada mode may be helpful in laying out the program. The
program text is a normal text file. We will suppose in our initial
example that you have used your editor to prepare the following
standard format text file:
with Text_IO; use Text_IO;
procedure Hello is
begin
Put_Line ("Hello WORLD!");
end Hello;
This file should be named `hello.adb'.
Using the normal default file naming conventions, By default, GNAT requires
that each file
contain a single compilation unit whose file name corresponds to the
unit name
with periods replaced by hyphens, and whose
extension is `.ads' for a
spec and `.adb' for a body.
This default file naming convention can be overridden by use of the
special pragma Source_File_Name see section Using Other File Names.
Alternatively, if you want to rename your files according to this default
convention, which is probably more convenient if you will be using GNAT
for all your compilation requirements, then the gnatchop utility
can be used to perform this renaming operation
(see section Renaming Files Using gnatchop).
You can compile the program using the following command:
$ gcc -c hello.adb
gcc is the command used to run the compiler. This compiler is
capable of compiling programs in several languages including Ada 95 and
C. It determines you have given it an Ada program by the extension
(`.ads' or `.adb'), and will call the GNAT compiler to compile
the specified file.
The -c switch is required. It tells gcc to only do a
compilation. (For C programs, gcc can also do linking, but this
capability is not used directly for Ada programs, so the -c
switch must always be present.)
This compile command generates a file
`hello.o' which is the object
file corresponding to your Ada program. It also generates a file
`hello.ali'
which contains additional information used to check
that an Ada program is consistent. To get an executable file,
we then use gnatbind to bind the program
and gnatlink to link it to produce the executable. The
argument to both gnatbind and gnatlink is the name of the
`ali' file, but the default extension of `.ali' can
be omitted. This means that in the most common case, the argument
is simply the name of the main program:
$ gnatbind hello $ gnatlink hello
A simpler method of carrying out these steps is to use
gnatmake, which
is a master program which invokes all of the required
compilation, binding and linking tools in the correct order. In particular,
gnatmake automatically recompiles any sources that have been modified
since they were last compiled, or sources that depend
on such modified sources, so that a consistent compilation is ensured.
$ gnatmake hello.adb
The result is an executable program called `hello', which can be run by entering:
$ ./hello
and, if all has gone well, you will see
Hello WORLD!
appear in response to this command.
Consider a slightly more complicated example that has three files: a main program, and the spec and body of a package:
package Greetings is
procedure Hello;
procedure Goodbye;
end Greetings;
with Text_IO; use Text_IO;
package body Greetings is
procedure Hello is
begin
Put_Line ("Hello WORLD!");
end Hello;
procedure Goodbye is
begin
Put_Line ("Goodbye WORLD!");
end Goodbye;
end Greetings;
with Greetings;
procedure Gmain is
begin
Greetings.Hello;
Greetings.Goodbye;
end Gmain;
Following the one-unit-per-file rule, place this program in the following three separate files:
Greetings
Greetings
To build an executable version of this program, we could use four separate steps to compile, bind, and link the program, as follows:
$ gcc -c gmain.adb $ gcc -c greetings.adb $ gnatbind gmain $ gnatlink gmain
Note that there is no required order of compilation when using GNAT.
In particular it is perfectly fine to compile the main program first.
Also, it is not necessary to compile package specs in the case where
there is a separate body, only the body need be compiled. If you want
to submit these programs to the compiler for semantic checking purposes,
then you use the
-gnatc switch:
$ gcc -c greetings.ads -gnatc
Although the compilation can be done in separate steps as in the
above example, in practice it is almost always more convenient
to use the gnatmake capability. All you need to know in this case
is the name of the main program source file. The effect of the above four
commands can be achieved with a single one:
$ gnatmake gmain.adb
In the next section we discuss the advantages of using gnatmake in
more detail.
gnatmake Utility
If you work on a program by compiling single components at a time using
gcc, you typically keep track of the units you modify. In order to
build a consistent system, you compile not only these units, but also any
units that depend on the units you have modified.
For example, in the preceding case,
if you edit `gmain.adb', you only need to recompile that file. But if
you edit `greetings.ads', you must recompile both
`greetings.adb' and `gmain.adb', because both files contain
units that depend on `greetings.ads'.
gnatbind will warn you if you forget one of these compilation
steps, so that it is impossible to generate an inconsistent program as a
result of forgetting to do a compilation. Nevertheless it is tedious and
error-prone to keep track of dependencies among units.
One approach to handle the dependency-bookkeeping is to use a
makefile. However, makefiles present maintenance problems of their own:
if the dependencies change as you change the program, you must make
sure that the makefile is kept up-to-date manually, which is also an
error-prone process.
The gnatmake utility takes care of these details automatically.
Invoke it using either one of the following forms:
$ gnatmake gmain.adb $ gnatmake gmain
The argument is the name of the file containing the main program from
which you may omit the extension. gnatmake
examines the environment, automatically recompiles any files that need
recompiling, and binds and links the resulting set of object files,
generating the executable file, `gmain'.
In a large program, it
can be extremely helpful to use gnatmake, because working out by hand
what needs to be recompiled can be difficult.
Note that gnatmake
takes into account all the intricate Ada 95 rules that
establish dependencies among units. These include dependencies that result
from inlining subprogram bodies, and from
generic instantiation. Unlike some other
Ada make tools, gnatmake does not rely on the dependencies that were
found by the compiler on a previous compilation, which may possibly
be wrong when sources change. gnatmake determines the exact set of
dependencies from scratch each time it is run.
This chapter describes the compilation model used by GNAT. Although similar to that used by other languages, such as C and C++, this model is substantially different from the traditional Ada compilation models, which are based on a library. The model is initially described without reference to the library-based model. If you have not previously used an Ada compiler, you need only read the first part of this chapter. The last section describes and discusses the differences between the GNAT model and the traditional Ada compiler models. If you have used other Ada compilers, this section will help you to understand those differences, and the advantages of the GNAT model.
Ada source programs are represented in standard text files, using Latin-1 coding. Latin-1 is an 8-bit code that includes the familiar 7-bit ASCII set, plus additional characters used for representing foreign languages (see section Foreign Language Representation for support of non-USA character sets). The format effector characters are represented using their standard ASCII encodings, as follows:
VT
16#0B#
HT
16#09#
CR
16#0D#
LF
16#0A#
FF
16#0C#
Source files are in standard text file format. In addition, GNAT will
recognize a wide variety of stream formats, in which the end of physical
physical lines is marked by any of the following sequences:
LF, CR, CR-LF, or LF-CR. This is useful
in accommodating files that are imported from other operating systems.
The end of a source file is normally represented by the physical end of
file. However, the control character 16#1A# (SUB) is also
recognized as signalling the end of the source file. Again, this is
provided for compatibility with other operating systems where this
code is used to represent the end of file.
Each file contains a single Ada compilation unit, including any pragmas associated with the unit. For example, this means you must place a package declaration (a package spec) and the corresponding body in separate files. An Ada compilation (which is a sequence of compilation units) is represented using a sequence of files. Similarly, you will place each subunit or child unit in a separate file.
GNAT supports the standard character sets defined in Ada 95 as well as several other non-standard character sets for use in localized versions of the compiler (see section Character Set Control).
The basic character set is Latin-1. This character set is defined by ISO
standard 8859, part 1. The lower half (character codes 16#00#
... 16#7F#) is identical to standard ASCII coding, but the upper half is
used to represent additional characters. These include extended letters
used by European languages, such as French accents, the vowels with umlauts
used in German, and the extra letter A-ring used in Swedish.
For a complete list of Latin-1 codes and their encodings, see the source
file of library unit Ada.Characters.Latin_1 in file
`a-chlat1.ads'.
You may use any of these extended characters freely in character or
string literals. In addition, the extended characters that represent
letters can be used in identifiers.
GNAT also supports several other 8-bit coding schemes:
For precise data on the encodings permitted, and the uppercase and lowercase equivalences that are recognized, see the file `csets.adb' in the GNAT compiler sources. You will need to obtain a full source release of GNAT to obtain this file.
GNAT allows wide character codes to appear in character and string literals, and also optionally in identifiers, by means of the following possible encoding schemes:
ESC a b c dWhere
a, b, c, d are the four hexadecimal
characters (using uppercase letters) of the wide character code. For
example, ESC A345 is used to represent the wide character with code
16#A345#.
This scheme is compatible with use of the full Wide_Character set.
16#abcd# where the upper bit is on (in
other words, "a" is in the range 8-F) is represented as two bytes,
16#ab# and 16#cd#. The second byte cannot be a format control
character, but is not required to be in the upper half. This method can
be also used for shift-JIS or EUC, where the internal coding matches the
external coding.
16#ab# and
16#cd#, with the restrictions described for upper-half encoding as
described above. The internal character code is the corresponding JIS
character according to the standard algorithm for Shift-JIS
conversion. Only characters defined in the JIS code set table can be
used with this encoding method.
16#ab# and
16#cd#, with both characters being in the upper half. The internal
character code is the corresponding JIS character according to the EUC
encoding algorithm. Only characters defined in the JIS code set table
can be used with this encoding method.
16#0000#-16#007f#: 2#0xxxxxxx# 16#0080#-16#07ff#: 2#110xxxxx# 2#10xxxxxx# 16#0800#-16#ffff#: 2#1110xxxx# 2#10xxxxxx# 2#10xxxxxx#where the xxx bits correspond to the left-padded bits of the 16-bit character value. Note that all lower half ASCII characters are represented as ASCII bytes and all upper half characters and other wide characters are represented as sequences of upper-half (The full UTF-8 scheme allows for encoding 31-bit characters as 6-byte sequences, but in this implementation, all UTF-8 sequences of four or more bytes length will be treated as illegal).
[ " a b c d " ]Where
a, b, c, d are the four hexadecimal
characters (using uppercase letters) of the wide character code. For
example, ["A345"] is used to represent the wide character with code
16#A345#. It is also possible (though not required) to use the
Brackets coding for upper half characters. For example, the code
16#A3# can be represented as ["A3"].
This scheme is compatible with use of the full Wide_Character set,
and is also the method used for wide character encoding in the standard
ACVC (Ada Compiler Validation Capability) test suite distributions.
Note: Some of these coding schemes do not permit the full use of the Ada 95 character set. For example, neither Shift JIS, nor EUC allow the use of the upper half of the Latin-1 set.
The default file name is determined by the name of the unit that the file contains. The name is formed by taking the full expanded name of the unit and replacing the separating dots with hyphens and using lowercase for all letters.
An exception arises if the file name generated by the above rules starts with one of the characters a,g,i, or s, and the second character is a minus. In this case, the character tilde is used in place of the minus. The reason for this special rule is to avoid clashes with the standard names for child units of the packages System, Ada, Interfaces, and GNAT, which use the prefixes s- a- i- and g- respectively.
The file extension is `.ads' for a spec and `.adb' for a body. The following list shows some examples of these rules.
Following these rules can result in excessively long
file names if corresponding
unit names are long (for example, if child units or subunits are
heavily nested). An option is available to shorten such long file names
(called file name "krunching"). This may be particularly useful when
programs being developed with GNAT are to be used on operating systems
with limited file name lengths. See section Using gnatkr.
Of course, no file shortening algorithm can guarantee uniqueness over
all possible unit names; if file name krunching is used, it is your
responsibility to ensure no name clashes occur. Alternatively you
can specify the exact file names that you want used, as described
in the next section. Finally, if your Ada programs are migrating from a
compiler with a different naming convention, you can use the gnatchop
utility to produce source files that follow the GNAT naming conventions.
(For details see section Renaming Files Using gnatchop.)
In the previous section, we have described the default rules used by GNAT to determine the file name in which a given unit resides. It is often convenient to follow these default rules, and if you follow them, the compiler knows without being explicitly told where to find all the files it needs.
However, in some cases, particularly when a program is imported from another Ada compiler environment, it may be more convenient for the programmer to specify which file names contain which units. GNAT allows arbitrary file names to be used by means of the Source_File_Name pragma. The form of this pragma is as shown in the following examples:
pragma Source_File_Name (My_Utilities.Stacks,
Spec_File_Name => "myutilst_a.ada");
pragma Source_File_name (My_Utilities.Stacks,
Body_File_Name => "myutilst.ada");
As shown in this example, the first argument for the pragma is the unit name (in this example a child unit). The second argument has the form of a named association. The identifier indicates whether the file name is for a spec or a body; the file name itself is given by a string literal.
The source file name pragma is a configuration pragma, which means that normally it will be placed in the `gnat.adc' file used to hold configuration pragmas that apply to a complete compilation environment. For more details on how the `gnat.adc' file is created and used see section Handling of Configuration Pragmas
GNAT allows completely arbitrary file names to be specified using the
source file name pragma. However, if the file name specified has an
extension other than `.ads' or `.adb' it is necessary to use a special
syntax when compiling the file. The name in this case must be preceded
by the special sequence -x followed by a space and the name of the
language, here ada, as in:
$ gcc -c -x ada peculiar_file_name.sim
gnatmake handles non-standard file names in the usual manner (the
non-standard file name for the main program is simply used as the
argument to gnatmake). Note that if the extension is also non-standard,
then it must be included in the gnatmake command, it may not be omitted.
An Ada program consists of a set of source files, and the first step in compiling the program is to generate the corresponding object files. These are generated by compiling a subset of these source files. The files you need to compile are the following:
The preceding rules describe the set of files that must be compiled to generate the object files for a program. Each object file has the same name as the corresponding source file, except that the extension is `.o' as usual.
You may wish to compile other files for the purpose of checking their syntactic and semantic correctness. For example, in the case where a package has a separate spec and body, you would not normally compile the spec. However, it is convenient in practice to compile the spec to make sure it is error-free before compiling clients of this spec, because such compilations will fail if there is an error in the spec.
GNAT provides an option for compiling such files purely for the
purposes of checking correctness; such compilations are not required as
part of the process of building a program. To compile a file in this
checking mode, use the -gnatc switch.
A given object file clearly depends on the source file which is compiled
to produce it. Here we are using depends in the sense of a typical
make utility; in other words, an object file depends on a source
file if changes to the source file require the object file to be
recompiled.
In addition to this basic dependency, a given object may depend on
additional source files as follows:
with's a unit X, the object file
depends on the file containing the spec of unit X. This includes
files that are with'ed implicitly either because they are parents
of with'ed child units or they are run-time units required by the
language constructs used in a particular unit.
Inline applies and inlining is activated with the
-gnatn switch, the object file depends on the file containing the
body of this subprogram as well as on the file containing the spec.
Similarly if the -gnatN switch is used, then the unit is
dependent on all body files.
These rules are applied transitively: if unit A with's
unit B, whose elaboration calls an inlined procedure in package
C, the object file for unit A will depend on the body of
C, in file `c.adb'.
The set of dependent files described by these rules includes all the files on which the unit is semantically dependent, as described in the Ada 95 Language Reference Manual. However, it is a superset of what the ARM describes, because it includes generic, inline, and subunit dependencies.
An object file must be recreated by recompiling the corresponding source
file if any of the source files on which it depends are modified. For
example, if the make utility is used to control compilation,
the rule for an Ada object file must mention all the source files on
which the object file depends, according to the above definition.
The determination of the necessary
recompilations is done automatically when one uses gnatmake.
Each compilation actually generates two output files. The first of these is the normal object file that has a `.o' extension. The second is a text file containing full dependency information. It has the same name as the source file, but an `.ali' extension. This file is known as the Ada Library Information (ALI) file.
Normally you need not be concerned with the contents of this file. This section is included in case you want to understand how these files are being used by the binder and other GNAT utilities. Each ALI file consists of a series of lines of the form:
Key_Character parameter parameter ...
The first two lines in the file identify the library output version and
Standard version. These are required to be consistent across the
entire set of compilation units in your program.
V "xxxxxxxxxxxxxxxx"
This line indicates the library output version, as defined in `gnatvsn.ads'. It ensures that separate object modules of a program are consistent. The library output version must be changed if anything in the compiler changes that would affect successful binding of modules compiled separately. Examples of such changes are modifications in the format of the library information described in this package, modifications to calling sequences, or to the way data is represented.
S "xxxxxxxxxxxxxxxx"
This line contains information regarding types declared in packages
Standard as stored in Gnatvsn.Standard_Version.
The purpose of this information is to ensure that all units in a
program are compiled with a consistent set of options.
This is critical on systems where, for example, the size of Integer
can be set by command line switches.
M type [priority]
This line is present only for a unit that can be a main program.
type is either P for a parameterless procedure or F
for a function returning a value of integral type. The latter is for
writing a main program that returns an exit status. priority is
present only if there was a valid pragma Priority in the
corresponding unit to set the main task priority. It is an unsigned
decimal integer.
F x
This line is present if a pragma Float_Representation or Long_Float is used to specify other than the default floating-point format. This option applies only to implementations of GNAT for the Digital Alpha Systems. The character x is 'I' for IEEE_Float, 'G' for VAX_Float with Long_Float using G_Float, and 'D' for VAX_Float for Long_Float with D_Float.
P L=x Q=x T=x
This line is present if the unit uses tasking directly or indirectly, and has one or more valid xxx_Policy pragmas that apply to the unit. The arguments are as follows
L=x (locking policy)
This is present if a valid Locking_Policy pragma applies to the unit. The single character indicates the policy in effect (e.g. `C' for Ceiling_Locking).
Q=x (queuing policy)
This is present if a valid Queuing_Policy pragma applies to the unit. The single character indicates the policy in effect (e.g. `P' for Priority_Queuing).
T=x (task_dispatching policy)
This is present if a valid Task_Dispatching_Policy pragma applies to the unit. The single character indicates the policy in effect (e.g. `F' for FIFO_Within_Priorities).
Following these header lines is a set of information lines, one per compilation unit. Each line lists a unit in the object file corresponding to this ALI file. In particular, when a package body or subprogram body is compiled there will be two such lines, one for the spec and one for the body, with the entry for the body appearing first. This is the only case in which a single ALI file contains more than one unit. Note that subunits do not count as compilation units for this purpose, and generate no library information, because they are inlined. The lines for each compilation unit have the following form:
U unit-name source-name version [attributes]
This line identifies the unit to which this section of the library
information file applies. unit-name is the unit name in internal
format, as described in package Uname, and source-name is
the name of the source file containing the unit.
version is the version, given by eight hexadecimal characters with lowercase letters. This value is a hash code that includes contributions from the time stamps of this unit and all the units on which it semantically depends.
The optional attributes are a series of two-letter codes indicating information about the unit. They indicate the nature of the unit and they summarize information provided by categorization pragmas.
EB
NE
NE set, depending on whether or not elaboration code is required.
PK
PU
Pure.
PR
Preelaborate.
RC
Remote_Call_Interface.
RT
Remote_Types.
SP
Shared_Passive.
SU
The attributes may appear in any order, separated by spaces. The next set of lines in the ALI file have the following form:
W unit-name [source-name lib-name [E] [EA] [ED]]
One of these lines is present for each unit mentioned in an explicit
with clause in the current unit. unit-name is the unit name
in internal format. source-name is the file name of the file that
must be compiled to compile that unit (usually the file for the body,
except for packages that have no body). lib-name is the file name
of the library information file that contains the results of compiling
the unit. The E and EA parameters are present if
pragma Elaborate or pragma Elaborate_All, respectively,
apply to this unit. ED is used to indicate that the compiler
has determined that a pragma Elaborate_All for this unit would be
desirable. For details on the use of the ED parameter see
See section Elaboration Order Handling in GNAT.
Following the unit information is an optional series of lines that
indicate the usage of pragma Linker_Options. For each appearance of
pragma Linker_Options in any of the units for which unit lines are
present, a line of the form
L string
appears. string is the string from the pragma enclosed in quotes. Within the quotes, the following can occur:
For further details, see Stringt.Write_String_Table_Entry in the
file `stringt.ads'. Note that wide characters of the form {hhhh}
cannot be produced, because pragma Linker_Option accepts only
String, not Wide_String.
Finally, the rest of the ALI file contains a series of lines that indicate the source files on which the compiled units depend. This is used by the binder for consistency checking and looks like:
D source-name time-stamp [comments]
where comments, if present, must be separated from the time stamp by at least one blank. Currently this field is unused.
Blank lines are ignored when the library information is read, and separate sections of the file are separated by blank lines to help readability. Extra blanks between fields are also ignored.
All compiled units are marked with a time stamp, which is derived from the source file. The binder uses these time stamps to ensure consistency of the set of units that constitutes a single program. Time stamps are fourteen-character strings of the form YYYYMMDDHHMMSS. The fields have the following meaning:
YYYY
MM
DD
HH
MM
SS
Time stamps may be compared lexicographically (in other words, the order of Ada comparison operations on strings) to determine which is later or earlier. However, in normal mode, only equality comparisons have any effect on the semantics of the library. Later/earlier comparisons are used only for determining the most informative error messages to be issued by the binder.
The time stamp is the actual stamp stored with the file without any
adjustment resulting from time zone comparisons. This avoids problems in
using libraries across networks with clients spread across multiple time
zones, but it means that the time stamp might differ from that displayed in a
directory listing. For example, in UNIX systems,
file time stamps are stored in Greenwich Mean Time (GMT), but the
ls command displays local times.
When using languages such as C and C++, once the source files have been compiled the only remaining step in building an executable program is linking the object modules together. This means that it is possible to link an inconsistent version of a program, in which two units have included different versions of the same header.
The rules of Ada do not permit such an inconsistent program to be built. For example, if two clients have different versions of the same package, it is illegal to build a program containing these two clients. These rules are enforced by the GNAT binder, which also determines an elaboration order consistent with the Ada rules.
The GNAT binder is run after all the object files for a program have been created. It is given the name of the main program unit, and from this it determines the set of units required by the program, by reading the corresponding ALI files. It generates error messages if the program is inconsistent or if no valid order of elaboration exists.
If no errors are detected, the binder produces a main program, in Ada by
default, that contains calls to the elaboration procedures of those
compilation unit that require them, followed by
a call to the main program. This Ada program is compiled to generate the
object file for the main program. The name of
the Ada file is b~xxx.adb (with the corresponding spec
b~xxx.ads) where xxx is the name of the
main program unit.
Finally, the linker is used to build the resulting executable program, using the object from the main program from the bind step as well as the object files for the Ada units of the program.
There are two ways to build a program that contains some Ada files and some other language files depending on whether the main program is in Ada or not. If the main program is in Ada, you should proceed as follows:
gcc -c file1.c gcc -c file2.c
gnatmake -c my_main.adb
gnatbind my_main
gnatlink my_main.ali file1.o file2.o
The three last steps can be grouped in a single command:
gnatmake my_main.adb -largs file1.o file2.o
If the main program is in some language other than Ada, Then you may have more than one entry point in the Ada subsystem. You must use a special option of the binder to generate callable routines to initialize and finalize the Ada units (see section Binding with Non-Ada Main Programs). Calls to the initialization and finalization routines must be inserted in the main program, or some other appropriate point in the code. The call to initialize the Ada units must occur before the first Ada subprogram is called, and the call to finalize the Ada units must occur after the last Ada subprogram returns. You use the same procedure for building the program as described previously. In this case, however, the binder only places the initialization and finalization subprograms into file `b~xxx.adb' instead of the main program. So, if the main program is not in Ada, you should proceed as follows:
gcc -c file1.c gcc -c file2.c
gnatmake -c entry_point1.adb gnatmake -c entry_point2.adb
gnatbind -n entry_point1 entry_point2
gnatlink entry_point2.ali file1.o file2.o
GNAT follows standard calling sequence conventions and will thus interface to any other language that also follows these conventions. The following Convention identifiers are recognized by GNAT:
Building a mixed application containing both Ada and C++ code may be a challenge for the unaware programmer. As a matter of fact, this interfacing has not been standardized in the Ada 95 reference manual due to the immaturity and lack of standard of C++ at the time. This section gives a few hints that should make this task easier. In particular the first section addresses the differences with interfacing with C. The second section looks into the delicate problem of linking the complete application from its Ada and C++ parts. The last section give some hints on how the GNAT runtime can be adapted in order to allow inter-language dispatching with a new C++ compiler.
GNAT supports interfacing with C++ compilers generating code that is compatible with the standard Application Binary Interface of the given platform.
Interfacing can be done at 3 levels: simple data, subprograms and classes. In the first 2 cases, GNAT offer a specific Convention CPP that behaves exactly like Convention C. Usually C++ mangle names of subprograms and currently GNAT does not provide any help to solve the demangling problem. This problem can be addressed in 2 ways:
Interfacing at the class level can be achieved by using the GNAT specific
pragmas such as CPP_Class and CPP_Virtual. See the GNAT
Reference Manual for additional information.
Usually the linker of the C++ development system must be used to link mixed applications because most C++ systems will resolve elaboration issues (such as calling constructors on global class instances) transparently during the link phase. GNAT has been adapted to ease the use of a foreign linker for the last phase. Three cases can be considered:
c++. Note that this setup is not
very common because it may request recompiling the whole GCC
tree from sources and it does not allow to upgrade easily to a new
version of one compiler for one of the two languages without taking the
risk of destabilizing the other.
$ c++ -c file1.C $ c++ -c file2.C $ gnatmake ada_unit -largs file1.o file2.o --LINK=c++
$ gnatbind ada_unit $ gnatlink -v -v ada_unit file1.o file2.o --LINK=c++If there is a problem due to interfering environment variables, it can be workaround by using an intermediate script:
$ gnatlink -v -v ada_unit file1.o file2.o --LINK=./my_script $ cat ./my_script #!/bin/sh unset C_INCLUDE_PATH unset GCC_EXEC_PREFIX c++ $*
$ gnatls -v $ Gdir=<the last directory on the object path> $ gnatlink ada_unit file1.o file2.o -L$Gdir -lgcc --LINK=<cpp_linker>
GNAT offers the capability to derive Ada 95 tagged types directly from
preexisting C++ classes and . See "Interfacing with C++" in the GNAT
reference manual. The mechanism used by GNAT for achieving such a goal
has been made user configurable through a GNAT library unit
Interfaces.CPP. The default version of this file is adapted to
the GNU c++ compiler. Internal knowledge of the virtual
table layout used by the new C++ compiler is needed to configure
properly this unit. The Interface of this unit is known by the compiler
and cannot be changed except for the value of the constants defining the
characteristics of the virtual table: CPP_DT_Prologue_Size, CPP_DT_Entry_Size,
CPP_TSD_Prologue_Size, CPP_TSD_Entry_Size. Read comments in the source
of this unit for more details.
The GNAT model of compilation is close to the C and C++ models. You can
think of Ada specs as corresponding to header files in C. As in C, you
don't need to compile specs; they are compiled when they are used. The
Ada with is similar in effect to the #include of a C
header.
One notable difference is that, in Ada, you may compile specs separately to check them for semantic and syntactic accuracy. This is not always possible with C headers because they are fragments of programs that have less specific syntactic or semantic rules.
The other major difference is the requirement for running the binder, which performs two important functions. First, it checks for consistency. In C or C++, the only defense against assembling inconsistent programs lies outside the compiler, in a makefile, for example. The binder satisfies the Ada requirement that it be impossible to construct an inconsistent program when the compiler is used in normal mode.
The other important function of the binder is to deal with elaboration
issues. There are also elaboration issues in C++ that are handled
automatically. This automatic handling has the advantage of being
simpler to use, but the C++ programmer has no control over elaboration.
Where gnatbind might complain there was no valid order of
elaboration, a C++ compiler would simply construct a program that
malfunctioned at run time.
This section is intended to be useful to Ada programmers who have previously used an Ada compiler implementing the traditional Ada library model, as described in the Ada 95 Language Reference Manual. If you have not used such a system, please go on to the next section.
In GNAT, there is no library in the normal sense. Instead, the set of source files themselves acts as the library. Compiling Ada programs does not generate any centralized information, but rather an object file and a ALI file, which are of interest only to the binder and linker. In a traditional system, the compiler reads information not only from the source file being compiled, but also from the centralized library. This means that the effect of a compilation depends on what has been previously compiled. In particular:
with'ed, the unit seen by the compiler corresponds
to the version of the unit most recently compiled into the library.
In GNAT, compiling one unit never affects the compilation of any other units because the compiler reads only source files. Only changes to source files can affect the results of a compilation. In particular:
with'ed, the unit seen by the compiler corresponds
to the source version of the unit that is currently accessible to the
compiler.
The most important result of these differences is that order of compilation is never significant in GNAT. There is no situation in which one is required to do one compilation before another. What shows up as order of compilation requirements in the traditional Ada library becomes, in GNAT, simple source dependencies; in other words, there is only a set of rules saying what source files must be present when a file is compiled.
gcc
This chapter discusses how to compile Ada programs using the gcc
command. It also describes the set of switches
that can be used to control the behavior of the compiler.
The first step in creating an executable program is to compile the units
of the program using the gcc command. You must compile the
following files:
You need not compile the following files
because they are compiled as part of compiling related units. GNAT package specs when the corresponding body is compiled, and subunits when the parent is compiled. If you attempt to compile any of these files, you will get one of the following error messages (where fff is the name of the file you compiled):
No code generated for file fff (package spec) No code generated for file fff (subunit)
The basic command for compiling a file containing an Ada unit is
$ gcc -c [switches] `file name'
where file name is the name of the Ada file (usually
having an extension
`.ads' for a spec or `.adb' for a body).
You specify the
-c switch to tell gcc to compile, but not link, the file.
The result of a successful compilation is an object file, which has the
same name as the source file but an extension of `.o' and an Ada
Library Information (ALI) file, which also has the same name as the
source file, but with `.ali' as the extension. GNAT creates these
two output files in the current directory, but you may specify a source
file in any directory using an absolute or relative path specification
containing the directory information.
gcc is actually a driver program that looks at the extensions of
the file arguments and loads the appropriate compiler. For example, the
GNU C compiler is `cc1', and the Ada compiler is `gnat1'.
These programs are in directories known to the driver program (in some
configurations via environment variables you set), but need not be in
your path. The gcc driver also calls the assembler and any other
utilities needed to complete the generation of the required object
files.
It is possible to supply several file names on the same gcc
command. This causes gcc to call the appropriate compiler for
each file. For example, the following command lists three separate
files to be compiled:
$ gcc -c x.adb y.adb z.c
calls gnat1 (the Ada compiler) twice to compile `x.adb' and
`y.adb', and cc1 (the C compiler) once to compile `z.c'.
The compiler generates three object files `x.o', `y.o' and
`z.o' and the two ALI files `x.ali' and `y.ali' from the
Ada compilations. Any switches apply to all the files listed,
except for
-gnatx switches, which apply only to Ada compilations.
gcc
The gcc command accepts numerous switches to control the
compilation process. These switches are fully described in this section.
-b target
-Bdir
gnat1, the Ada compiler)
from dir instead of the default location. Only use this switch
when multiple versions of the GNAT compiler are available. See the
gcc manual page for further details. You would normally use the
-b or -V switch instead.
-c
gcc without a -c switch to
compile and link in one step. This is because the binder must be run,
and currently gcc cannot be used to run the GNAT binder.
-g
-g switch
if you plan on using the debugger.
-Idir
-I-
-o file
gcc to redirect the generated object file
and its associated ALI file. Beware of this switch with GNAT, because it may
cause the object file and ALI file to have different names which in turn
may confuse the binder and the linker.
-O[n]
-O appears
-O without
an operand.
-S
-c to
cause the assembler source file to be
generated, using `.s' as the extension,
instead of the object file.
This may be useful if you need to examine the generated assembly code.
-v
gcc driver. Normally used only for
debugging purposes or if you need to be sure what version of the
compiler you are executing.
-V ver
gcc
version, not the GNAT version.
-Wuninitialized
-O switch (in other words, This switch works only if
optimization is turned on).
-gnata
Pragma Assert and pragma Debug to be
activated.
-gnatb
stderr even if verbose mode set.
-gnatc
-gnatD
-gnate
-gnatE
-gnatf
-gnatg
-gnatG
-gnatic
-gnath
stdout.
-gnatkn
k = krunch).
-gnatl
-gnatmn
-gnatn
inline is specified.
-gnatN
inline is specified. This is equivalent
to using -gnatn and adding a pragma inline for every
subprogram in the program.
-fno-inline
-gnato
-gnatp
-gnatq
-gnatP
-gnatR
-gnats
-gnatt
-gnatu
-gnatU
-gnatv
stdout.
-gnatwm
s,e,l for suppress, treat as error, elaboration
warnings).
-gnatWe
-gnatx
-gnatwm
-gnaty
-gnatzm
-gnat83
-gnat95
You may combine a sequence of GNAT switches into a single switch. For example, the combined switch
-gnatcfi3
is equivalent to specifying the following sequence of switches:
-gnatc -gnatf -gnati3
The standard default format for error messages is called "brief format."
Brief format messages are written to stdout (the standard output
file) and have the following form:
e.adb:3:04: Incorrect spelling of keyword "function" e.adb:4:20: ";" should be "is"
The first integer after the file name is the line number in the file,
and the second integer is the column number within the line.
emacs can parse the error messages
and point to the referenced character.
The following switches provide control over the error message
format:
-gnatv
stdout. The same program compiled with the
-gnatv switch would generate:
3. funcion X (Q : Integer)
|
>>> Incorrect spelling of keyword "function"
4. return Integer;
|
>>> ";" should be "is"
The vertical bar indicates the location of the error, and the `>>>'
prefix can be used to search for error messages. When this switch is
used the only source lines output are those with errors.
-gnatl
l stands for list.
This switch causes a full listing of
the file to be generated. The output might look as follows:
1. procedure E is
2. V : Integer;
3. funcion X (Q : Integer)
|
>>> Incorrect spelling of keyword "function"
4. return Integer;
|
>>> ";" should be "is"
5. begin
6. return Q + Q;
7. end;
8. begin
9. V := X + X;
10.end E;
When you specify the -gnatv or -gnatl switches and
standard output is redirected, a brief summary is written to
stderr (standard error) giving the number of error messages and
warning messages generated.
-gnatU
-gnatb
b stands for brief.
This switch causes GNAT to generate the
brief format error messages to stdout as well as the verbose
format message or full listing.
-gnatmn
m stands for maximum.
n is a decimal integer in the
range of 1 to 999 and limits the number of error messages to be
generated. For example, using -gnatm2 might yield
e.adb:3:04: Incorrect spelling of keyword "function" e.adb:5:35: missing ".." fatal error: maximum errors reached compilation abandoned
-gnatf
f stands for full.
Normally, the compiler suppresses error messages that are likely to be
redundant. This switch causes all error
messages to be generated. In particular, in the case of
references to undefined variables. If a given variable is referenced
several times, the normal format of messages is
e.adb:7:07: "V" is undefined (more references follow)where the parenthetical comment warns that there are additional references to the variable
V. Compiling the same program with the
-gnatf switch yields
e.adb:7:07: "V" is undefined e.adb:8:07: "V" is undefined e.adb:8:12: "V" is undefined e.adb:8:16: "V" is undefined e.adb:9:07: "V" is undefined e.adb:9:12: "V" is undefined
-gnatq
q stands for quit (really "don't quit").
In normal operation mode, the compiler first parses the program and
determines if there are any syntax errors. If there are, appropriate
error messages are generated and compilation is immediately terminated.
This switch tells
GNAT to continue with semantic analysis even if syntax errors have been
found. This may enable the detection of more errors in a single run. On
the other hand, the semantic analyzer is more likely to encounter some
internal fatal error when given a syntactically invalid tree.
-gnate
-gnate may cause error messages to be
generated out of sequence and also disconnects a number of useful
error message processing circuits. This switch should be used only
in error situations where the compiler terminates with no output
at all, or goes into an infinite loop. In such cases, the
-gnate switch may be used to see if any error situations
were detected before the compiler crash (see section GNAT Abnormal Termination).
In addition to error messages, which correspond to illegalities as defined in the Ada 95 Reference Manual, the compiler detects two kinds of warning situations.
First, the compiler considers some constructs suspicious and generates a warning message to alert you to a possible error. Second, if the compiler detects a situation that is sure to raise an exception at run time, it generates a warning message. The following shows an example of warning messages:
e.adb:4:24: warning: creation of object may raise Storage_Error e.adb:10:17: warning: static value out of range e.adb:10:17: warning: "Constraint_Error" will be raised at run time
GNAT considers a large number of situations as appropriate
for the generation of warning messages. As always, warnings are not
definite indications of errors. For example, if you do an out-of-range
assignment with the deliberate intention of raising a
Constraint_Error exception, then the warning that may be
issued does not indicate an error. Some of the situations for which GNAT
issues warnings (at least some of the time) are:
Four switches are available to control the handling of warning messages:
-gnatwe (treat warnings as errors)
-gnatws (suppress warnings)
-gnatwl (warn on elaboration order errors)
-gnatwu (warn on unused entities)
with'ed
and not
referenced. In the case of packages, a warning is also generated if
no entities in the package are referenced. This means that if the package
is referenced but the only references are in use
clauses or renames
declarations, a warning is still generated. A warning is also generated
for a generic package that is with'ed but never instantiated.
-gnatR
-gnatR causes the compiler to output a listing
showing representation information for declared array and record types,
including record representation clauses.
-gnatx
gnatfind and gnatxref. The -gnatx switch
suppresses this information. This saves some space and may slightly
speed up compilation, but means that these tools cannot be used.
-gnata
Assert and Debug normally have no effect and
are ignored. This switch, where `a' stands for assert, causes
Assert and Debug pragmas to be activated.
The pragmas have the form:
pragma Assert (Boolean-expression [, static-string-expression]) pragma Debug (procedure call)The
Assert pragma causes Boolean-expression to be tested.
If the result is True, the pragma has no effect (other than
possible side effects from evaluating the expression). If the result is
False, the exception Assert_Error declared in the package
System.Assertions is
raised (passing static-string-expression, if present, as the
message associated with the exception). If no string expression is
given the default is a string giving the file name and line number
of the pragma.
The Debug pragma causes procedure to be called. Note that
pragma Debug may appear within a declaration sequence, allowing
debugging procedures to be called between declarations.
The -gnatyx switch causes the compiler to enforce specified style rules. A limited set of style rules has been used in writing the GNAT sources themselves. This switch allows user programs to activate all or some of these checks. If the source program fails a specified style check, an appropriate error message is given, preceded by the character sequence "(style)", and the program is considered illegal. The string x is a sequence of letters or digits indicating the particular style checks to be performed. The following checks are defined:
1-9 (specify indentation level)
-gnaty then proper
indentation is checked, with the digit indicating the indentation level
required. The general style of required indentation is as specified by
the examples in the Ada Reference Manual. Full line comments must be
aligned with the -- starting on a column that is a multiple of
the alignment level.
a (check attribute casing)
-gnaty then
attribute names, including the case of keywords such as digits
used as attributes names, must be written in mixed case, that is, the
initial letter and any letter following an underscore must be uppercase.
All other letters must be lowercase.
b (blanks not allowed at statement end)
-gnaty then
trailing blanks are not allowed at the end of statements. The purpose of this
rule, together with h (no horizontal tabs), is to enforce a canonical format
for the use of blanks to separate source tokens.
c (check comments)
-gnaty then
comments must meet the following set of rules:
gnatprep tool.
--------------------------- -- This is a box comment -- -- with two text lines. -- ---------------------------
e (check end labels)
-gnaty then
optional labels on end statements ending subprograms are required to be
present.
f (no form feeds or vertical tabs)
-gnaty then
neither form feeds nor vertical tab characters are not permitted
in the source text.
h (no horizontal tabs)
-gnaty then
horizontal tab characters are not permitted in the source text.
Together with the b (no blanks at end of line) check, this
enforces a canonical form for the use of blanks to separate
source tokens.
i (check if-then layout)
-gnaty,
then the keyword then must appear either on the same
line as corresponding if, or on a line on its own, lined
up under the if with at least one non-blank line in between
containing all or part of the condition to be tested.
k (check keyword casing)
-gnaty then
all keywords must be in lower case (with the exception of keywords
such as digits used as attribute names to which this check
does not apply).
l (check layout)
-gnaty then
layout of statement and declaration constructs must follow the
recommendations in the Ada Reference Manual, as indicated by the
form of the syntax rules. For example an else keyword must
be lined up with the corresponding if keyword.
There are two respects in which the style rule enforced by this check
option are more liberal than those in the Ada Reference Manual. First
in the case of record declarations, it is permissible to put the
record keyword on the same line as the type keyword, and
then the end in end record must line up under type.
For example, either of the following two layouts is acceptable:
type q is record
a : integer;
b : integer;
end record;
type q is
record
a : integer;
b : integer;
end record;
Second, in the case of a block statement, a permitted alternative
is to put the block label on the same line as the declare or
begin keyword, and then line the end keyword up under
the block label. For example both the following are permitted:
Block : declare
A : Integer := 3;
begin
Proc (A, A);
end Block;
Block :
declare
A : Integer := 3;
begin
Proc (A, A);
end Block;
The same alternative format is allowed for loops. For example, both of
the following are permitted:
Clear : while J < 10 loop
A (J) := 0;
end loop Clear;
Clear :
while J < 10 loop
A (J) := 0;
end loop Clear;
m (check maximum line length)
-gnaty
then the length of source lines must not exceed 79 characters, including
any trailing blanks. The value of 79 allows convenient display on an
80 character wide device or window, allowing for possible special
treatment of 80 character lines.
Mnnn (set maximum line length)
-gnaty then the length of lines must not exceed the
given value.
p (check pragma casing)
-gnaty then
pragma names must be written in mixed case, that is, the
initial letter and any letter following an underscore must be uppercase.
All other letters must be lowercase.
r (check references)
-gnaty
then all identifier references must be cased in the same way as the
corresponding declaration. No specific casing style is imposed on
identifiers. The only requirement is for consistency of references
with declarations.
s (check separate specs)
-gnaty then
separate declarations ("specs") are required for subprograms (a
body is not allowed to serve as its own declaration). The only
exception is that parameterless library level procedures are
not required to have a separate declaration. This exception covers
the most frequent form of main program procedures.
t (check token spacing)
-gnaty then
the following token spacing rules are enforced:
abs and not must be followed by a space.
=> must be surrounded by spaces.
<> must be preceded by a space or a left parenthesis.
** must be surrounded by spaces.
There is no restriction on the layout of the ** binary operator.
The switch
-gnaty on its own (that is not followed by any letters or digits),
is equivalent to gnaty3abcefhiklmprst, that is all checking
options are enabled, with an indentation level of 3. This is the standard
checking option that is used for the GNAT sources.
If you compile with the default options, GNAT will insert many run-time
checks into the compiled code, including code that performs range
checking against constraints, but not arithmetic overflow checking for
integer operations (including division by zero) or checks for access
before elaboration on subprogram calls. All other run-time checks, as
required by the Ada 95 Reference Manual, are generated by default.
The following gcc switches refine this default behavior:
-gnatp
pragma Suppress (all_checks)
had been present in the source. Use this switch to improve the performance
of the code at the expense of safety in the presence of invalid data or
program bugs.
-gnato
Constraint_Error as required by Ada
semantics).
Note that the -gnato switch does not affect the code generated
for any floating-point operations; it applies only to integer
operations. For floating-point, GNAT has the Machine_Overflows
attribute set to False and the normal mode of operation is to
generate IEEE NaN and infinite values on overflow or invalid operations
(such as dividing 0.0 by 0.0).
-gnatE
gcc.
The setting of these switches only controls the default setting of the
checks. You may modify them using either Suppress (to remove
checks) or Unsuppress (to add back suppressed checks) pragmas in
the program source.
gcc for Syntax Checking-gnats
s stands for syntax.
Run GNAT in syntax checking only mode. For
example, the command
$ gcc -c -gnats x.adbcompiles file `x.adb' in syntax-check-only mode. You can check a series of files in a single command , and can use wild cards to specify such a group of files. Note that you must specify the
-c (compile
only) flag in addition to the -gnats flag.
.
You may use other switches in conjunction with -gnats. In
particular, -gnatl and -gnatv are useful to control the
format of any generated error messages.
The output is simply the error messages, if any. No object file or ALI
file is generated by a syntax-only compilation. Also, no units other
than the one specified are accessed. For example, if a unit X
with's a unit Y, compiling unit X in syntax
check only mode does not access the source file containing unit
Y.
Normally, GNAT allows only a single unit in a source file. However, this
restriction does not apply in syntax-check-only mode, and it is possible
to check a file containing multiple compilation units concatenated
together. This is primarily used by the gnatchop utility
(see section Renaming Files Using gnatchop).
gcc for Semantic Checking-gnatc
c stands for check.
Causes the compiler to operate in semantic check mode,
with full checking for all illegalities specified in the
Ada 95 Reference Manual, but without generation of any source code (no object
or ALI file generated).
Because dependent files must be accessed, you must follow the GNAT
semantic restrictions on file structuring to operate in this mode:
-gnat83
-gnat83 switch can be ported easily to an Ada 83
compiler. This is the main use of the switch.
With few exceptions (most notably the need to use <> on
unconstrained generic formal parameters, the use of the new Ada 95
keywords, and the use of packages
with optional bodies), it is not necessary to use the
-gnat83 switch when compiling Ada 83 programs, because, with rare
exceptions, Ada 95 is upwardly compatible with Ada 83. This
means that a correct Ada 83 program is usually also a correct Ada 95
program.
-gnat95
-gnatr
else
must line up with an if and code in the then and
else parts must be indented. The compiler treats violations of
the layout rules as syntax errors if you specify this switch.
-gnatg
-gnatg switch specified.
You can find the full documentation for the style conventions imposed by
-gnatg in the body of the package Style in the
compiler sources (in the file `style.adb').
You should not normally use the -gnatg switch. However, you
must use -gnatg for compiling any language-defined unit,
or for adding children to any language-defined unit other than
Standard.
-gnatic
1
2
3
4
p
8
f
n
w
-gnatWe
h
u
s
e
8
b
-gnatW8 specifies that both
brackets and UTF-8 encodings will be recognized. The units that are
with'ed directly or indirectly will be scanned using the specified
representation scheme, and so if one of the non-brackets scheme is
used, it must be used consistently throughout the program. However,
since brackets encoding is always recognized, it may be conveniently
used in standard libraries, allowing these libraries to be used with
any of the available coding schemes.
scheme. If no -gnatW? parameter is present, then the default
representation is Brackets encoding only.
Note that the wide character representation that is specified (explicitly
or by default) for the main program also acts as the default encoding used
for Wide_Text_IO files if not specifically overridden by a WCEM form
parameter.
-gnatkn
-gnatn
n here is intended to suggest the first syllable of the
word "inline".
GNAT recognizes and processes Inline pragmas. However, for the
inlining to actually occur, optimization must be enabled. To enable
inlining across unit boundaries, this is, inlining a call in one unit of
a subprogram declared in a with'ed unit, you must also specify
this switch.
In the absence of this switch, GNAT does not attempt
inlining across units and does not need to access the bodies of
subprograms for which pragma Inline is specified if they are not
in the current unit.
If you specify this switch the compiler will access these bodies,
creating an extra source dependency for the resulting object file, and
where possible, the call will be inlined. For further details on when inlining is possible
see See section Inlining of Subprograms.
-gnatN
Inline for every subprogram referenced by the compiled
unit.
-gnatt
-gnatu
stdout.
The listing includes all units on which the unit being compiled depends
either directly or indirectly.
-gnatdx
Debug unit in the compiler source
file `debug.adb'.
-gnatG
-gnatG you can identify
these cases, and consider whether it may be desirable to modify the coding
approach to improve efficiency.
The format of the output is very similar to standard Ada source, and is
easily understood by an Ada programmer. The following special syntactic
additions correspond to low level features used in the generated code that
do not have any exact analogies in pure Ada source form:
-gnatD
-gnatG to cause the expanded
source, as described above to be written to files with names
`xxx.dg', where `xxx' is the normal file name,
for example, if the source file name is `hello.adb',
then a file `hello.adb.dg' will be written.
The debugging information generated
by the gcc -g switch will refer to the generated
`xxx.dg' file. This allows you to do source level debugging using
the generated code which is sometimes useful for complex code, for example
to find out exactly which part of a complex construction raised an
exception.
new xxx [storage_pool = yyy]
at end procedure-name;
(if expr then expr else expr)
x?y:z construction in C.
target^(source)
target?(source)
target?^(source)
x #/ y
x #mod y
x #* y
x #rem y
free expr [storage_pool = xxx]
free statement.
freeze typename [actions]
reference itype
function-name! (arg, arg, arg)
labelname : label
expr && expr && expr ... && expr
[constraint_error]
Constraint_Error exception.
expression'reference
target-type!(source-expression)
[numerator/denominator]
With the GNAT source-based library system, the compiler must be able to find source files for units that are needed by the unit being compiled. Search paths are used to guide this process.
The compiler compiles one source file whose name must be given explicitly on the command line. In other words, no searching is done for this file. To find all other source files that are needed (the most common being the specs of units), the compiler examines the following directories, in the following order:
-I switch given on the gcc
command line, in the order given.
ADA_INCLUDE_PATH environment variable.
Construct this value
exactly as the PATH environment variable: a list of directory
names separated by colons.
Specifying the switch -I-
inhibits the use of the directory
containing the source file named in the command line. You can still
have this directory on your search path, but in this case it must be
explicitly requested with a -I switch.
Specifying the switch -nostdinc
inhibits the search of the default location for the GNAT Run Time
Library (RTL) source files.
The compiler outputs its object files and ALI files in the current
working directory.
Caution: The object file can be redirected with the -o switch;
however, gcc and gnat1 have not been coordinated on this
so the ALI file will not go to the right place. Therefore, you should
avoid using the -o switch.
The packages Ada, System, and Interfaces and their
children make up the GNAT RTL, together with the simple System.IO
package used in the "Hello World" example. The sources for these units
are needed by the compiler and are kept together in one directory. Not
all of the bodies are needed, but all of the sources are kept together
anyway. In a normal installation, you need not specify these directory
names when compiling or binding. Either the environment variables or
the built-in defaults cause these files to be found.
In addition to the language-defined hierarchies (System, Ada and Interfaces), the GNAT distribution provides a fourth hierarchy, consisting of child units of GNAT. This is a collection of generally useful routines. See the GNAT Reference Manual for further details.
Besides simplifying access to the RTL, a major use of search paths is in compiling sources from multiple directories. This can make development environments much more flexible.
If, in our earlier example, there was a spec for the hello
procedure, it would be contained in the file `hello.ads'; yet this
file would not have to be explicitly compiled. This is the result of the
model we chose to implement library management. Some of the consequences
of this model are as follows:
with's, all its subunits, and the bodies of any generics it
instantiates must be available (reachable by the search-paths mechanism
described above), or you will receive a fatal error message.
The following are some typical Ada compilation command line examples:
$ gcc -c xyz.adb
$ gcc -c -O2 -gnata xyz-def.adb
Assert/Debug statements
enabled.
$ gcc -c -gnatc abc-def.adb
gnatbind
This chapter describes the GNAT binder, gnatbind, which is used
to bind compiled GNAT objects. The gnatbind program performs
four separate functions:
gnatlink utility used to link the Ada application.
gnatbind
The form of the gnatbind command is
$ gnatbind [switches] mainprog[.ali] [switches]
where mainprog.adb is the Ada file containing the main program
unit body. If no switches are specified, gnatbind constructs an Ada
package in two files whose names are
`b~ada_main.ads', and
`b~ada_main.adb'.
For example, if given the
parameter `hello.ali', for a main program contained in file
`hello.adb', the binder output files would be `b~hello.ads'
and `b~hello.adb'.
When doing consistency checking, the binder takes any source files it
can locate into consideration. For example, if the binder determines
that the given main program requires the package Pack, whose ALI
file is `pack.ali' and whose corresponding source spec file is
`pack.ads', it attempts to locate the source file `pack.ads'
(using the same search path conventions as previously described for the
gcc command). If it can locate this source file, it checks that
the time stamps
or source checksums of the source and its references to in `ali' files
match. In other words, any `ali' files that mentions this spec must have
resulted from compiling this version of the source file (or in the case
where the source checksums match, a version close enough that the
difference does not matter).
The effect of this consistency checking, which includes source files, is that the binder ensures that the program is consistent with the latest version of the source files that can be located at bind time. Editing a source file without compiling files that depend on the source file cause error messages to be generated by the binder.
For example, suppose you have a main program `hello.adb' and a
package P, from file `p.ads' and you perform the following
steps:
gcc -c hello.adb to compile the main program.
gcc -c p.ads to compile package P.
gnatbind hello.
At this point, the file `p.ali' contains an out-of-date time stamp because the file `p.ads' has been edited. The attempt at binding fails, and the binder generates the following error messages:
error: "hello.adb" must be recompiled ("p.ads" has been modified)
error: "p.ads" has been modified and must be recompiled
Now both files must be recompiled as indicated, and then the bind can succeed, generating a main program. You need not normally be concerned with the contents of this file, but it is similar to the following (when using -C):
extern int gnat_argc;
extern char **gnat_argv;
extern char **gnat_envp;
extern int gnat_exit_status;
void adafinal ();
void adainit ()
{
__gnat_set_globals (
-1, /* Main_Priority */
-1, /* Time_Slice_Value */
' ', /* Locking_Policy */
' ', /* Queuing_Policy */
' ', /* Tasking_Dispatching_Policy */
adafinal);
system___elabs ();
/* system__standard_library___elabs (); */
/* system__task_specific_data___elabs (); */
/* system__tasking_soft_links___elabs (); */
system__tasking_soft_links___elabb ();
/* system__task_specific_data___elabb (); */
/* system__standard_library___elabb (); */
/* m___elabb (); */
}
void adafinal () {
}
int main (argc, argv, envp)
int argc;
char **argv;
char **envp;
{
gnat_argc = argc;
gnat_argv = argv;
gnat_envp = envp;
__gnat_initialize();
adainit();
_ada_m ();
adafinal();
__gnat_finalize();
exit (gnat_exit_status);
}
unsigned mB = 0x2B0EB17F;
unsigned system__standard_libraryB = 0x0122ED49;
unsigned system__standard_libraryS = 0x79B018CE;
unsigned systemS = 0x08FBDA7E;
unsigned system__task_specific_dataB = 0x6CC7367B;
unsigned system__task_specific_dataS = 0x47178527;
unsigned system__tasking_soft_linksB = 0x5A75A73C;
unsigned system__tasking_soft_linksS = 0x3012AFCB;
/* BEGIN Object file/option list
./system.o
./s-tasoli.o
./s-taspda.o
./s-stalib.o
./m.o
END Object file/option list */
The call to __gnat_set_globals establishes program parameters,
including the priority of the main task, and parameters for tasking
control. It also passes the address of the finalization routine so
that it can be called at the end of program execution.
Next there is code to save the argc and argv values for
later access by the Ada.Command_Line package. The variable
gnat_exit_status saves the exit status set by calls to
Ada.Command_Line.Set_Exit_Status and is used to return an exit
status to the system.
The call to __gnat_initialize and the corresponding call at the
end of execution to __gnat_finalize allow any specialized
initialization and finalization code to be hooked in. The default
versions of these routines do nothing.
The calls to xxx___elabb and
xxx___elabs perform necessary elaboration of the bodies and
specs respectively of units in the program. These calls are commented
out if the unit in question has no elaboration code.
The call to
m
is the call to the main program.
The list of unsigned constants gives the version number information.
Version numbers are computed by combining time stamps of a unit and all
units on which it depends. These values are used for implementation of
the Version and Body_Version attributes.
Finally, a set of comments gives the full names of all the object files that must be linked to provide the Ada component of the program. As seen in the previous example, this list includes the files explicitly supplied and referenced by the user as well as implicitly referenced run-time unit files. The latter are omitted if the corresponding units reside in shared libraries. The directory names for the run-time units depend on the system configuration. See section Output Control.
As described in the previous section, by default gnatbind checks
that object files are consistent with one another and are consistent
with any source files it can locate. The following switches control binder
access to sources.
-s
-x
gnatmake because in this
case the checking against sources has already been performed by
gnatmake in the course of compilation (i.e. before binding).
The following switches provide control over the generation of error messages from the binder:
-v
stderr. If this switch is present, a header is written
to stdout and any error messages are directed to stdout.
All that is written to stderr is a brief summary message.
-b
stderr even if verbose mode is
specified. This is relevant only when used with the
-v switch.
-mn
-Mxxx
main to xxx.
This is useful in the case of some cross-building environments, where
the actual main program is separate from the one generated
by gnatbind.
-ws
-we
-t
-t should be used only in unusual situations,
with extreme care.
The following switches provide additional control over the elaboration order. For full details see See section Elaboration Order Handling in GNAT.
-f
Elaborate_All pragmas, and to use full Ada 95 Reference
Manual semantics in an attempt to find a legal elaboration order,
even if it seems likely that this order will cause an elaboration
exception.
-p
Program_Error exception. This switch reverses the
action of the binder, and requests that it deliberately choose an order
that is likely to maximize the likelihood of an elaboration error.
This is useful in ensuring portability and avoiding dependence on
accidental fortuitous elaboration ordering.
The following switches allow additional control over the output generated by the binder.
-A
-o gnatbind option.
-C
-o gnatbind
option.
-e
stdout.
-h
stdout.
-l
stdout.
-O
stdout.
This list includes the files explicitly supplied and referenced by the user
as well as implicitly referenced run-time unit files. The latter are
omitted if the corresponding units reside in shared libraries. The
directory names for the run-time units depend on the system configuration.
-o file
-c
In our description so far we have assumed that the main
program is in Ada, and that the task of the binder is to generate a
corresponding function main that invokes this Ada main
program. GNAT also supports the building of executable programs where
the main program is not in Ada, but some of the called routines are
written in Ada and compiled using GNAT (see section Mixed Language Programming).
The following switch is used in this situation:
-n
In this case, most of the functions of the binder are still required, but instead of generating a main program, the binder generates a file containing the following callable routines:
adainit
adainit is
required before the first call to an Ada subprogram.
adafinal
adafinal is required
after the last call to an Ada subprogram, and before the program
terminates.
If the -n switch
is given, more than one ALI file may appear on
the command line for gnatbind. The normal closure
calculation is performed for each of the specified units. Calculating
the closure means finding out the set of units involved by tracing
with references. The reason it is necessary to be able to
specify more than one ALI file is that a given program may invoke two or
more quite separate groups of Ada units.
The binder takes the name of its output file from the last specified ALI
file, unless overridden by the use of the \-o
file\/OUTPUT=file\. The output is an Ada unit in source form that can
be compiled with GNAT unless the -C switch is used in which case the
output is a C source file, which must be compiled using the C compiler.
This compilation occurs automatically as part of the gnatlink
processing.
It is possible to have an Ada program which does not have a main subprogram. This program will call the elaboration routines of all the packages, then the finalization routines.
The following switch is used to bind programs organized in this manner:
-z
The following are the switches available with gnatbind:
-aO
-aI
-A
-b
stderr even if verbose mode set.
-c
-C
-e
-E
gcc flag -funwind-tables when
compiling every file in your application. See also the packages
GNAT.Traceback and GNAT.Traceback.Symbolic
-f
-h
-I
-I-
gnatbind was
invoked, and do not look for ALI files in the directory containing the
ALI file named in the gnatbind command line.
-l
-Mxyz
-mn
-n
-nostdinc
-nostdlib
-o file
-O
-p
-s
-static
-shared
-t
-Tn
-v
stdout.
-wx
-x
-z
You may obtain this listing by running the program gnatbind with
no arguments.
The package Ada.Command_Line provides access to the command-line
arguments and program name. In order for this interface to operate
correctly, the two variables
int gnat_argc; char **gnat_argv;
are declared in one of the GNAT library routines. These variables must
be set from the actual argc and argv values passed to the
main program. With no n present, gnatbind
generates the C main program to automatically set these variables.
If the n switch is used, there is no automatic way to
set these variables. If they are not set, the procedures in
Ada.Command_Line will not be available, and any attempt to use
them will raise Constraint_Error. If command line access is
required, your main program must set gnat_argc and
gnat_argv from the argc and argv values passed to
it.
gnatbindThe binder takes the name of an ALI file as its argument and needs to locate source files as well as other ALI files to verify object consistency.
For source files, it follows exactly the same search rules as gcc
(see section Search Paths and the Run-Time Library (RTL)). For ALI files the
directories searched are:
-I- is specified.
-I
switches on the gnatbind
command line, in the order given.
ADA_OBJECTS_PATH environment variable.
Construct this value
exactly as the PATH environment variable: a list of directory
names separated by colons.
-nostdlib is specified.
In the binder the switch -I
is used to specify both source and
library file paths. Use -aI
instead if you want to specify
source paths only, and -aO
if you want to specify library paths
only. This means that for the binder
-Idir is equivalent to
-aIdir
-aOdir.
The binder generates the bind file (a C language source file) in the
current working directory.
The packages Ada, System, and Interfaces and their
children make up the GNAT Run-Time Library, together with the package
GNAT and its children, which contain a set of useful additional
library functions provided by GNAT. The sources for these units are
needed by the compiler and are kept together in one directory. The ALI
files and object files generated by compiling the RTL are needed by the
binder and the linker and are kept together in one directory, typically
different from the directory containing the sources. In a normal
installation, you need not specify these directory names when compiling
or binding. Either the environment variables or the built-in defaults
cause these files to be found.
Besides simplifying access to the RTL, a major use of search paths is in compiling sources from multiple directories. This can make development environments much more flexible.
gnatbind Usage
This section contains a number of examples of using the GNAT binding
utility gnatbind.
gnatbind hello
Hello (source program in `hello.adb') is
bound using the standard switch settings. The generated main program is
`b~hello.adb'. This is the normal, default use of the binder.
gnatbind hello -o mainprog.adb
Hello (source program in `hello.adb') is
bound using the standard switch settings. The generated main program is
`mainprog.adb' with the associated spec in
`mainprog.ads'. Note that you must specify the body here not the
spec, in the case where the output is in Ada. Note that if this option
is used, then linking must be done manually, since gnatlink will not
be able to find the generated file.
gnatbind main -C -o mainprog.c -x
Main (source program in
`main.adb') is bound, excluding source files from the
consistency checking, generating
the file `mainprog.c'.
gnatbind -x main_program -C -o mainprog.c
gnatbind -n math dbase -C -o ada-control.c
Math and Dbase appear. This call
to gnatbind generates the file `ada-control.c' containing
the adainit and adafinal routines to be called before and
after accessing the Ada units.
gnatlink
This chapter discusses gnatlink, a utility program used to link
Ada programs and build an executable file. This is a simple program
that invokes the UNIX linker (via the gcc
command) with a correct list of object files and library references.
gnatlink automatically determines the list of files and
references for the Ada part of a program. It uses the binder file
generated by the binder to determine this list.
gnatlink
The form of the gnatlink command is
$ gnatlink [switches] mainprog[.ali] [non-Ada objects] [linker options]
`mainprog.ali' references the ALI file of the main program.
The `.ali' extension of this file can be omitted. From this
reference, gnatlink locates the corresponding binder file
`b~mainprog.adb' and, using the information in this file along
with the list of non-Ada objects and linker options, constructs a UNIX
linker command file to create the executable.
The arguments following `mainprog.ali' are passed to the
linker uninterpreted. They typically include the names of object files
for units written in other languages than Ada and any library references
required to resolve references in any of these foreign language units,
or in pragma Import statements in any Ada units. This list may
also include linker switches.
gnatlink determines the list of objects required by the Ada
program and prepends them to the list of objects passed to the linker.
gnatlink also gathers any arguments set by the use of
pragma Linker_Options and adds them to the list of arguments
presented to the linker.
gnatlink
The following switches are available with the gnatlink utility:
-A
-C
gnatlink that the binder has generated C code rather than
Ada code.
-g
-g.
In addition, the binder does not delete the `b~mainprog.adb',
`b~mainprog.o' and `b~mainprog.ali' files.
Without -g, the binder removes these files by
default. The same procedure apply if a C bind file was generated using
-C gnatbind option, in this case the filenames are
`b_mainprog.c' and `b_mainprog.o'.
-n
-v
-v -v
-o exec-name
gnatlink try.ali creates
an executable called `try'.
-b target
-Bdir
gnat1, the Ada compiler)
from dir instead of the default location. Only use this switch
when multiple versions of the GNAT compiler are available. See the
gcc manual page for further details. You would normally use the
-b or -V switch instead.
--GCC=compiler_name
gcc'. You need to use quotes around compiler_name if
compiler_name contains spaces or other separator characters. As
an example --GCC="foo -x -y" will instruct gnatlink to use
foo -x -y as your compiler. Note that switch -c is always
inserted after your command name. Thus in the above example the compiler
command that will be used by gnatlink will be foo -c -x -y.
--LINK=name
gnatmakeA typical development cycle when working on an Ada program consists of the following steps:
The third step can be tricky, because not only do the modified files
have to be compiled, but any files depending on these files must also be
recompiled. The dependency rules in Ada can be quite complex, especially
in the presence of overloading, use clauses, generics and inlined
subprograms.
gnatmake automatically takes care of the third and fourth steps
of this process. It determines which sources need to be compiled,
compiles them, and binds and links the resulting object files.
Unlike some other Ada make programs, the dependencies are always
accurately recomputed from the new sources. The source based approach of
the GNAT compilation model makes this possible. This means that if
changes to the source program cause corresponding changes in
dependencies, they will always be tracked exactly correctly by
gnatmake.
gnatmake
The form of the gnatmake command is
$ gnatmake [switches] file_name [mode_switches]
The only required argument is file_name, which specifies
the compilation unit that is the main program. If switches are
present, they can be placed before of after file_name.
If mode_switches are present, they must always be placed after
file_name and all switches.
If you are using standard file extensions (.adb and .ads), then the
extension may be omitted from the file_name argument. However, if
you are using non-standard extensions, then it is required that the
extension be given. A relative or absolute directory path can be
specified in file_name, in which case, the input source file will
be searched for in the specified directory only. Otherwise, the input
source file will first be searched in the directory where
gnatmake was invoked and if it is not found, it will be search on
the source path of the compiler as described in section Search Paths and the Run-Time Library (RTL).
All gnatmake output (except when you specify
-M) is to
stderr. The output produced by the
-M switch is send to
stdout.
gnatmake
You may specify any of the following switches to gnatmake:
--GCC=compiler_name
gcc'. You need to use
quotes around compiler_name if compiler_name contains
spaces or other separator characters. As an example --GCC="foo -x
-y" will instruct gnatmake to use foo -x -y as your
compiler. Note that switch -c is always inserted after your
command name. Thus in the above example the compiler command that will
be used by gnatmake will be foo -c -x -y.
--GNATBIND=binder_name
gnatbind'. You need to
use quotes around binder_name if binder_name contains spaces
or other separator characters. As an example --GNATBIND="bar -x
-y" will instruct gnatmake to use bar -x -y as your
binder. Binder switches that are normally appended by gnatmake to
gnatbind' are now appended to the end of bar -x -y.
--GNATLINK=linker_name
gnatlink'. You need to
use quotes around linker_name if linker_name contains spaces
or other separator characters. As an example --GNATLINK="lan -x
-y" will instruct gnatmake to use lan -x -y as your
linker. Linker switches that are normally appended by gnatmake to
gnatlink' are now appended to the end of lan -x -y.
-a
gnatmake does not check these files,
because the assumption is that the GNAT internal files are properly up
to date, and also that any write protected ALI files have been properly
installed. Note that if there is an installation problem, such that one
of these files is not up to date, it will be properly caught by the
binder.
You may have to specify this switch if you are working on GNAT
itself. -f is also useful in conjunction with
-f
if you need to recompile an entire application,
including run-time files, using special configuration pragma settings,
such as a non-standard Float_Representation pragma.
By default
gnatmake -a compiles all GNAT
internal files with
gcc -c -gnatg rather than gcc -c.
-c
gnatmake will attempt binding and linking
unless all objects are up to date and the executable is more recent than
the objects.
-f
-a switch is also specified.
-i
gnatmake compiles all object files and ALI files
into the current directory. If the -i switch is used,
then instead object files and ALI files that already exist are overwritten
in place. This means that once a large project is organized into separate
directories in the desired manner, then gnatmake will automatically
maintain and update this organization. If no ALI files are found on the
Ada object path (section Search Paths and the Run-Time Library (RTL)),
the new object and ALI files are created in the
directory containing the source being compiled. If another organization
is desired, where objects and sources are kept in different directories,
a useful technique is to create dummy ALI files in the desired directories.
When detecting such a dummy file, gnatmake will be forced to recompile
the corresponding source file, and it will be put the resulting object
and ALI files in the directory where it found the dummy file.
-jn
gnatmake will give you the full ordered
list of failing compiles at the end). If this is problematic, rerun
the make process with n set to 1 to get a clean list of messages.
-k
gnatmake
terminates.
-m
gnatmake ignores time
stamp differences when the only
modifications to a source file consist in adding/removing comments,
empty lines, spaces or tabs. This means that if you have changed the
comments in a source file or have simply reformatted it, using this
switch will tell gnatmake not to recompile files that depend on it
(provided other sources on which these files depend have undergone no
semantic modifications).
-M
stdout in a form that can be directly exploited in
a `Makefile'. By default, each source file is prefixed with its
(relative or absolute) directory name. This name is whatever you
specified in the various -aI
and -I switches. If you use
gnatmake -M
-q
(see below), only the source file names,
without relative paths, are output. If you just specify the
-M
switch, dependencies of the GNAT internal system files are omitted. This
is typically what you want. If you also specify
the -a switch,
dependencies of the GNAT internal files are also listed. Note that
dependencies of the objects in external Ada libraries (see switch
-aLdir in the following list) are never reported.
-n
-o exec_name
-o switch is omitted the default
name for the executable will be the name of the input file in appropriate form
for an executable file on the host system.
-q
gnatmake are displayed.
-v
gnatmake
decides are necessary.
-z
gcc switches-g or any uppercase switch (other than -A, or
-L) or any switch that is more than one character is passed to
gcc (e.g. -O, -gnato, etc.)
Source and library search path switches:
-aIdir
-aLdir
gnatmake to skip compilation units whose `.ali'
files have been located in directory dir. This allows you to have
missing bodies for the units in dir. You still need to specify
the location of the specs for these units by using the switches
-aIdir
or -Idir.
Note: this switch is provided for compatibility with previous versions
of gnatmake. The easier method of causing standard libraries
to be excluded from consideration is to write-protect the corresponding
ALI files.
-aOdir
gnatbind.
-Adir
-aLdir
-aIdir.
-Idir
-aOdir
-aIdir.
-I-
gnatmake was invoked.
-Ldir
-largs -Ldir.
-nostdinc
-nostdlib
gnatmake
The mode switches (referred to as mode_switches) allow the
inclusion of switches that are to be passed to the compiler itself, the
binder or the linker. The effect of a mode switch is to cause all
subsequent switches up to the end of the switch list, or up to the next
mode switch, to be interpreted as switches to be passed on to the
designated component of GNAT.
-cargs switches
gcc. They will be passed on to
all compile steps performed by gnatmake.
-bargs switches
gcc. They will be passed on to
all bind steps performed by gnatmake.
-largs switches
gcc. They will be passed on to
all link steps performed by gnatmake.
This section contains some additional useful notes on the operation
of the gnatmake command.
gnatmake finds no ALI files, it recompiles the main program
and all other units required by the main program.
This means that gnatmake
can be used for the initial compile, as well as during subsequent steps of
the development cycle.
gnatmake file.adb, where `file.adb'
is a subunit or body of a generic unit, gnatmake recompiles
`file.adb' (because it finds no ALI) and stops, issuing a
warning.
gnatmake the switch -I
is used to specify both source and
library file paths. Use -aI
instead if you just want to specify
source paths only and -aO
if you want to specify library paths
only.
gnatmake examines both an ALI file and its corresponding object file
for consistency. If an ALI is more recent than its corresponding object,
or if the object file is missing, the corresponding source will be recompiled.
Note that gnatmake expects an ALI and the corresponding object file
to be in the same directory.
gnatmake will ignore any files whose ALI file is write-protected.
This may conveniently be used to exclude standard libraries from
consideration and in particular it means that the use of the
-f switch will not recompile these files
unless -a is also specified.
gnatmake has been designed to make the use of Ada libraries
particularly convenient. Assume you have an Ada library organized
as follows: obj-dir contains the objects and ALI files for
of your Ada compilation units,
whereas include-dir contains the
specs of these units, but no bodies. Then to compile a unit
stored in main.adb, which uses this Ada library you would just type
$ gnatmake -aIinclude-dir -aLobj-dir main
gnatmake along with the
-m (minimal recompilation)
switch provides an
extremely powerful tool: you can freely update the comments/format of your
source files without having to recompile everything. Note, however, that
adding or deleting lines in a source files may render its debugging
info obsolete. If the file in question is a spec, the impact is rather
limited, as that debugging info will only be useful during the
elaboration phase of your program. For bodies the impact can be more
significant. In all events, your debugger will warn you if a source file
is more recent than the corresponding object, and therefore obsolescence of
debugging information will go unnoticed.
gnatmake Works
Generally gnatmake automatically performs all necessary
recompilations and you don't need to worry about how it works. However,
it may be useful to have some basic understanding of the gnatmake
approach and in particular to understand how it uses the results of
previous compilations without incorrectly depending on them.
First a definition: an object file is considered up to date if the corresponding ALI file exists and its time stamp predates that of the object file and if all the source files listed in the dependency section of this ALI file have time stamps matching those in the ALI file. This means that neither the source file itself nor any files that it depends on have been modified, and hence there is no need to recompile this file.
gnatmake works by first checking if the specified main unit is up
to date. If so, no compilations are required for the main unit. If not,
gnatmake compiles the main program to build a new ALI file that
reflects the latest sources. Then the ALI file of the main unit is
examined to find all the source files on which the main program depends,
and gnatmake recursively applies the above procedure on all these files.
This process ensures that gnatmake only trusts the dependencies
in an existing ALI file if they are known to be correct. Otherwise it
always recompiles to determine a new, guaranteed accurate set of
dependencies. As a result the program is compiled "upside down" from what may
be more familiar as the required order of compilation in some other Ada
systems. In particular, clients are compiled before the units on which
they depend. The ability of GNAT to compile in any order is critical in
allowing an order of compilation to be chosen that guarantees that
gnatmake will recompute a correct set of new dependencies if
necessary.
gnatmake Usagegnatmake hello.adb
Hello) and bind and link the
resulting object files to generate an executable file `hello'.
gnatmake -q Main_Unit -cargs -O2 -bargs -l
Main_Unit (from file `main_unit.adb'). All compilations will
be done with optimization level 2 and the order of elaboration will be
listed by the binder. gnatmake will operate in quiet mode, not
displaying commands it is executing.
Complex project organizations can be handled in a very powerful way by using GNU make combined with gnatmake. Here is for instance a Makefile which allows to build each subsystem of a big project into a separate shared library. Such a makefile allows to significantly reduce the link time of very bug applications while maintaining a complete coherence at each step of the build process.
## This Makefile is intended to be used with the following directory
## configuration:
## - The sources are split into a series of csc (computer software components)
## Each of these csc is put in its own directory.
## Their name are referenced by the directory names.
## They will be compiled into shared library (although this would also work
## with static libraries
## - The main program (and possibly other packages that do not belong to any
## csc is put in the top level directory (where the Makefile is).
## toplevel_dir __ first_csc (sources) __ lib (will contain the library)
## \_ second_csc (sources) __ lib (will contain the library)
## \_ ...
## Although this Makefile is build for shared library, it is easy to modify
## to build partial link objects instead (modify the lines with -shared and
## gnatlink below)
##
## With this makefile, you can change any file in the system or add any new
## file, and everything will be recompiled correctly (only the relevant shared
## objects will be recompiled, and the main program will be re-linked).
# The list of computer software component for your project
CSC_LIST=aa bb cc
# Name of the main program (no extension)
MAIN=main
# If we need to build objects with -fPIC, uncomment the following line
#NEED_FPIC=-fPIC
# The following variable should give the directory containing libgnat.so
# You can get this directory through 'gnatls -v'. This is usually the last
# directory in the Object_Path.
GLIB=...
# The directories for the libraries
# (This macro expands the list of CSC to the list of shared libraries, you
# could simply use the expanded form :
# LIB_DIR=aa/lib/libaa.so bb/lib/libbb.so cc/lib/libcc.so
LIB_DIR=${foreach dir,${CSC_LIST},${dir}/lib/lib${dir}.so}
${MAIN}: objects ${LIB_DIR}
gnatbind ${MAIN} ${CSC_LIST:%=-aO%/lib} -shared
gnatlink ${MAIN} ${CSC_LIST:%=-l%}
objects::
# recompile the sources
gnatmake -c -i ${MAIN}.adb ${NEED_FPIC} ${CSC_LIST:%=-I%}
# Note about the rules below: if your csc are not split into multiple
# directories, but simply by their name, you need to replace *.o and
# *.ali with the appropriate list of files
# Note: In a future version of GNAT, the following commands will be simplified
# by a new tool, gnatmlib
${LIB_DIR}:
mkdir -p ${dir $ }
cd ${dir $ }; gcc -shared -o ${notdir $ } ../*.o -L${GLIB} -lgnat
cd ${dir $ }; cp -f ../*.ali .
# The dependencies for the modules
aa/lib/libaa.so: aa/*.o
bb/lib/libbb.so: bb/*.o
bb/lib/libcc.so: cc/*.o
run::
LD_LIBRARY_PATH=pwd/aa/lib:pwd/bb/lib:pwd/cc/lib ./${MAIN}
clean::
${RM} -rf ${CSC_LIST:%=%/lib}
${RM} ${CSC_LIST:%=%/*.ali}
${RM} ${CSC_LIST:%=%/*.o}
${RM} *.o *.ali ${MAIN}
gnatchop
This chapter discusses how to handle files with multiple units by using
the gnatchop utility. This utility is also useful in renaming
files to meet the standard GNAT default file naming conventions.
The basic compilation model of GNAT requires that a file submitted to the compiler have only one unit and there be a strict correspondence between the file name and the unit name.
The gnatchop utility allows both of these rules to be relaxed,
allowing GNAT to process files which contain multiple compilation units
and files with arbitrary file names. gnatchop
reads the specified file and generates one or more output files,
containing one unit per file. The unit and the file name correspond,
as required by GNAT.
If you want to permanently restructure a set of "foreign" files so that
they match the GNAT rules, and do the remaining development using the
GNAT structure, you can simply use gnatchop once, generate the
new set of files and work with them from that point on.
Alternatively, if you want to keep your files in the "foreign" format,
perhaps to maintain compatibility with some other Ada compilation
system, you can set up a procedure where you use gnatchop each
time you compile, regarding the source files that it writes as temporary
files that you throw away.
The basic function of gnatchop is to take a file with multiple units
and split it into separate files. The boundary between files is reasonably
clear, except for the issue of comments and pragmas. In default mode, the
rule is that any pragmas between units belong to the previous unit, except
that configuration pragmas always belong to the following unit. Any comments
belong to the following unit. These rules
almost always result in the right choice of
the split point without needing to mark it explicitly and most users will
find this default to be what they want. In this default mode it is incorrect to
submit a file containing only configuration pragmas, or one that ends in
configuration pragmas, to gnatchop.
However, using a special option to activate "compilation mode",
gnatchop
can perform another function, which is to provide exactly the semantics
required by the RM for handling of configuration pragmas in a compilation.
In the absence of configuration pragmas (at the main file level), this
option has no effect, but it causes such configuration pragmas to be handled
in a quite different manner.
First, in compilation mode, if gnatchop is given a file that consists of
only configuration pragmas, then this file is appended to the
`gnat.adc' file in the current directory. This behavior provides
the required behavior described in the RM for the actions to be taken
on submitting such a file to the compiler, namely that these pragmas
should apply to all subsequent compilations in the same compilation
environment. Using GNAT, the current directory, possibly containing a
`gnat.adc' file is the representation
of a compilation environment. For more information on the
`gnat.adc' file, see the section on handling of configuration
pragmas see section Handling of Configuration Pragmas.
Second, in compilation mode, if gnatchop
is given a file that starts with
configuration pragmas, and contains one or more units, then these
configuration pragmas are prepended to each of the chopped files. This
behavior provides the required behavior described in the RM for the
actions to be taken on compiling such a file, namely that the pragmas
apply to all units in the compilation, but not to subsequently compiled
units.
Finally, if configuration pragmas appear between units, they are appended to the previous unit. This results in the previous unit being illegal, since the compiler does not accept configuration pragmas that follow a unit. This provides the required RM behavior that forbids configuration pragmas other than those preceding the first compilation unit of a compilation.
For most purposes, gnatchop will be used in default mode. The
compilation mode described above is used only if you need exactly
accurate behavior with respect to compilations, and you have files
that contain multiple units and configuration pragmas. In this
circumstance the use of gnatchop with the compilation mode
switch provides the required behavior, and is for example the mode
in which GNAT processes the ACVC tests.
gnatchop
The gnatchop command has the form:
$ gnatchop switches file name [file name file name ...] [directory]
The only required argument is the file name of the file to be chopped. There are no restrictions on the form of this file name. The file itself contains one or more Ada units, in normal GNAT format, concatenated together. As shown, more than one file may be presented to be chopped.
When run in default mode, gnatchop generates one output file in
the current directory for each unit in each of the files.
directory, if specified, gives the name of the directory to which the output files will be written. If it is not specified, all files are written to the current directory.
For example, given a file called `hellofiles' containing
procedure hello;
with Text_IO; use Text_IO;
procedure hello is
begin
Put_Line ("Hello");
end hello;
the command
$ gnatchop hellofiles
generates two files in the current directory, one called `hello.ads' containing the single line that is the procedure spec, and the other called `hello.adb' containing the remaining text. The original file is not affected. The generated files can be compiled in the normal manner.
gnatchop
gnatchop recognizes the following switches:
-c
gnatchop to operate in compilation mode, in which
configuration pragmas are handled according to strict RM rules. See
previous section for a full description of this mode.
-gnatxxx
-gnatxxx switch to gnat which is
used to parse the given file. Not all xxx options make sense,
but for example, the use of -gnati2 allows gnatchop to
process a source file that uses Latin-2 coding for identifiers.
-h
gnatchop to generate a brief help summary to the standard
output file showing usage information.
-kmm
mm
of characters.
This is useful if the
resulting set of files is required to be interoperable with systems
which limit the length of file names.
No space is allowed between the -k and the numeric value. The numeric
value may be omitted in which case a default of -k8,
suitable for use
with DOS-like file systems, is used. If no -k switch
is present then
there is no limit on the length of file names.
-q
-r
Source_Reference pragmas. Use this switch if the output
files are regarded as temporary and development is to be done in terms
of the original unchopped file. This switch causes
Source_Reference pragmas to be inserted into each of the
generated files to refers back to the original file name and line number.
The result is that all error messages refer back to the original
unchopped file.
In addition, the debugging information placed into the object file (when
the -g switch of gcc or gnatmake is specified) also
refers back to this original file so that tools like profilers and
debuggers will give information in terms of the original unchopped file.
If the original file to be chopped itself contains
a Source_Reference
pragma referencing a third file, then gnatchop respects
this pragma, and the generated Source_Reference pragmas
in the chopped file refer to the original file, with appropriate
line numbers. This is particularly useful when gnatchop
is used in conjunction with gnatprep to compile files that
contain preprocessing statements and multiple units.
-v
gnatchop to operate in verbose mode. The version
number and copyright notice are output, as well as exact copies of
the gnat1 commands spawned to obtain the chop control information.
-w
gnatchop regards it as a
fatal error if there is already a file with the same name as a
file it would otherwise output, in other words if the files to be
chopped contain duplicated units. This switch bypasses this
check, and causes all but the last instance of such duplicated
units to be skipped.
gnatchop Usagegnatchop -w hello_s.ada ichbiah/files
gnatchop archive
gnatchop is in sending sets of sources
around, for example in email messages. The required sources are simply
concatenated (for example, using a UNIX cat
command), and then
gnatchop is used at the other end to reconstitute the original
file names.
gnatchop file1 file2 file3 direc
-w switch,
in which case the last occurrence in the last file will
be the one that is output, and earlier duplicate occurrences for a given
unit will be skipped.
In Ada 95, configuration pragmas include those pragmas described as
such in the Ada 95 Reference Manual, as well as
implementation-dependent pragmas that are configuration pragmas. See the
individual descriptions of pragmas in the GNAT Reference Manual for
details on these additional GNAT-specific configuration pragmas. Most
notably, the pragma Source_File_Name, which allows
specifying non-default names for source files, is a configuration
pragma.
Configuration pragmas may either appear at the start of a compilation unit, in which case they apply only to that unit, or they may apply to all compilations performed in a given compilation environment.
GNAT also provides the gnatchop utility to provide an automatic
way to handle configuration pragmas following the semantics for
compilations (that is, files with multiple units), described in the RM.
See section see section Operating gnatchop in Compilation Mode for details.
However, for most purposes, it will be more convenient to edit the
`gnat.adc' file that contains configuration pragmas directly,
as described in the following section.
In GNAT a compilation environment is defined by the current directory at the time that a compile command is given. This current directory is searched for a file whose name is `gnat.adc'. If this file is present, it is expected to contain one or more configuration pragmas that will be applied to the current compilation.
Configuration pragmas may be entered into the `gnat.adc' file
either by running gnatchop on a source file that consists only of
configuration pragmas, or more conveniently by
direct editing of the `gnat.adc' file, which is a standard format
source file.
This chapter describes the handling of elaboration code in Ada 95 and in GNAT, and discusses how the order of elaboration of program units can be controlled in GNAT, either automatically or with explicit programming features.
Ada 95 provides rather general mechanisms for executing code at elaboration time, that is to say before the main program starts executing. Such code arises in three contexts:
Sqrt_Half : Float := Sqrt (0.5);
BEGIN-END section at the outer level of a package body is
executed as part of the package body elaboration code.
Subprogram calls are possible in any of these contexts, which means that any arbitrary part of the program may be executed as part of the elaboration code. It is even possible to write a program which does all its work at elaboration time, with a null main program, although stylistically this would usually be considered an inappropriate way to structure a program.
An important concern arises in the context of elaboration code:
we have to be sure that it is executed in an appropriate order. What we
have is numerous sections of elaboration code, potentially one section
for each unit in the program. It is important that these execute
in the correct order. Correctness here means that, taking the above
example of the declaration of Sqrt_Half,
that if some other piece of
elaboration code references Sqrt_Half,
then it must run after the
section of elaboration code that contains the declaration of
Sqrt_Half.
There would never be any order of elaboration problem if we made a rule
that whenever you with a unit, you must elaborate both the spec and body
of that unit before elaborating the unit doing the with'ing:
with Unit_1; package Unit_2 is ...
would require that both the body and spec of Unit_1 be elaborated
before the spec of Unit_2. However, a rule like that would be far too
restrictive. In particular, it would make it impossible to have routines
in separate packages that were mutually recursive.
You might think that a clever enough compiler could look at the actual elaboration code and determine an appropriate correct order of elaboration, but in the general case, this is not possible. Consider the following example.
In the body of Unit_1, we have a procedure Func_1
that references
the variable Sqrt_1, which is declared in the elaboration code
of the body of Unit_1:
Sqrt_1 : Float := Sqrt (0.1);
The elaboration code of the body of Unit_1 also contains:
if expression_1 = 1 then
Q := Unit_2.Func_2;
end if;
Unit_2 is exactly parallel,
it has a procedure Func_2 that references
the variable Sqrt_2, which is declared in the elaboration code of
the body Unit_2:
Sqrt_2 : Float := Sqrt (0.1);
The elaboration code of the body of Unit_2 also contains:
if expression_2 = 2 then
Q := Unit_1.Func_1;
end if;
Now the question is, which of the following orders of elaboration is acceptable:
Spec of Unit_1 Spec of Unit_2 Body of Unit_1 Body of Unit_2
or
Spec of Unit_2 Spec of Unit_1 Body of Unit_2 Body of Unit_1
If you carefully analyze the flow here, you will see that you cannot tell
at compile time the answer to this question.
If expression_1 is not equal to 1,
and expression_2 is not equal to 2,
then either order is acceptable, because neither of the function calls is
executed. If both tests evaluate to true, then neither order is acceptable
and in fact there is no correct order.
If one of the two expressions is true, and the other is false, then one
of the above orders is correct, and the other is incorrect. For example,
if expression_1 = 1 and expression_2 /= 2,
then the call to Func_2
will occur, but not the call to Func_1.
This means that it is essential
to elaborate the body of Unit_1 before
the body of Unit_2, so the first
order of elaboration is correct and the second is wrong.
By making expression_1 and expression_2
depend on input data, or perhaps
the time of day, we can make it impossible for the compiler or binder
to figure out which of these expressions will be true, and hence it
is impossible to guarantee a safe order of elaboration at run time.
In some languages that involve the same kind of elaboration problems, e.g. Java and C++, the programmer is expected to worry about these ordering problems himself, and it is common to write a program in which an incorrect elaboration order gives surprising results, because it references variables before they are initialized. Ada 95 is designed to be a safe language, and a programmer-beware approach is clearly not sufficient. Consequently, the language provides three lines of defense:
with a unit, then its spec is always
elaborated before the unit doing the with. Similarly, a parent
spec is always elaborated before the child spec, and finally
a spec is always elaborated before its corresponding body.
Program_Error) is raised.
Let's look at these facilities in more detail. First, the rules for dynamic checking. One possible rule would be simply to say that the exception is raised if you access a variable which has not yet been elaborated. The trouble with this approach is that it could require expensive checks on every variable reference. Instead Ada 95 has two rules which are a little more restrictive, but easier to check, and easier to state:
Program_Error is raised.
Program_Error is raised.
The idea is that if the body has been elaborated, then any variables it references must have been elaborated; by checking for the body being elaborated we guarantee that none of its references causes any trouble. As we noted above, this is a little too restrictive, because a subprogram that has no non-local references in its body may in fact be safe to call. However, it really would be unsafe to rely on this, because it would mean that the caller was aware of details of the implementation in the body. This goes against the basic tenets of Ada.
A plausible implementation can be described as follows.
A Boolean variable is associated with each subprogram
and each generic unit. This variable is initialized to False, and is set to
True at the point body is elaborated. Every call or instantiation checks the
variable, and raises Program_Error if the variable is False.
In the previous section we discussed the rules in Ada 95 which ensure
that Program_Error is raised if an incorrect elaboration order is
chosen. This prevents erroneous executions, but we need mechanisms to
specify a correct execution and avoid the exception altogether.
To achieve this, Ada 95 provides a number of features for controlling
the order of elaboration. We discuss these features in this section.
First, there are several ways of indicating to the compiler that a given unit has no elaboration problems:
package Definitions is
generic
type m is new integer;
package Subp is
type a is array (1 .. 10) of m;
type b is array (1 .. 20) of m;
end Subp;
end Definitions;
A package that with's Definitions may safely instantiate
Definitions.Subp because the compiler can determine that there
definitely is no package body to worry about in this case
A has such a pragma,
and unit B does
a with of unit A. Recall that the standard rules require
the spec of unit A
to be elaborated before the with'ing unit; given the pragma in
A, we also know that the body of A
will be elaborated before B, so
that calls to A are safe and do not need a check.
Note that,
unlike pragma Pure and pragma Preelaborate,
the use of
Elaborate_Body does not guarantee that the program is
free of elaboration problems, because it may not be possible
to satisfy the requested elaboration order.
Let's go back to the example with Unit_1 and Unit_2.
If a programmer
marks Unit_1 as Elaborate_Body,
and not Unit_2, then the order of
elaboration will be:
Spec of Unit_2 Spec of Unit_1 Body of Unit_1 Body of Unit_2
Now that means that the call to Func_1 in Unit_2
need not be checked,
it must be safe. But the call to Func_2 in
Unit_1 may still fail if
Expression_1 is equal to 1,
and the programmer must still take
responsibility for this not being the case.
If all units carry a pragma Elaborate_Body, then all problems are
eliminated, except for calls entirely within a body, which are
in any case fully under programmer control. However, using the pragma
everywhere is not always possible.
In particular, for our Unit_1/Unit_2 example, if
we marked both of them as having pragma Elaborate_Body, then
clearly there would be no possible elaboration order.
The above pragmas allow a server to guarantee safe use by clients, and
clearly this is the preferable approach. Consequently a good rule in
Ada 95 is to mark units as Pure or Preelaborate if possible,
and if this is not possible,
mark them as Elaborate_Body if possible.
As we have seen, there are situation where neither of these
three pragmas can be used.
So we also provide methods for clients to control the
order of elaboration of the servers on which they depend:
with statement,
and it requires that the body of the named unit be elaborated before
the unit in which the pragma occurs. The idea is to use this pragma
if the current unit calls at elaboration time, directly or indirectly,
some subprogram in the named unit.
Unit ANow if we put a pragmawith's unit B and calls B.Func in elaboration code Unit Bwith's unit C, and B.Func calls C.Func
Elaborate (B)
in unit A, this ensures that the
body of B is elaborated before the call, but not the
body of C, so
the call to C.Func could still cause Program_Error to
be raised.
The effect of a pragma Elaborate_All is stronger, it requires
not only that the body of the named unit be elaborated before the
unit doing the with, but also the bodies of all units that the
named unit uses, following with links transitively. For example,
if we put a pragma Elaborate_All (B) in unit A,
then it requires
not only that the body of B be elaborated before A,
but also the
body of C, because B with's C.
We are now in a position to give a usage rule in Ada 95 for avoiding elaboration problems, at least if dynamic dispatching and access to subprogram values are not used. We will handle these cases separately later.
The rule is simple. If a unit has elaboration code that can directly or
indirectly make a call to a subprogram in a with'ed unit, or instantiate
a generic unit in a with'ed unit,
then if the with'ed unit does not have
pragma Pure, Preelaborate, or Elaborate_Body, then the client should have
an Elaborate_All for the with'ed unit. By following this rule a client is
assured that calls can be made without risk of an exception.
If this rule is not followed, then a program may be in one of four
states:
Note that one additional advantage of following our Elaborate_All rule is that the program continues to stay in the ideal (all orders OK) state even if maintenance changes some bodies of some subprograms. Conversely, if a program that does not follow this rule happens to be safe at some point, this state of affairs may deteriorate silently as a result of maintenance changes.
In the case of internal calls, i.e. calls within a single package, the programmer has full control over the order of elaboration, and it is up to the programmer to elaborate declarations in an appropriate order. For example writing:
function One return Float;
Q : Float := One;
function One return Float is
begin
return 1.0;
end One;
will obviously raise Program_Error at run time, because function One will be called before its body is elaborated. In this case GNAT will generate a warning that the call will raise Program_Error:
1. procedure y is
2. function One return Float;
3.
4. Q : Float := One;
|
>>> warning: cannot call "One" before body is elaborated
>>> warning: Program_Error will be raised at run time
5.
6. function One return Float is
7. begin
8. return 1.0;
9. end One;
10.
11. begin
12. null;
13. end;
Note that in this particular case, it is likely that the call is safe, because
the function One does not access any global variables.
Nevertheless in Ada 95, we do not want the validity of the check to depend on
the contents of the body (think about the separate compilation case), so this
is still wrong, as we discussed in the previous sections.
The error is easily corrected by rearranging the declarations so that the body of One appears before the declaration containing the call (note that in Ada 95, declarations can appear in any order, so there is no restriction that would prevent this reordering, and if we write:
function One return Float;
function One return Float is
begin
return 1.0;
end One;
Q : Float := One;
then all is well, no warning is generated, and no
Program_Error exception
will be raised.
Things are more complicated when a chain of subprograms is executed:
function A return Integer; function B return Integer; function C return Integer; function B return Integer is begin return A; end; function C return Integer is begin return B; end; X : Integer := C; function A return Integer is begin return 1; end;
Now the call to C
at elaboration time in the declaration of X is correct, because
the body of C is already elaborated,
and the call to B within the body of
C is correct, but the call
to A within the body of B is incorrect, because the body
of A has not been elaborated, so Program_Error
will be raised on the call to A.
In this case GNAT will generate a
warning that Program_Error may be
raised at the point of the call. Let's look at the warning:
1. procedure x is
2. function A return Integer;
3. function B return Integer;
4. function C return Integer;
5.
6. function B return Integer is begin return A; end;
|
>>> warning: call to "A" before body is elaborated may
raise Program_Error
>>> warning: "B" called at line 7
>>> warning: "C" called at line 9
7. function C return Integer is begin return B; end;
8.
9. X : Integer := C;
10.
11. function A return Integer is begin return 1; end;
12.
13. begin
14. null;
15. end;
Note that the message here says "may raise", instead of the direct case,
where the message says "will be raised". That's because whether
A is
actually called depends in general on run-time flow of control.
For example, if the body of B said
function B return Integer is
begin
if some-condition-depending-on-input-data then
return A;
else
return 1;
end if;
end B;
then we could not know until run time whether the incorrect call to A would
actually occur, so Program_Error might
or might not be raised. It is possible for a compiler to
do a better job of analyzing bodies, to
determine whether or not Program_Error
might be raised, but it certainly
couldn't do a perfect job (that would require solving the halting problem
and is provably impossible), and because this is a warning anyway, it does
not seem worth the effort to do the analysis. Cases in which it
would be relevant are rare.
In practice, warnings of either of the forms given above will usually correspond to real errors, and should be examined carefully and eliminated. In the rare case where a warning is bogus, it can be suppressed by any of the following methods:
-gnatws switch set
Elaboration_Checks for the called subprogram
Warnings_Off to turn warnings off for the call
For the internal elaboration check case,
GNAT by default generates the
necessary run-time checks to ensure
that Program_Error is raised if any
call fails an elaboration check. Of course this can only happen if a
warning has been issued as described above. The use of pragma
Suppress (Elaboration_Checks) may (but is not guaranteed) to suppress
some of these checks, meaning that it may be possible (but is not
guaranteed) for a program to be able to call a subprogram whose body
is not yet elaborated, without raising a Program_Error exception.
The previous section discussed the case in which the execution of a particular thread of elaboration code occurred entirely within a single unit. This is the easy case to handle, because a programmer has direct and total control over the order of elaboration, and furthermore, checks need only be generated in cases which are rare and which the compiler can easily detect. The situation is more complex when separate compilation is taken into account. Consider the following:
package Math is
function Sqrt (Arg : Float) return Float;
end Math;
package body Math is
function Sqrt (Arg : Float) return Float is
begin
...
end Sqrt;
end Math;
with Math;
package Stuff is
X : Float := Math.Sqrt (0.5);
end Stuff;
with Stuff;
procedure Main is
begin
...
end Main;
where Main is the main program. When this program is executed, the
elaboration code must first be executed, and one of the jobs of the
binder is to determine the order in which the units of a program are
to be elaborated. In this case we have four units: the spec and body
of Math,
the spec of Stuff and the body of Main).
In what order should the four separate sections of elaboration code
be executed?
There are some restrictions in the order of elaboration that the binder
can choose. In particular, if unit U has a with
for a package X, then you
are assured that the spec of X
is elaborated before U , but you are
not assured that the body of X
is elaborated before U.
This means that in the above case, the binder is allowed to choose the
order:
spec of Math spec of Stuff body of Math body of Main
but that's not good, because now the call to Math.Sqrt
that happens during
the elaboration of the Stuff
spec happens before the body of Math.Sqrt is
elaborated, and hence causes Program_Error exception to be raised.
At first glance, one might say that the binder is misbehaving, because
obviously you want to elaborate the body of something you with
first, but
that is not a general rule that can be followed in all cases. Consider
package X is ... package Y is ... with X; package body Y is ... with Y; package body X is ...
This is a common arrangement, and, apart from the order of elaboration
problems that might arise in connection with elaboration code, this works fine.
A rule that says that you must first elaborate the body of anything you
with cannot work in this case
(the body of X with's Y,
which means you would have to
elaborate the body of Y first, but that with's X,
which means
you have to elaborate the body of X first, but ... and we have a
loop that cannot be broken.
It is true that the binder can in many cases guess an order of elaboration
that is unlikely to cause a Program_Error
exception to be raised, and it tries to do so (in the
above example of Math/Stuff/Spec, the GNAT binder will
in fact always
elaborate the body of Math right after its spec, so all will be well).
However, a program that blindly relies on the binder to be helpful can get into trouble, as we discussed in the previous sections, so GNAT provides a number of facilities for assisting the programmer in developing programs that are robust with respect to elaboration order.
The default behavior in GNAT ensures elaboration safety. In its default mode GNAT implements the rule we previously described as the right approach. Let's restate it:
If a unit has elaboration code that can directly or indirectly make a
call to a subprogram in a with'ed unit, or instantiate a generic unit
in a with'ed unit, then if the with'ed unit
does not have pragma Pure,
Preelaborate, or Elaborate_Body,
then the client should have an
Elaborate_All for the with'ed unit. By following this rule a client
is assured that calls and instantiations can be made without risk of an exception.
In this mode GNAT traces all calls that are potentially made from
elaboration code, and put in any missing implicit Elaborate_All
pragmas.
The advantage of this approach is that no elaboration problems
are possible if the binder can find an elaboration order that is
consistent with these implicit Elaborate_All pragmas. The
disadvantage of this approach is that no such order may exist.
If the binder does not generate any diagnostics, then it means that it
has found an elaboration order that is guaranteed to be safe. However,
the binder may still be relying on implicitly generated
Elaborate_All pragmas so portability to other compilers than
GNAT is not guaranteed.
If it is important to guarantee portability, then the compilations should
use the
-gnatwl
(warn on elaboration problems) switch. This will cause warning messages
to be generated indicating the missing Elaborate_All pragmas.
Consider the following source program:
with k;
package j is
m : integer := k.r;
end;
where it is clear that there
should be a pragma Elaborate_All
for unit k. An implicit pragma will be generated, and it is
likely that the binder will be able to honor it. However,
it is safer to include the pragma explicitly in the source. If this
unit is compiled with the
-gnatwl
switch, then the compiler outputs a warning:
1. with k;
2. package j is
3. m : integer := k.r;
|
>>> warning: call to "r" may raise Program_Error
>>> warning: missing pragma Elaborate_All for "k"
4. end;
and these warnings can be used as a guide for supplying manually the missing pragmas.
If the binder cannot find an acceptable order, it outputs detailed diagnostics. For example:
error: elaboration circularity detected info: "proc (body)" must be elaborated before "pack (body)" info: reason: Elaborate_All probably needed in unit "pack (body)" info: recompile "pack (body)" with -gnatwl info: for full details info: "proc (body)" info: is needed by its spec: info: "proc (spec)" info: which is withed by: info: "pack (body)" info: "pack (body)" must be elaborated before "proc (body)" info: reason: pragma Elaborate in unit "proc (body)"
In this case we have a cycle that the binder cannot break. On the one
hand, there is an explicit pragma Elaborate in proc for
pack. This means that the body of pack must be elaborated
before the body of proc. On the other hand, there is elaboration
code in pack that calls a subprogram in proc. This means
that for maximum safety, there should really be a pragma
Elaborate_All in pack for proc which would require that
the body of proc be elaborated before the body of
pack. Clearly both requirements cannot be satisfied.
Faced with a circularity of this kind, you have three different options.
-gnatE
(dynamic elaboration check) switch, then GNAT behaves in
a quite different manner. Dynamic checks are generated for all calls
that could possibly result in raising an exception. With this switch,
the compiler does not generate implicit Elaborate_All pragmas.
The behavior then is exactly as specified in the Ada 95 Reference Manual.
The binder will generate an executable program that may or may not
raise Program_Error, and then it is the programmer's job to ensure
that it does not raise an exception. Note that it is important to
compile all units with the switch, it cannot be used selectively.
-f
switch for the
gnatbind
step, or
-bargs -f
if you are using
gnatmake.
This switch tells the binder to ignore any implicit Elaborate_All
pragmas that were generated by the compiler, and suppresses any
circularity messages that they cause. The resulting executable will work
properly if there are no elaboration problems, but if there are some order of
elaboration problems they will not be detected, and unexpected
results may occur.
It is hard to generalize on which of these three approaches should be
taken. Obviously if it is possible to fix the program so that the default
treatment works, this is preferable, but this may not always be practical.
It is certainly simple enough to use
-gnatE
or
-f
but the danger in either case is that, even if the GNAT binder
finds a correct elaboration order, it may not always do so,
and certainly a binder from another Ada compiler might not. A
combination of testing and analysis (for which the warnings generated
with the
-gnatwl
switch can be useful) must be used to ensure that the program is free
of errors. One switch that is useful in this testing is the
-h (horrible elaboration order)
switch for
gnatbind.
Normally the binder tries to find an order that has the best chance of
of avoiding elaboration problems. With this switch, the binder
plays a devil's advocate role, and tries to choose the order that
has the best chance of failing. If your program works even with this
switch, then it has a better chance of being error free, but this is still
not a guarantee.
For an example of this approach in action, consider the C-tests (executable tests) from the ACVC suite. If these are compiled and run with the default treatment, then all but one of them succeed without generating any error diagnostics from the binder. However, there is one test that fails, and this is not surprising, because the whole point of this test is to ensure that the compiler can handle cases where it is impossible to determine a correct order statically, and it checks that an exception is indeed raised at run time.
This one test must be compiled and run using the
-gnatE
switch, and then it passes. Alternatively, the entire suite can
be run using this switch. It is never wrong to run with the dynamic
elaboration switch if your code is correct, and we assume that the
C-tests are indeed correct (it is less efficient, but efficiency is
not a factor in running the ACVC tests.)
The introduction of access-to-subprogram types in Ada 95 complicates the handling of elaboration. The trouble is that it becomes impossible to tell at compile time which procedure is being called. This means that it is not possible for the binder to analyze the elaboration requirements in this case.
If at the point at which the access value is created, the body of the subprogram is
known to have been elaborated, then the access value is safe, and its use
does not require a check. This may be achieved by appropriate arrangement
of the order of declarations if the subprogram is in the current unit,
or, if the subprogram is in another unit, by using pragma
Pure, Preelaborate, or Elaborate_Body
on the referenced unit.
If the referenced body is not known to have been elaborated at the point
the access value is created, then any use of the access value must do a
dynamic check, and this dynamic check will fail and raise a
Program_Error exception if the body has not been elaborated yet.
GNAT will generate the necessary checks, and in addition, if the
-gnatwl
switch is set, will generate warnings that such checks are required.
The use of dynamic dispatching for tagged types similarly generates
a requirement for dynamic checks, and premature calls to any primitive
operation of a tagged type before the body of the operation has been elaborated,
will result in the raising of Program_Error.
First, compile your program with the default options, using none of
the special elaboration control switches. If the binder successfully
binds your program, then you can be confident that, apart from issues
raised by the use of access-to-subprogram types and dynamic dispatching,
the program is free of elaboration errors. If it is important that the
program be portable, then use the
-gnatwl
switch to generate warnings about missing Elaborate_All
pragmas, and supply the missing pragmas.
If the program fails to bind using the default static elaboration
handling, then you can fix the program to eliminate the binder
message, or recompile the entire program with the
-gnatE switch to generate dynamic elaboration checks,
or, if you are sure there really are no elaboration problems,
use the
-f
switch for the binder to cause it to ignore implicit Elaborate_All
pragmas generated by the compiler.
gnatxref and gnatfindThe compiler generates cross-referencing information (unless you set the `-gnatx' switch), which are saved in the `.ali' files. This information indicates where in the source each entity is declared and referenced.
Before using any of these two tools, you need to compile successfully your application, so that GNAT gets a chance to generate the cross-referencing information.
The two tools gnatxref and gnatfind take advantage of this
information to provide the user with the capability to easily locate the
declaration and references to an entity. These tools are quite similar,
the difference being that gnatfind is intended for locating
definitions and/or references to a specified entity or entities, whereas
gnatxref is oriented to generating a full report of all
cross-references.
To use these tools, you must not compile your application using the `-gnatx' switch on the `gnatmake' command line (See Info file `gnat_ug', node `The GNAT Make Program gnatmake'). Otherwise, cross-referencing information will not be generated.
The command lines for gnatxref is:
$ gnatxref [switches] sourcefile1 [sourcefile2 ...]
where
sourcefile1, sourcefile2
The switches can be :
-a
gnatfind and gnatxref will parse
the read-only files found in the library search path. Otherwise, these files
will be ignored. This option can be used to protect Gnat sources or your own
libraries from being parsed, thus making gnatfind and gnatxref
much faster, and their output much smaller.
-aIDIR
-aODIR
-f
-g
gnatfind and gnatxref.
-IDIR
-pFILE
gnatxref and gnatfind will try to locate a
project file in the current directory.
If a project file is either specified or found by the tools, then the content
of the source directory and object directory lines are added as if they
had been specified respectively by `-aI'
and `-aO'.
-u
gnatxref will then
display every unused entity and 'with'ed package.
-v
gnatxref will generate a
`tags' file that can be used by vi. For examples how to use this
feature, see See section Examples of gnatxref usage. The tags file is output
to the standard output, thus you will have to redirect it to a file.
All these switches may be in any order on the command line, and may even appear after the file names. They need not be separated by spaces, thus you can say `gnatxref -ag' instead of `gnatxref -a -g'.
The command line for gnatfind is:
$ gnatfind [switches] pattern[:sourcefile[:line[:column]]] [file1 file2 ...]
where
pattern
sourcefile
gnatfind will look for references, bodies or declarations
of symbols referenced in `sourcefile', at line `line'
and column `column'. See see section Examples of gnatfind usage
for syntax examples.
line
column
file1 file2 ...
gnatfind may
sometimes not be able to find the body of the subprograms...
At least one of 'sourcefile' or 'pattern' has to be present on the command line.
The following switches are available:
-a
gnatfind and gnatxref will parse
the read-only files found in the library search path. Otherwise, these files
will be ignored. This option can be used to protect Gnat sources or your own
libraries from being parsed, thus making gnatfind and gnatxref
much faster, and their output much smaller.
-aIDIR
-aODIR
-e
gnatfind accept the simple regular expression set for
`pattern'. If this switch is set, then the pattern will be
considered as full Unix-style regular expression.
-f
-g
gnatfind and gnatxref.
-IDIR
-pFILE
gnatxref and gnatfind will try to locate a
project file in the current directory.
If a project file is either specified or found by the tools, then the content
of the source directory and object directory lines are added as if they
had been specified respectively by `-aI' and
`-aO'.
-r
gnatfind will output only the information about the
declaration, body or type completion of the entities. If this switch is
set, the gnatfind will locate every reference to the entities in
the files specified on the command line (or in every file in the search
path if no file is given on the command line).
-s
gnatfind will output the content
of the Ada source file lines were the entity was found.
All these switches may be in any order on the command line, and may even appear after the file names. They need not be separated by spaces, thus you can say `gnatxref -ag' instead of `gnatxref -a -g'.
As stated previously, gnatfind will search in every directory in the
search path. You can force it to look only in the current directory if
you specify * at the end of the command line.
The project files allows a programmer to specify how to compile its
application, where to find sources,... These files are used primarily by
the Emacs Ada mode, but they can also be used by the two tools
gnatxref and gnatfind.
A project file name must end with `.adp'. If a single one is
present in the current directory, then gnatxref and gnatfind will
extract the information from it. If multiple project files are found, none of
them is read, and you have to use the `-p' switch to specify the one
you want to use.
The following lines can be included, even though most of them have default values which can be used in most cases. The lines can be entered in any order in the file. Except for `src_dir' and `obj_dir', you can only have one instance of each line. If you have multiple instances, only the last one is taken into account.
gnatxref and gnatfind only take into account the `src_dir'
and `obj_dir' lines, and ignore the others.
As specified in the section about gnatfind, the pattern can be a
regular expression. Actually, there are to set of regular expressions
which are recognized by the program :
regexp ::= term
term ::= elmt -- matches elmt
term ::= elmt elmt -- concatenation (elmt then elmt)
term ::= * -- any string of 0 or more characters
term ::= ? -- matches any character
term ::= [char {char}] -- matches any character listed
term ::= [char - char] -- matches any character in range
regexp ::= term {| term} -- alternation (term or term ...)
term ::= item {item} -- concatenation (item then item)
item ::= elmt -- match elmt
item ::= elmt * -- zero or more elmt's
item ::= elmt + -- one or more elmt's
item ::= elmt ? -- matches elmt or nothing
elmt ::= nschar -- matches given character
elmt ::= [nschar {nschar}] -- matches any character listed
elmt ::= [^ nschar {nschar}] -- matches any character not listed
elmt ::= [char - char] -- matches chars in given range
elmt ::= \ char -- matches given character
elmt ::= . -- matches any single character
elmt ::= ( regexp ) -- parens used for grouping
char ::= any character, including special characters
nschar ::= any character except ()[].*+?^
Following are a few examples :
gnatxref usageFor the following examples, we will consider the following units :
main.ads: 1: with Bar; 2: package Main is 3: procedure Foo (B : in Integer); 4: C : Integer; 5: private 6: D : Integer; 7: end Main; main.adb: 1: package body Main is 2: procedure Foo (B : in Integer) is 3: begin 4: C := B; 5: D := B; 6: Bar.Print (B); 7: Bar.Print (C); 8: end Foo; 9: end Main; bar.ads: 1: package Bar is 2: procedure Print (B : Integer); 3: end bar;
gnatxref main.adb
gnatxref generates cross-reference information for main.adb
and every unit 'with'ed by main.adb.
The output would be:
B Type: Integer
Decl: bar.ads 2:22
B Type: Integer
Decl: main.ads 3:20
Body: main.adb 2:20
Ref: main.adb 4:13 5:13 6:19
Bar Type: Unit
Decl: bar.ads 1:9
Ref: main.adb 6:8 7:8
main.ads 1:6
C Type: Integer
Decl: main.ads 4:5
Modi: main.adb 4:8
Ref: main.adb 7:19
D Type: Integer
Decl: main.ads 6:5
Modi: main.adb 5:8
Foo Type: Unit
Decl: main.ads 3:15
Body: main.adb 2:15
Main Type: Unit
Decl: main.ads 2:9
Body: main.adb 1:14
Print Type: Unit
Decl: bar.ads 2:15
Ref: main.adb 6:12 7:12
that is the entity Main is declared in main.ads, line 2, column 9,
its body is in main.adb, line 1, column 14 and is not referenced any where.
The entity Print is declared in bar.ads, line 2, column 15 and it
it referenced in main.adb, line 6 column 12 and line 7 column 12.
gnatxref package1.adb package2.ads
gnatxref will generates cross-reference information for
package1.adb, package2.ads and any other package 'with'ed by any
of these.
gnatxref can generate a tags file output, which can be used
directly from `vi'. Note that the standard version of `vi'
will not work properly with overloaded symbols. Consider using another
free implementation of `vi', such as `vim'.
$ gnatxref -v gnatfind.adb > tags
will generate the tags file for gnatfind itself (if the sources
are in the search path!).
From `vi', you can then use the command `:tag entity' (replacing entity by whatever you are looking for), and vi will display a new file with the corresponding declaration of entity.
gnatfind usagegnatfind -f xyz:main.adb
directory/main.ads:106:14: xyz <= declaration directory/main.adb:24:10: xyz <= body directory/foo.ads:45:23: xyz <= declarationthat is to say, one of the entities xyz found in main.adb is declared at line 12 of main.ads (and its body is in main.adb), and another one is declared at line 45 of foo.ads
gnatfind -fs xyz:main.adb
gnatfind will
display the content of the Ada source file lines.
The output will look like:
directory/main.ads:106:14: xyz <= declaration
procedure xyz;
directory/main.adb:24:10: xyz <= body
procedure xyz is
directory/foo.ads:45:23: xyz <= declaration
xyz : Integer;
This can make it easier to find exactly the location your are looking
for.
gnatfind -r "*x*":main.ads:123 foo.adb
gnatfind main.ads:123
gnatfind "*":main.adb:123.
gnatfind mydir/main.adb:123:45
gnatkr
This chapter discusses the method used by the compiler to shorten
the default file names chosen for Ada units so that they do not
exceed the maximum length permitted. It also describes the
gnatkr utility that can be used to determine the result of
applying this shortening.
gnatkrThe default file naming rule in GNAT is that the file name must be derived from the unit name. The exact default rule is as follows:
The reason for this exception is to avoid clashes with the standard names for children of System, Ada, Interfaces, and GNAT, which use the prefixes s- a- i- and g- respectively.
The -gnatknn
switch of the compiler activates a "krunching"
circuit that limits file names to nn characters (where nn is a decimal
integer). For example, using OpenVMS,
where the maximum file name length is
39, the value of nn is usually set to 39, but if you want to generate
a set of files that would be usable if ported to a system with some
different maximum file length, then a different value can be specified.
The default value of 39 for OpenVMS need not be specified.
The gnatkr utility can be used to determine the krunched name for
a given file, when krunched to a specified maximum length.
gnatkr
The gnatkr command has the form
$ gnatkr name [length]
name can be an Ada name with dots or the GNAT name of the unit,
where the dots representing child units or subunit are replaced by
hyphens. The only confusion arises if a name ends in .ads or
.adb. gnatkr takes this to be an extension if there are
no other dots in the name and the whole name is in lowercase.
length represents the length of the krunched name. The default when no argument is given is 8 characters. A length of zero stands for unlimited, in other words do not chop except for system files which are always 8.
The output is the krunched name. The output has an extension only if the original argument was a file name with an extension.
The initial file name is determined by the name of the unit that the file
contains. The name is formed by taking the full expanded name of the
unit and replacing the separating dots with hyphens and
using lowercase
for all letters, except that a hyphen in the second character position is
replaced by a tilde if the first character is
a, i, g, or s.
The extension is .ads for a
specification and .adb for a body.
Krunching does not affect the extension, but the file name is shortened to
the specified length by following these rules:
our-strings-wide_fixed 22 our strings wide fixed 19 our string wide fixed 18 our strin wide fixed 17 our stri wide fixed 16 our stri wide fixe 15 our str wide fixe 14 our str wid fixe 13 our str wid fix 12 ou str wid fix 11 ou st wid fix 10 ou st wi fix 9 ou st wi fi 8 Final file name: oustwifi.adb
ada-strings-wide_fixed 22 a- strings wide fixed 18 a- string wide fixed 17 a- strin wide fixed 16 a- stri wide fixed 15 a- stri wide fixe 14 a- str wide fixe 13 a- str wid fixe 12 a- str wid fix 11 a- st wid fix 10 a- st wi fix 9 a- st wi fi 8 Final file name: a-stwifi.adb
Of course no file shortening algorithm can guarantee uniqueness over all
possible unit names, and if file name krunching is used then it is your
responsibility to ensure that no name clashes occur. The utility
program gnatkr is supplied for conveniently determining the
krunched name of a file.
gnatkr Usage$ gnatkr very_long_unit_name.ads --> velounna.ads $ gnatkr grandparent-parent-child.ads --> grparchi.ads $ gnatkr Grandparent.Parent.Child --> grparchi $ gnatkr very_long_unit_name.ads/count=6 --> vlunna.ads $ gnatkr very_long_unit_name.ads/count=0 --> very_long_unit_name.ads
gnatprep
The gnatprep utility provides
a simple preprocessing capability for Ada programs.
It is designed for use with GNAT, but is not dependent on any special
features of GNAT.
gnatprep
To call gnatprep use
$ gnatprep [-bcrsu] [-Dsymbol=value] infile outfile [deffile]
where
infile
outfile
deffile
-D switch.
switches
gnatprep-b
-c
-Dsymbol=value
True. This switch
can be used in place of a definition file.
-r
Source_Reference pragma to be generated that
references the original input file, so that error messages will use
the file name of this original file. The use of this switch implies
that preprocessor lines are not to be removed from the file, so its
use will force -b mode if
-c
has not been specified explicitly.
Note that if the file to be preprocessed contains multiple units, then
it will be necessary to gnatchop the output file from
gnatprep. If a Source_Reference pragma is present
in the preprocessed file, it will be respected by
gnatchop -r
so that the final chopped files will correctly refer to the original
input source file for gnatprep.
-s
-u
#if or #elsif test will be treated as an error.
Note: if neither -b nor -c is present,
then preprocessor lines and
deleted lines are completely removed from the output, unless -r is
specified, in which case -b is assumed.
The definitions file contains lines of the form
symbol := value
where symbol is an identifier, following normal Ada (case-insensitive) rules for its syntax, and value is one of the following:
Comment lines may also appear in the definitions file, starting with
the usual --,
and comments may be added to the definitions lines.
gnatprepThe input text may contain preprocessor conditional inclusion lines, as well as general symbol substitution sequences. The preprocessor conditional inclusion commands have the form
#if expression [then]
lines
#elsif expression [then]
lines
#elsif expression [then]
lines
...
#else
lines
#end if;
In this example, expression is defined by the following grammar:
expression ::= <symbol> expression ::= <symbol> = "<value>" expression ::= <symbol> = <symbol> expression ::= <symbol> 'Defined expression ::= not expression expression ::= expression and expression expression ::= expression or expression expression ::= expression and then expression expression ::= expression or else expression expression ::= ( expression )
For the first test (expression ::= <symbol>) the symbol must have
either the value true or false, that is to say the right-hand of the
symbol definition must be one of the (case-insensitive) literals
True or False. If the value is true, then the
corresponding lines are included, and if the value is false, they are
excluded.
The test (expression ::= <symbol> 'Defined) is true only if
the symbol has been defined in the definition file or by a -D
switch on the command line. Otherwise, the test is false.
The equality tests are case insensitive, as are all the preprocessor lines.
If the symbol referenced is not defined in the symbol definitions file,
then the effect depends on whether or not switch -u
is specified. If so, then the symbol is treated as if it had the value
false and the test fails. If this switch is not specified, then
it is an error to reference an undefined symbol. It is also an error to
reference a symbol that is defined with a value other than True
or False.
The use of the not operator inverts the sense of this logical test, so
that the lines are included only if the symbol is not defined.
The then keyword is optional as shown
The # must be in column one, but otherwise the format is free form.
Spaces or tabs may appear between the # and the keyword. The keywords
and the symbols are case insensitive as in normal Ada code. Comments
may be used on a preprocessor line, but other than that, no other
tokens may appear on a preprocessor line.
Any number of elsif clauses can be present, including none at all.
The else is optional, as in Ada.
The # marking the start of a preprocessor line must be the first
non-blank character on the line, i.e. it must be preceded only by
spaces or horizontal tabs.
Symbol substitution outside of preprocessor lines is obtained by using the sequence
$symbol
anywhere within a source line, except in a comment. The identifier
following the $ must match one of the symbols defined in the symbol
definition file, and the result is to substitute the value of the
symbol in place of $symbol in the output file.
gnatls
gnatls is a tool that outputs information about compiled
units. It gives the relationship between objects, unit names and source
files. It can also be used to check the source dependencies of a unit
as well as various characteristics.
gnatls
The gnatls command has the form
$ gnatls switches object_or_ali_file
The main argument is the list of object or `ali' files (see section The Ada Library Information Files) for which information is requested.
In normal mode, without additional option, gnatls produces a
four-column listing. Each line represents information for a specific
object. The first column gives the full path of the object, the second
column gives the name of the principal unit in this object, the third
column gives the status of the source and the fourth column gives the
full path of the source representing this unit.
Here is a simple example of use:
$ gnatls *.o ./demo1.o demo1 DIF demo1.adb ./demo2.o demo2 OK demo2.adb ./hello.o h1 OK hello.adb ./instr-child.o instr.child MOK instr-child.adb ./instr.o instr OK instr.adb ./tef.o tef DIF tef.adb ./text_io_example.o text_io_example OK text_io_example.adb ./tgef.o tgef DIF tgef.adb
The first line can be interpreted as follows: the main unit which is contained in object file `demo1.o' is demo1, whose main source is in `demo1.adb'. Furthermore, the version of the source used for the compilation of demo1 has been modified (DIF). Each source file has a status qualifier which can be:
OK (unchanged)
MOK (slightly modified)
-m (minimal recompilation), a file marked
MOK will not be recompiled.
DIF (modified)
??? (file not found)
HID (hidden, unchanged version not first on PATH)
gnatls
gnatls recognizes the following switches:
-a
-d.
-d
-h
-o
-s
-u
-aOdir
-aIdir
-Idir
-I-
-nostdinc
gnatmake
-v
Preelaborable
No_Elab_Code
Pure
Elaborate_Body
Remote_Types
Shared_Passive
Predefined
Remote_Call_Interface
gnatls UsageExample of using the verbose switch. Note how the source and object paths are affected by the -I switch.
$ gnatls -v -I.. demo1.o
GNATLS 3.10w (970212) Copyright 1999 Free Software Foundation, Inc.
Source Search Path:
<Current_Directory>
../
/home/comar/local/adainclude/
Object Search Path:
<Current_Directory>
../
/home/comar/local/lib/gcc-lib/mips-sni-sysv4/2.7.2/adalib/
./demo1.o
Unit =>
Name => demo1
Kind => subprogram body
Flags => No_Elab_Code
Source => demo1.adb modified
The following is an example of use of the dependency list. Note the use of the -s switch which gives a straight list of source files. This can be useful for building specialized scripts.
$ gnatls -d demo2.o
./demo2.o demo2 OK demo2.adb
OK gen_list.ads
OK gen_list.adb
OK instr.ads
OK instr-child.ads
$ gnatls -d -s -a demo1.o
demo1.adb
/home/comar/local/adainclude/ada.ads
/home/comar/local/adainclude/a-finali.ads
/home/comar/local/adainclude/a-filico.ads
/home/comar/local/adainclude/a-stream.ads
/home/comar/local/adainclude/a-tags.ads
gen_list.ads
gen_list.adb
/home/comar/local/adainclude/gnat.ads
/home/comar/local/adainclude/g-io.ads
instr.ads
/home/comar/local/adainclude/system.ads
/home/comar/local/adainclude/s-exctab.ads
/home/comar/local/adainclude/s-finimp.ads
/home/comar/local/adainclude/s-finroo.ads
/home/comar/local/adainclude/s-secsta.ads
/home/comar/local/adainclude/s-stalib.ads
/home/comar/local/adainclude/s-stoele.ads
/home/comar/local/adainclude/s-stratt.ads
/home/comar/local/adainclude/s-tasoli.ads
/home/comar/local/adainclude/s-unstyp.ads
/home/comar/local/adainclude/unchconv.ads
It may be useful to recompile the GNAT library in various contexts, the
most important one being the use of partition wide configuration pragmas
such as Normalize_Scalar. A special Makefile called
Makefile.adalib is provided to that effect and can be found in
the directory containing the GNAT library. The location of this
directory depends on how the GNAT environment has been installed and can
be located with the command
$ gnatls -v
The last line of the Object Search Path usually contains the gnat library. This Makefile contains its own documentation and in particular the set of instructions needed to rebuild a new library and to use it.
gnatmem
gnatmem, is a tool that monitors dynamic allocation and
deallocation activity in a program, and displays information about
incorrect deallocations and possible sources of memory leaks. Gnatmem
provides three type of information:
gnatmem
The gnatmem command has the form
$ gnatmem [n] [-o file] user_program [program_arg]* or $ gnatmem [n] -i file
Gnatmem must be supplied with the executable to examine, followed by its run-time inputs. For example, if a program is executed with the command:
$ my_program arg1 arg2
then it can be run under gnatmem control using the command:
$ gnatmem my_program arg1 arg2
The program is transparently executed under the control of the debugger
section The GNAT Debugger GDB. This does not affect the behavior
of the program, except for sensitive real-time programs. When the program
has completed execution, gnatmem outputs a report containing general
allocation/deallocation information and potential memory leak.
For better results, the user program should be compiled with
debugging options section Switches for gcc.
Here is a simple example of use:
*************** debut cc
$ gnatmem test_gm
Global information
------------------
Total number of allocations : 45
Total number of deallocations : 6
Final Water Mark (non freed mem) : 11.29 Kilobytes
High Water Mark : 11.40 Kilobytes
.
.
.
Allocation Root # 2
-------------------
Number of non freed allocations : 11
Final Water Mark (non freed mem) : 1.16 Kilobytes
High Water Mark : 1.27 Kilobytes
Backtrace :
test_gm.adb:23 test_gm.alloc
.
.
.
The first block of output give general information. In this case, the Ada construct "new" was executed 45 times, and only 6 calls to an unchecked deallocation routine occurred.
Subsequent paragraphs display information on all allocation roots. An allocation root is a specific point in the execution of the program that generates some dynamic allocation, such as a "new" construct. This root is represented by an execution backtrace (or subprogram call stack). By default the backtrace depth for allocations roots is 1, so that a root corresponds exactly to a source location. The backtrace can be made deeper, to make the root more specific.
gnatmem
gnatmem recognizes the following switches:
-qn-o file-i filegnatmem
processing starting from file which has been generated
by a previous call to gnatmem
with the -o switch. This is useful for
post mortem processing.
gnatmem Usage
The first example shows the use of gnatmem
on a simple leaking program.
Suppose that we have the following Ada program:
with Unchecked_Deallocation;
procedure Test_Gm is
type T is array (1..1000) of Integer;
type Ptr is access T;
procedure Free is new Unchecked_Deallocation (T, Ptr);
A : Ptr;
procedure My_Alloc is
begin
A := new T;
end My_Alloc;
procedure My_DeAlloc is
B : Ptr := A;
begin
Free (B);
end My_DeAlloc;
begin
My_Alloc;
for I in 1 .. 5 loop
for J in I .. 5 loop
My_Alloc;
end loop;
My_Dealloc;
end loop;
end;
The program needs to be compiled with debugging option:
$ gnatmake -g test_gm
gnatmem is invoked simply with
$ gnatmem test_gm
which produces the following output:
Global information
------------------
Total number of allocations : 18
Total number of deallocations : 5
Final Water Mark (non freed mem) : 53.00 Kilobytes
High Water Mark : 56.90 Kilobytes
Allocation Root # 1
-------------------
Number of non freed allocations : 11
Final Water Mark (non freed mem) : 42.97 Kilobytes
High Water Mark : 46.88 Kilobytes
Backtrace :
test_gm.adb:11 test_gm.my_alloc
Allocation Root # 2
-------------------
Number of non freed allocations : 1
Final Water Mark (non freed mem) : 10.02 Kilobytes
High Water Mark : 10.02 Kilobytes
Backtrace :
s-secsta.adb:81 system.secondary_stack.ss_init
Allocation Root # 3
-------------------
Number of non freed allocations : 1
Final Water Mark (non freed mem) : 12 Bytes
High Water Mark : 12 Bytes
Backtrace :
s-secsta.adb:181 system.secondary_stack.ss_init
Note that the GNAT run time contains itself a certain number of allocations that have no corresponding deallocation, as shown here for root #2 and root #1. This is a normal behavior when the number of non freed allocations is one, it locates dynamic data structures that the run time needs for the complete lifetime of the program. Note also that there is only one allocation root in the user program with a single line back trace: test_gm.adb:11 test_gm.my_alloc, whereas a careful analysis of the program shows that 'My_Alloc' is called at 2 different points in the source (line 21 and line 24). If those two allocation roots need to be distinguished, the backtrace depth parameter can be used:
$ gnatmem 3 test_gm
which will give the following output:
Global information
------------------
Total number of allocations : 18
Total number of deallocations : 5
Final Water Mark (non freed mem) : 53.00 Kilobytes
High Water Mark : 56.90 Kilobytes
Allocation Root # 1
-------------------
Number of non freed allocations : 10
Final Water Mark (non freed mem) : 39.06 Kilobytes
High Water Mark : 42.97 Kilobytes
Backtrace :
test_gm.adb:11 test_gm.my_alloc
test_gm.adb:24 test_gm
b_test_gm.c:52 main
Allocation Root # 2
-------------------
Number of non freed allocations : 1
Final Water Mark (non freed mem) : 10.02 Kilobytes
High Water Mark : 10.02 Kilobytes
Backtrace :
s-secsta.adb:81 system.secondary_stack.ss_init
s-secsta.adb:283 <system__secondary_stack___elabb>
b_test_gm.c:33 adainit
Allocation Root # 3
-------------------
Number of non freed allocations : 1
Final Water Mark (non freed mem) : 3.91 Kilobytes
High Water Mark : 3.91 Kilobytes
Backtrace :
test_gm.adb:11 test_gm.my_alloc
test_gm.adb:21 test_gm
b_test_gm.c:52 main
Allocation Root # 4
-------------------
Number of non freed allocations : 1
Final Water Mark (non freed mem) : 12 Bytes
High Water Mark : 12 Bytes
Backtrace :
s-secsta.adb:181 system.secondary_stack.ss_init
s-secsta.adb:283 <system__secondary_stack___elabb>
b_test_gm.c:33 adainit
The allocation root #1 of the first example has been split in 2 roots #1 and #3 thanks to the more precise associated backtrace.
gnatmem executes the user program under the control of gdb using
a script that sets breakpoints and gathers information on each dynamic
allocation and deallocation. The output of the script is then analyzed
by gnatmem
in order to locate memory leaks and their origin in the
program. Gnatmem works by recording each address returned by the
allocation procedure (__gnat_malloc)
along with the backtrace at the
allocation point. On each deallocation, the deallocated address is
matched with the corresponding allocation. At the end of the processing,
the unmatched allocations are considered potential leaks. All the
allocations associated with the same backtrace are grouped together and
form an allocation root. The allocation roots are then sorted so that
those with the biggest number of unmatched allocation are printed
first. A delicate aspect of this technique is to distinguish between the
data produced by the user program and the data produced by the gdb
script. Currently, on systems that allow probing the terminal, the gdb
command "tty" is used to force the program output to be redirected to the
current terminal while the gdb output is directed to a file or to a
pipe in order to be processed subsequently by gnatmem.
Some of the tools distributed with GNAT are based on the ASIS implementation for GNAT (ASIS-for-GNAT). Binary executables for such tools do not require ASIS-for-GNAT to be around and they have a command-line interface similar to other GNAT tools. The main specific feature of ASIS-based tools is that they process tree output files.
The ASIS implementation for GNAT, called ASIS-for-GNAT, is the implementation if the Ada Semantic Interface Specification (ASIS). It is a separate product which is not included in the standard GNAT distribution. However, the binary executables for tools created on top of ASIS-for-GNAT do not require ASIS-for-GNAT installed on your system and they can be used for a standard GNAT distribution along with other GNAT tools
The ASIS implementation for GNAT is based on tree output files (or, simply, tree files). A tree file stores a snapshot of the compiler internal data structures in the very end of a successful compilation. It contains all the syntactical and semantic information about the unit being compiled and all the units upon which it depends semantically. ASIS-for-GNAT (and, therefore, any tool based on its top) processes tree files, extracts this information from it and converts it into the format prescribing by the ASIS definition.
To use some ASIS-based tools, a user should take care of producing the right set of tree files for the tool, some other ASIS tools produce a needed set of tree files themselves.
GNAT produces a tree file if -gnatt option is set when calling gcc. ASIS needs tree files created in "compile-only" GNAT mode set by -gnatc gcc switch. Names of the tree files are obtained by replacing 'd' with 't' in the extension of the name of the source file being compiled.
Therefore, to produce a tree file for the body of a procedure Foo contained in the source file named 'foo.adb', you can compile it using
$ gcc -c -gnatc -gnatt foo.adb
and you will get the tree file named 'foo.atb' as a result of this compilation.
gnatstub
gnatstub creates body samples - that is, empty but compilable bodies for
library unit declarations.
gnatstub is an ASIS-based tool, but it creates a needed tree
file itself, so it can be considered as a usual command-line utility
program when using with GNAT.
To create a body sampler, gnatstub has to compile the library
unit declaration. Therefore, bodies can be created only for legal
library units. Moreover, if a library unit depends semantically upon
units located not only in the current directory, you have to provide
a source search path when calling gnatstub, see the description of
gnatstub switches below.
gnatstub
gnatstub has the command-line interface of the form
$ gnatstub [switches] filename [directory]
where
filename
directory
switches
gnatstub-f
-f switch is set or leaves it intact
otherwise.
-hs
-hg
-IDIR
-I-
-in
-k
-k prevents deleting the
tree file.
-ln
-q
-r
-r
also implies -k, whether or not
-k is set explicitly.
-t
-t option is set
-v
gnatelimgnatelimWhen a program shares a set of Ada packages with other programs, it may happen that this program uses only a fraction of the subprograms defined in these packages. The code created for those unused subprograms increases the size of the executable.
gnatelim is a utility tracking unused subprograms in an Ada
program. Its output consists of a list of Eliminate pragmas
marking all the subprograms that are declared, but never called in a
given program. Eliminate is a GNAT-specific pragma, it is
described in the next section. By placing the list of
Eliminate pragmas in the GNAT configuration file
`gnat.adc' and recompiling your program, you may decrease the
size of its executable, because the compiler will not generate code
for those unused subprograms.
gnatelim is an ASIS-based tool, and it needs as its input data
a set of tree files representing all the components of a program to
process. It also needs a bind file for a main subprogram. (See
section Preparing Tree and Bind Files for gnatelim for full details)
Eliminate pragmaThe syntax of Eliminate pragma is
pragma Eliminate (Library_Unit_Name, Subprogram_Name);
Library_Unit_Name
Subprogram_Name
The effect of an Eliminate pragma placed in the GNAT configuration file `gnat.adc' is:
Subprogram_Name is declared within
the library unit having Library_Unit_Name as its defining program
unit name, then the compiler will not generate code for this subprogram.
It applies to all overloaded subprograms denoted by
Subprogram_Name.
Eliminate pragma as unused is
actually used (called) in a program, then the compiler will produce a
diagnosis in place where it is called.
gnatelim
gnatelim can process only full Ada programs (partitions) and
it needs a set of tree files representing the whole program
(partition) to be presented in the current directory. It also needs a
bind file for the main subprogram of the program (partition) to be
presented in the current directory.
Let Main_Prog be the name of a main subprogram, and suppose
this subprogram is in a file named `main_prog.ads' or
`main_prog.adb'.
To create a minimal set of tree files covering the whole program, call
gnatmake for this program as follows:
$ gnatmake -c -f -gnatc -gnatt Main_Prog
The -c gnatmake option turns off the bind and link phases,
which are impossible anyway, because sources are compiled with
-gnatc option, which turns off code generation.
the -f gnatmake option is used to force
recompilation of all the needed sources.
To create a bind file for gnatelim, run gnatbind for
the main subprogram. gnatelim can work with either an Ada or a C
bind file, if both are present, it works with the Ada bind file.
To avoid problems with creating a consistent data for
gnatelim, it is advised to use the following procedure. It creates all
the data needed by gnatelim from scratch and therefore
guarantees their consistency:
$ gnatmake -c Main_Prog $ gnatbind main_prog
$ gnatmake -f -c -gnatc -gnatt Main_Prog
Note, that gnatelim needs neither object nor ALI files, so they
can be deleted at this stage.
gnatelimgnatelim has the following command-line interface:
$ gnatelim [options] name
name should be a full expanded Ada name of a main subprogram
of a program (partition).
gnatelim options:
-v
-a
-m
gnatelim directs its output to the standard output,
so to produce a proper GNAT configuration file
`gnat.adc', redirection can be used:
$ gnatelim Main_Prog > gnat.adc
or
$ gnatelim Main_Prog >> gnat.adc
In order to append the gnatelim output to the existing contents of `gnat.adc'.
It may happen that gnatelim try to eliminate subprograms which
cannot really be eliminated because they are actually called in the
program although this only happens in very rare cases. In this case, the
compiler will generate an error message of the form:
file.adb:106:07: cannot call eliminated subprogram "My_Prog"
You have to correct the `gnat.adc' file manually by suppressing the faulty Eliminate pragmas. It is advised to recompile your program from scratch after that, because you need a consistent `gnat.adc' file during the complete compilation in order to get an meaningful result.
To get a smaller executable for your program, you have to recompile
the program completely, having the `gnat.adc' file with a set of
Eliminate pragmas created by gnatelim in your current
directory:
$ gnatmake -f Main_Prog
(you will need -f option for gnatmake to
recompile everything
with the set of pragmas Eliminate you have got from
gnatelim).
Be aware that a set of Eliminate pragmas is specific to each
program. Therefore, it is not advised to merge sets of Eliminate
pragmas created for different programs in one `gnat.adc' file.
Here is a summary of the steps to be taken in order to reduce the size of
your executables with gnatelim. You may use
other GNAT options to control the optimization level,
to produce the debugging information, to set search path, etc.
$ gnatmake -c Main_Prog $ gnatbind main_prog $ gnatmake -f -c -gnatc -gnatt Main_Prog
Eliminate pragmas
$ gnatelim Main_Prog >[>] gnat.adc
$ gnatmake -f Main_Prog
This chapter discusses some other utility programs available in the Ada environment.
The object files generated by GNAT are in standard system format and in particular the debugging information uses this format. This means programs generated by GNAT can be used with existing utilities that depend on these formats.
In general, any utility program that works with C will also often work with Ada programs generated by GNAT. This includes software utilities such as gprof (a profiling program), gdb (the FSF debugger), and utilities such as Purify.
gnatpsys Utility ProgramMany of the definitions in package System are implementation-dependent. Furthermore, although the source of the package System is available for inspection, it uses special attributes for parameterizing many of the critical values, so the source is not informative for the casual user.
The gnatpsys utility is designed to deal with this situation.
It is an Ada program that dynamically determines the
values of all the relevant parameters in System, and prints them
out in the form of an Ada source listing for System, displaying all
the values of interest. This output is generated to
`stdout'.
To determine the value of any parameter in package System, simply
run gnatpsys with no qualifiers or arguments, and examine
the output. This is preferable to consulting documentation, because
you know that the values you are getting are the actual ones provided
by the executing system.
gnatpsta Utility ProgramMany of the definitions in package Standard are implementation-dependent. However, the source of this package does not exist as an Ada source file, so these values cannot be determined by inspecting the source. They can be determined by examining in detail the coding of `cstand.adb' which creates the image of Standard in the compiler, but this is awkward and requires a great deal of internal knowledge about the system.
The gnatpsta utility is designed to deal with this situation.
It is an Ada program that dynamically determines the
values of all the relevant parameters in Standard, and prints them
out in the form of an Ada source listing for Standard, displaying all
the values of interest. This output is generated to
`stdout'.
To determine the value of any parameter in package Standard, simply
run gnatpsta with no qualifiers or arguments, and examine
the output. This is preferable to consulting documentation, because
you know that the values you are getting are the actual ones provided
by the executing system.
In order to interpret the output from GNAT, when using tools that are originally intended for use with other languages, it is useful to understand the conventions used to generate link names from the Ada entity names.
All link names are in all lowercase letters. With the exception of library procedure names, the mechanism used is simply to use the full expanded Ada name with dots replaced by double underscores. For example, suppose we have the following package spec:
package QRS is
MN : Integer;
end QRS;
The variable MN has a full expanded Ada name of QRS.MN, so
the corresponding link name is qrs__mn.
Of course if a pragma Export is used this may be overridden:
package Exports is
Var1 : Integer;
pragma Export (Var1, C, External_Name => "var1_name");
Var2 : Integer;
pragma Export (Var2, C, Link_Name => "var2_link_name");
end Exports;
In this case, the link name for Var1 is var1_name, and the link name for Var2 is var2_link_name.
One exception occurs for library level procedures. A potential ambiguity
arises between the required name _main for the C main program,
and the name we would otherwise assign to an Ada library level procedure
called Main (which might well not be the main program).
To avoid this ambiguity, we attach the prefix _ada_ to such
names. So if we have a library level procedure such as
procedure Hello (S : String);
the external name of this procedure will be _ada_hello.
emacsThe Emacs mode for programming in Ada (both, Ada83 and Ada95) helps the user in understanding existing code and facilitates writing new code. It furthermore provides some utility functions for easier integration of standard Emacs features when programming in Ada.
For more information, please see See section Ada Mode for emacs.
This Perl script allows Ada source files to be browsed using
standard Web browsers. For installation procedure, see the section
See section Installing gnathtml.
Ada reserved keywords are highlighted in a bold font and Ada comments in
a blue font. Unless your program was compiled with the gcc -gnatx
switch to suppress the generation of cross-referencing information, user
defined variables and types will appear in a different color; you will
be able to click on any identifier and go to its declaration.
The command line is as follow:
$ perl gnathtml.pl [switches] ada-files
You can pass it as many Ada files as you want. gnathtml will generate
an html file for every ada file, and a global file called `index.htm'.
This file is an index of every identifier defined in the files.
The available switches are the following ones :
-83
-cc color
-d
with command, the latter will also be converted to html.
Only the files in the user project will be converted to html, not the files
in the runtime library itself.
-D
gnathtml will also look
for files in the runtime library, and generate html files for them.
-f
-f on the command line, then links will be generated for local
entities too.
-l number
gnathtml
will number the html files every number line.
-I dir
-o dir
-p file
-sc color
Perl needs to be installed on your machine to run this script.
Perl is freely available for almost every architecture and
Operating System via the Internet.
On Unix systems, you may want to modify the first line of the script
gnathtml, to explicitly tell the Operating system where Perl
is. The syntax of this line is :
#!full_path_name_to_perl
Alternatively, you may run the script using the following command line:
$ perl gnathtml.pl [switches] files
This chapter discusses how to debug Ada programs. An incorrect Ada program may be handled in three ways by the GNAT compiler:
GDB is a general purpose, platform-independent debugger that can be used to debug mixed-language programs compiled with GCC, and in particular is capable of debugging Ada programs compiled with GNAT. The latest versions of GDB are Ada-aware and can handle complex Ada data structures. The manual Debugging with GDB contains full details on the usage of GDB, including a section on its usage on programs. This manual should be consulted for full details. The section that follows is a brief introduction to the philosophy and use of GDB.
When GNAT programs are compiled, the compiler optionally writes debugging information into the generated object file, including information on line numbers, and on declared types and variables. This information is separate from the generated code. It makes the object files considerably larger, but it does not add to the size of the actual executable that will be loaded into memory, and has no impact on run-time performance. The generation of debug information is triggered by the use of the -g switch in the gcc or gnatmake command used to carry out the compilations. It is important to emphasize that the use of these options does not change the generated code.
The debugging information is written in standard system formats that are used by many tools, including debuggers and profilers. The format of the information is typically designed to describe C types and semantics, but GNAT implements a translation scheme which allows full details about Ada types and variables to be encoded into these standard C formats. Details of this encoding scheme may be found in the file exp_dbug.ads in the GNAT source distribution. However, the details of this encoding are, in general, of no interest to a user, since GDB automatically performs the necessary decoding.
When a program is bound and linked, the debugging information is collected from the object files, and stored in the executable image of the program. Again, this process significantly increases the size of the generated executable file, but it does not increase the size of the executable program itself. Furthermore, if this program is run in the normal manner, it runs exactly as if the debug information were not present, and takes no more actual memory.
However, if the program is run under control of GDB, the debugger is activated. The image of the program is loaded, at which point it is ready to run. If a run command is given, then the program will run exactly as it would have if GDB were not present. This is a crucial part of the GDB design philosophy. GDB is entirely non-intrusive until a breakpoint is encountered. If no breakpoint is ever hit, the program will run exactly as it would if no debugger were present. When a breakpoint is hit, GDB accesses the debugging information and can respond to user commands to inspect variables, and more generally to report on the state of execution.
The debugger can be launched directly and simply from emacs which allows
to browse and modify directly the source code during the debugging
session, See section Ada Mode for emacs. Here is described the basic use of
GDB is text mode.
The command to run GDB is
$ gdb program
where program is the name of the executable file. This
activates the debugger and results in a prompt for debugger commands.
The simplest command is simply run, which causes the program to run
exactly as if the debugger were not present. The following section
describes some of the additional commands that can be given to GDB.
GDB contains a large repertoire of commands. The manual Debugging with GDB includes extensive documentation on the use of these commands, together with examples of their use. Furthermore, the command help invoked from within GDB activates a simple help facility which summarizes the available commands and their options. In this section we summarize a few of the most commonly used commands to give an idea of what GDB is about. You should create a simple program with debugging information and experiment with the use of these GDB commands on the program as you read through the following section.
set args arguments
set args
command is not needed if the program does not require arguments.
run
run command causes execution of the program to start from the
beginning. If the program is already running, that is to say if you
are currently positioned at a breakpoint,
then a prompt will ask for confirmation that you want
to abandon the current execution and restart.
breakpoint location
file:linenumber,
or it is the name of a subprogram. If you request that a breakpoint be set on
a subprogram that is overloaded, a prompt will ask you to specify on which of
those subprograms you want to breakpoint. You can also
specify that all of them should be breakpointed. If the program is run
and execution encounters the breakpoint, then the program
stops and GDB signals that the breakpoint was encountered by printing the
line of code before which the program is halted.
breakpoint exception name
print expression
continue
step
next
list
backtrace
up
up can be used to
examine the contents of other active frames, by moving the focus up
the stack, that is to say from callee to caller, one frame at a time.
down
frame n
The above list is a very short introduction to the commands that GDB provides. Important additional capabilities, including conditional breakpoints, the ability to execute command sequences on a breakpoint, the ability to debug at the machine instruction level and many other features are described in detail in Debugging with GDB. Note that most commands can be abbreviated (for example, c for continue, bt for backtrace).
GDB supports a fairly large subset of Ada expression syntax, with some extensions. The philosophy behind the design of this subset is
Thus, for brevity, the debugger acts as if there were
implicit with and use clauses in effect for all user-written
packages, thus making it unnecessary to fully qualify most names with
their packages, regardless of context. Where this causes ambiguity,
GDB asks the user's intent.
For details on the supported Ada syntax Debugging with GDB.
An important capability of GDB is the ability to call user-defined subprograms while debugging. This is achieved simply by entering a subprogram call statement in the form:
call subprogram-name (parameters)
The keyword call can be omitted in the normal case where the
subprogram-name does not coincide with any of the predefined
GDB commands.
The effect is to invoke the given subprogram, passing it the list of parameters that is supplied. The parameters can be expressions and can include variables from the program being debugged. The subprogram must be defined at the library level within your program, and GDB will call the subprogram within the environment of your program execution (which means that the subprogram is free to access or even modify variables within your program).
The most important use of this facility is in allowing the inclusion of
debugging routines that are tailored to particular data structures
in your program. Such debugging routines can be written to provide a suitably
high-level description of an abstract type, rather than a low-level dump
of its physical layout. After all, the standard
GDB print command only knows the physical layout of your
types, not their abstract meaning. Debugging routines can provide information
at the desired semantic level and are thus enormously useful.
For example, when debugging GNAT itself, it is crucial to have access to
the contents of the tree nodes used to represent the program internally.
But tree nodes are represented simply by an integer value (which in turn
is an index into a table of nodes).
Using the print command on a tree node would simply print this integer
value, which is not very useful. But the PN routine (defined in file
treepr.adb in the GNAT sources) takes a tree node as input, and displays
a useful high level representation of the tree node, which includes the
syntactic category of the node, its position in the source, the integers
that denote descendant nodes and parent node, as well as varied
semantic information. To study this example in more detail, you might want to
look at the body of the PN procedure in the stated file.
You can set breakpoints that trip when your program raises selected exceptions.
break exception
break exception name
break exception unhandled
info exceptions
info exceptions regexp
info exceptions command permits the user to examine all defined
exceptions within Ada programs. With a regular expression, regexp, as
argument, prints out only those exceptions whose name matches regexp.
GDB allows the following task-related commands:
info tasks
(gdb) info tasks
ID TID P-ID Thread Pri State Name
1 8088000 0 807e000 15 Child Activation Wait main_task
2 80a4000 1 80ae000 15 Accept/Select Wait b
3 809a800 1 80a4800 15 Child Activation Wait a
* 4 80ae800 3 80b8000 15 Running c
In this listing, the asterisk before the first task indicates it to be the
currently running task. The first column lists the task ID that is used
to refer to tasks in the following commands.
break linespec task taskid
break linespec task taskid if ...
break ... thread ....
linespec specifies source lines.
Use the qualifier `task taskid' with a breakpoint command
to specify that you only want GDB to stop the program when a
particular Ada task reaches this breakpoint. taskid is one of the
numeric task identifiers assigned by GDB, shown in the first
column of the `info tasks' display.
If you do not specify `task taskid' when you set a
breakpoint, the breakpoint applies to all tasks of your
program.
You can use the task qualifier on conditional breakpoints as
well; in this case, place `task taskid' before the
breakpoint condition (before the if).
task taskno
For more detailed information on the tasking support Debugging with GDB.
GNAT always uses code expansion for generic instantiation. This means that each time an instantiation occurs, a complete copy of the original code is made, with appropriate substitutions of formals by actuals.
It is not possible to refer to the original generic entities in GDB, but it is always possible to debug a particular instance of a generic, by using the appropriate expanded names. For example, if we have
procedure g is
generic package k is
procedure kp (v1 : in out integer);
end k;
package body k is
procedure kp (v1 : in out integer) is
begin
v1 := v1 + 1;
end kp;
end k;
package k1 is new k;
package k2 is new k;
var : integer := 1;
begin
k1.kp (var);
k2.kp (var);
k1.kp (var);
k2.kp (var);
end;
Then to break on a call to procedure kp in the k2 instance, simply use the command:
(gdb) break g.k2.kp
When the breakpoint occurs, you can step through the code of the instance in the normal manner and examine the values of local variables, as for other units.
When presented with programs that contain serious errors in syntax or semantics, GNAT may on rare occasions experience problems in operation, such as aborting with a segmentation fault or illegal memory access, raising an internal exception, or terminating abnormally. In such cases, you can activate various features of GNAT that can help you pinpoint the construct in your program that is the likely source of the problem.
The following strategies are presented in increasing order of difficulty, corresponding to your programming skills and your familiarity with compiler internals.
gcc with the -gnatf and -gnate switches. The first
switch causes all errors on a given line to be reported. In its absence,
only the first error on a line is displayed.
The -gnate switch causes errors to be displayed as soon as they
are encountered, rather than after compilation is terminated. If GNAT
terminates prematurely, the last error message displayed is likely to
pinpoint the culprit.
gcc with the -v (verbose) switch. In this mode,
gcc produces ongoing information about the progress of the
compilation and provides the name of each procedure as code is
generated. This switch allows you to find which Ada procedure was being
compiled when it encountered a code generation problem.
gcc with the -gnatdc switch. This is a GNAT specific
switch that does for the front-end what -v does for the back end.
The system prints the name of each unit, either a compilation unit or
nested unit, as it is being analyzed.
gdb directly on the gnat1 executable. gnat1 is the
front-end of GNAT, and can be run independently (normally it is just
called from gcc). You can use gdb on gnat1 as you
would on a C program (but see section The GNAT Debugger GDB for caveats). The
where command is the first line of attack; the variable
lineno (seen by print lineno), used by the second phase of
gnat1 and by the gcc backend, indicates the source line at
which the execution stopped, and input_file name indicates the name of
the source file.
In order to examine the workings of the GNAT system, the following brief description of its organization may be helpful:
Ada, as
defined in Annex A.
Interfaces, as
defined in Annex B.
System. This includes
both language-defined children and GNAT run-time routines.
GNAT. These are useful
general-purpose packages, fully documented in their specifications. All
the other `.c' files are modifications of common gcc files.
Most compilers have internal debugging switches and modes. GNAT does also, except GNAT internal debugging switches and modes are not secret. A summary and full description of all the compiler and binder debug flags are in the file `debug.adb'. You must obtain the sources of the compiler to see the full detailed effects of these flags.
The switches that print the source of the program (reconstructed from the internal tree) are of general interest for user programs, as are the options to print the full internal tree, and the entity table (the symbol table information). The reconstructed source provides a readable version of the program after the front-end has completed analysis and expansion, and is useful when studying the performance of specific constructs. For example, constraint checks are indicated, complex aggregates are replaced with loops and assignments, and tasking primitives are replaced with run-time calls.
The GNAT system provides a number of options that allow a trade-off between
The defaults (if no options are selected) aim at improving the speed of compilation and minimizing dependences, at the expense of performance of the generated code:
These options are suitable for most program development purposes. This chapter describes how you can modify these choices.
By default, GNAT produces all run-time checks, except arithmetic overflow
checking for integer operations (that includes division by zero) and checks
for access before elaboration on subprogram calls.
Two gnat switches, -gnatp and -gnato allow this default to
be modified. See section Run-time Checks.
Our experience is that the default is suitable for most development purposes.
We treat integer overflow specially because these are quite expensive and in our experience are not as important as other run-time checks in the development process.
Elaboration checks are off by default, and also not needed by default, since GNAT uses a static elaboration analysis approach that avoids the need for run-time checking. This manual contains a full chapter discussing the issue of elaboration checks, and if the default is not satisfactory for your use, you should read this chapter.
Note that the setting of the switches controls the default setting of
the checks. They may be modified using either pragma Suppress (to
remove checks) or pragma Unsuppress (to add back suppressed
checks) in the program source.
The default is optimization off. This results in the fastest compile
times, but GNAT makes absolutely no attempt to optimize, and the
generated programs are considerably larger and slower than when
optimization is enabled. You can use the
-On switch, where n is an integer from 0 to 3,
on the gcc command line to control the optimization level:
-O0
-O1
-O2
-O3
Higher optimization levels perform more global transformations on the program and apply more expensive analysis algorithms in order to generate faster and more compact code. The price in compilation time, and the resulting improvement in execution time, both depend on the particular application and the hardware environment. You should experiment to find the best level for your application.
Note: Unlike some other compilation systems, gcc has
been tested extensively at all optimization levels. There are some bugs
which appear only with optimization turned on, but there have also been
bugs which show up only in unoptimized code. Selecting a lower
level of optimization does not improve the reliability of the code
generator, which in practice is highly reliable at all optimization
levels.
A call to a subprogram in the current unit is inlined if all the following conditions are met:
-O1.
gcc
cannot support in inlined subprograms.
pragma Inline applies to the subprogram or it is
small and automatic inlining (optimization level -O3) is
specified.
Calls to subprograms in with'ed units are normally not inlined.
To achieve this level of inlining, the following conditions must all be
true:
-O1.
gcc cannot
support in inlined subprograms.
pragma Inline for the subprogram.
-gnatn switch
is used in the gcc command line
Note that specifying the -gnatn switch causes additional
compilation dependencies. Consider the following:
package R is
procedure Q;
pragma Inline (Q);
end R;
package body R is
...
end R;
with R;
procedure Main is
begin
...
R.Q;
end Main;
With the default behavior (no -gnatn switch specified), the
compilation of the Main procedure depends only on its own source,
`main.adb', and the spec of the package in file `r.ads'. This
means that editing the body of R does not require recompiling
Main.
On the other hand, the call R.Q is not inlined under these
circumstances. If the -gnatn switch is present when Main
is compiled, the call will be inlined if the body of Q is small
enough, but now Main depends on the body of R in
`r.adb' as well as on the spec. This means that if this body is edited,
the main program must be recompiled. Note that this extra dependency
occurs whether or not the call is in fact inlined by gcc.
Note: The -fno-inline switch
can be used to prevent
all inlining. This switch overrides all other conditions and ensures
that no inlining occurs. The extra dependences resulting from
-gnatn will still be active, even if
this switch is used to suppress the resulting inlining actions.
--GCC=compiler_name (gnatlink)
--GCC=compiler_name (gnatmake)
--GNATBIND=binder_name (gnatmake)
--GNATLINK=linker_name (gnatmake)
--LINK= (gnatlink)
-83 (gnathtml)
-A (gnatbind)
-A (gnatlink)
-a (gnatls)
-A (gnatmake)
-a (gnatmake)
-aI (gnatmake)
-aL (gnatmake)
-aO (gnatmake)
-B (gcc)
-b (gcc)
-b (gnatbind)
-B (gnatlink)
-b (gnatlink)
-bargs (gnatmake)
-c (gcc)
-C (gnatbind)
-c (gnatbind)
-c (gnatchop)
-C (gnatlink)
-c (gnatmake)
-cargs (gnatmake)
-d (gnathtml)
-d (gnatls)
-e (gnatbind)
-f (gnatbind)
-f (gnathtml)
-f (gnatmake)
-fno-inline (gcc)
-g (gcc)
-g (gnatlink)
-gnat83 (gcc)
-gnat95 (gcc)
-gnata (gcc)
-gnatb (gcc)
-gnatc (gcc)
-gnatD (gcc)
-gnatdc switch
-gnatE (gcc)
-gnate (gcc)
-gnatf (gcc)
-gnatG (gcc)
-gnatg (gcc)
-gnati (gcc)
-gnatk (gcc)
-gnatl (gcc)
-gnatm (gcc)
-gnatn switch
-gnatn (gcc), -gnatn (gcc)
-gnato (gcc), -gnato (gcc)
-gnatp (gcc), -gnatp (gcc)
-gnatq (gcc)
-gnatR (gcc)
-gnatr (gcc)
-gnats (gcc)
-gnatt (gcc)
-gnatU (gcc)
-gnatu (gcc)
-gnatv (gcc)
-gnatW (gcc)
-gnatwe (gcc)
-gnatwl (gcc)
-gnatws (gcc)
-gnatwu (gcc)
-gnatx (gcc)
-h (gnatbind), -h (gnatbind)
-h (gnatls)
-I- (gcc)
-I- (gnatmake)
-I (gcc)
-I (gnathtml)
-I (gnatmake)
-i (gnatmake)
-i (gnatmem)
-j (gnatmake)
-k (gnatchop)
-k (gnatmake)
-l (gnatbind)
-l (gnathtml)
-L (gnatmake)
-largs (gnatmake)
-M (gnatbind)
-m (gnatbind)
-M (gnatmake)
-m (gnatmake)
-n (gnatbind)
-n (gnatlink)
-n (gnatmake)
-nostdinc (gnatmake)
-nostdlib (gnatmake)
-O (gcc), -O (gcc)
-o (gcc)
-o (gnatbind)
-O (gnatbind)
-o (gnathtml)
-o (gnatlink)
-o (gnatls)
-o (gnatmake)
-o (gnatmem)
-p (gnathtml)
-q (gnatchop)
-q (gnatmake)
-q (gnatmem)
-r (gnatchop)
-S (gcc)
-s (gnatbind)
-s (gnatls), -s (gnatls)
-t (gnatbind)
-u (gnatls)
-v -v (gnatlink)
-V (gcc)
-v (gcc)
-v (gnatbind)
-v (gnatchop)
-v (gnatlink)
-v (gnatmake)
-w (gnatchop)
-we (gnatbind)
-ws (gnatbind)
-Wuninitialized (gcc)
-x (gnatbind)
-z (gnatbind)
-z (gnatmake)
n (gnatmem)
gnatmake
gnatmake
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