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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.
3.1 Compiling Programs 3.2 Switches for gcc3.3 Search Paths and the Run-Time Library (RTL) 3.4 Order of Compilation Issues 3.5 Examples
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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):
cannot generate code for file fff (package spec) to check package spec, use -gnatc cannot generate code for file fff (missing subunits) to check parent unit, use -gnatc cannot generate code for file fff (subprogram spec) to check subprogram spec, use -gnatc cannot generate code for file fff (subunit) to check subunit, use -gnatc |
As indicated by the above error messages, if you want to submit one of these files to the compiler to check for correct semantics without generating code, then use the `-gnatc' switch.
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.
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gcc
The gcc command accepts switches that control the
compilation process. These switches are fully described in this section.
First we briefly list all the switches, in alphabetical order, then we
describe the switches in more detail in functionally grouped sections.
More switches exist for GCC than those documented here, especially for specific targets. However, their use is not recommended as they may change code generation in ways that are incompatible with the Ada run-time library, or can cause inconsistencies between compilation units.
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.
Note: for some other languages when using gcc, notably in
the case of C and C++, it is possible to use
use gcc without a `-c' switch to
compile and link in one step. In the case of GNAT, you
cannot use this approach, because the binder must be run
and gcc cannot be used to run the GNAT binder.
Inline_Always.
See also `-gnatn' and `-gnatN'.
Pragma Assert and pragma Debug to be
activated.
k = krunch).
setjmp/longjmp exception mechanism.
inline is specified. This inlining is performed
by the GCC back-end.
Inline is specified. This inlining is performed
by the front end and will be visible in the
`-gnatG' output.
In some cases, this has proved more effective than the back end
inlining resulting from the use of
`-gnatn'.
Note that
`-gnatN' automatically implies
`-gnatn' so it is not necessary
to specify both options. There are a few cases that the back-end inlining
catches that cannot be dealt with in the front-end.
case statements.
This may result in less efficient code, but is sometimes necessary
(for example on HP-UX targets)
in order to compile large and/or nested case statements.
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.
Inline. This applies only to
inlining within a unit. For details on control of inlining
see 3.2.13 Subprogram Inlining Control.
gnatmake flag (see section 6.2 Switches for gnatmake).
gcc driver. Normally used only for
debugging purposes or if you need to be sure what version of the
compiler you are executing.
gcc
version, not the GNAT version.
You may combine a sequence of GNAT switches into a single switch. For example, the combined switch
-gnatofi3 |
is equivalent to specifying the following sequence of switches:
-gnato -gnatf -gnati3 |
The following restrictions apply to the combination of switches in this manner:
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The standard default format for error messages is called "brief format". Brief format messages are written to `stderr' (the standard error 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.
glide can parse the error messages
and point to the referenced character.
The following switches provide control over the error message
format:
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.
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.
b stands for brief.
This switch causes GNAT to generate the
brief format error messages to `stderr' (the standard error
file) as well as the verbose
format message or full listing (which as usual is written to
`stdout' (the standard output file).
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 |
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 |
The `-gnatf' switch also generates additional information for some error messages. Some examples are:
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.
In addition, if `-gnatt' is also specified, then the tree file is generated even if there are illegalities. It may be useful in this case to also specify `-gnatq' to ensure that full semantic processing occurs. The resulting tree file can be processed by ASIS, for the purpose of providing partial information about illegal units, but if the error causes the tree to be badly malformed, then ASIS may crash during the analysis.
When `-gnatQ' is used and the generated `ALI' file is marked as
being in error, gnatmake will attempt to recompile the source when it
finds such an `ALI' file, including with switch `-gnatc'.
Note that `-gnatQ' has no effect if `-gnats' is specified, since ALI files are never generated if `-gnats' is set.
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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 given in the following
list. This list is not complete, and new warnings are often added to
subsequent versions of GNAT. The list is intended to give a general idea
of the kinds of warnings that are generated.
accept statement
select
return statement along some execution path in a function
with clauses
Bit_Order usage that does not have any effect
Standard.Duration used to resolve universal fixed expression
with'ed by application unit
for loop that is known to be null or might be null
The following switches are available to control the handling of warning messages:
.all will generate a warning. With this warning
enabled, access checks occur only at points where an explicit
.all appears in the source code (assuming no warnings are
generated as a result of this switch). The default is that such
warnings are not generated.
Note that `-gnatwa' does not affect the setting of
this warning option.
with of an internal GNAT
implementation unit, defined as any unit from the Ada,
Interfaces, GNAT,
or System
hierarchies that is not
documented in either the Ada Reference Manual or the GNAT
Programmer's Reference Manual. Such units are intended only
for internal implementation purposes and should not be with'ed
by user programs. The default is that such warnings are generated
This warning can also be turned on using `-gnatwa'.
with of an internal GNAT
implementation unit.
pragma Obsolescent and
for use of features in Annex J of the Ada Reference Manual. In the
case of Annex J, not all features are flagged. In particular use
of the renamed packages (like Text_IO) and use of package
ASCII are not flagged, since these are very common and
would generate many annoying positive warnings. The default is that
such warnings are not generated.
In addition to the above cases, warnings are also generated for
GNAT features that have been provided in past versions but which
have been superceded (typically by features in the new Ada standard).
For example, pragma Ravenscar will be flagged since its
function is replaced by pragma Profile(Ravenscar).
Note that this warning option functions differently from the
restriction No_Obsolescent_Features in two respects.
First, the restriction applies only to annex J features.
Second, the restriction does flag uses of package ASCII.
pragma Elaborate_All statements.
See the section in this guide on elaboration checking for details on
when such pragma should be used. Warnings are also generated if you
are using the static mode of elaboration, and a pragma Elaborate
is encountered. The default is that such warnings
are not generated.
This warning is not automatically turned on by the use of `-gnatwa'.
Base where typ'Base is the same
as typ.
Pack when all components are placed by a record
representation clause.
gcc back end.
To suppress these back end warnings as well, use the switch `-w'
in addition to `-gnatws'.
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.
In the case where a package or subprogram body is compiled, and there
is a with on the corresponding spec
that is only referenced in the body,
a warning is also generated, noting that the
with can be moved to the body. The default is that
such warnings are not generated.
This switch also activates warnings on unreferenced formals
(it includes the effect of `-gnatwf').
This warning can also be turned on using `-gnatwa'.
A string of warning parameters can be used in the same parameter. For example:
-gnatwaLe |
will turn on all optional warnings except for elaboration pragma warnings, and also specify that warnings should be treated as errors. When no switch `-gnatw' is used, this is equivalent to:
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The pragmas 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_Failure 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.
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The Ada 95 Reference Manual has specific requirements for checking for invalid values. In particular, RM 13.9.1 requires that the evaluation of invalid values (for example from unchecked conversions), not result in erroneous execution. In GNAT, the result of such an evaluation in normal default mode is to either use the value unmodified, or to raise Constraint_Error in those cases where use of the unmodified value would cause erroneous execution. The cases where unmodified values might lead to erroneous execution are case statements (where a wild jump might result from an invalid value), and subscripts on the left hand side (where memory corruption could occur as a result of an invalid value).
The `-gnatVx' switch allows more control over the validity
checking mode.
The x argument is a string of letters that
indicate validity checks that are performed or not performed in addition
to the default checks described above.
IEEE infinity mode is not allowed. The exact contexts
in which floating-point values are checked depends on the setting of other
options. For example,
`-gnatVif' or
`-gnatVfi'
(the order does not matter) specifies that floating-point parameters of mode
in should be validity checked.
in mode parameters
Arguments for parameters of mode in are validity checked in function
and procedure calls at the point of call.
in out mode parameters.
Arguments for parameters of mode in out are validity checked in
procedure calls at the point of call. The 'm' here stands for
modify, since this concerns parameters that can be modified by the call.
Note that there is no specific option to test out parameters,
but any reference within the subprogram will be tested in the usual
manner, and if an invalid value is copied back, any reference to it
will be subject to validity checking.
Standard,
the shift operators defined as intrinsic in package Interfaces
and operands for attributes such as Pos. Checks are also made
on individual component values for composite comparisons.
return statements in functions is validity
checked.
if, while or exit
statements are checked, as well as guard expressions in entry calls.
The `-gnatV' switch may be followed by
a string of letters
to turn on a series of validity checking options.
For example,
`-gnatVcr'
specifies that in addition to the default validity checking, copies and
function return expressions are to be validity checked.
In order to make it easier
to specify the desired combination of effects,
the upper case letters CDFIMORST may
be used to turn off the corresponding lower case option.
Thus
`-gnatVaM'
turns on all validity checking options except for
checking of in out procedure arguments.
The specification of additional validity checking generates extra code (and
in the case of `-gnatVa' the code expansion can be substantial.
However, these additional checks can be very useful in detecting
uninitialized variables, incorrect use of unchecked conversion, and other
errors leading to invalid values. The use of pragma Initialize_Scalars
is useful in conjunction with the extra validity checking, since this
ensures that wherever possible uninitialized variables have invalid values.
See also the pragma Validity_Checks which allows modification of
the validity checking mode at the program source level, and also allows for
temporary disabling of validity checks.
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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 warning message is given, preceded by the character sequence "(style)". The string x is a sequence of letters or digits indicating the particular style checks to be performed. The following checks are defined:
-- starting on a column that is a multiple of
the alignment level.
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.
--" that starts the column must either start in column one,
or else at least one blank must precede this sequence.
--" at the start of the comment.
--" that
starts the comment, with the following exceptions.
--" characters, possibly preceded
by blanks is permitted.
--x" where x is a special character
is permitted.
This allows proper processing of the output generated by specialized tools
including gnatprep (where "--!" is used) and the SPARK
annotation
language (where "--#" is used). For the purposes of this rule, a
special character is defined as being in one of the ASCII ranges
16#21#..16#2F# or 16#3A#..16#3F#.
Note that this usage is not permitted
in GNAT implementation units (i.e. when `-gnatg' is used).
--" is permitted as long as at
least one blank follows the initial "--". Together with the preceding
rule, this allows the construction of box comments, as shown in the following
example:
--------------------------- -- This is a box comment -- -- with two text lines. -- --------------------------- |
end statements ending subprograms and on
exit statements exiting named loops, are required to be present.
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.
digits used as attribute names to which this check
does not apply).
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;
|
Integer and ASCII.NUL).
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.
if statements, while statements and
exit statements.
In the above rules, appearing in column one is always permitted, that is, counts as meeting either a requirement for a required preceding space, or as meeting a requirement for no preceding space.
Appearing at the end of a line is also always permitted, that is, counts as meeting either a requirement for a following space, or as meeting a requirement for no following space.
If any of these style rules is violated, a message is generated giving
details on the violation. The initial characters of such messages are
always "(style)". Note that these messages are treated as warning
messages, so they normally do not prevent the generation of an object
file. The `-gnatwe' switch can be used to treat warning messages,
including style messages, as fatal errors.
The switch
`-gnaty' on its own (that is not
followed by any letters or digits),
is equivalent to gnaty3abcefhiklmnprst, that is all checking
options enabled with the exception of `-gnatyo',
`-gnatyd', `-gnatyu', and `-gnatyx'.
an indentation level of 3 is set. This is similar to the standard
checking option that is used for the GNAT sources.
The switch `-gnatyN' clears any previously set style checks.
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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:
pragma Suppress (all_checks)
had been present in the source. Validity checks are also suppressed (in
other words `-gnatp' also implies `-gnatVn'.
Use this switch to improve the performance
of the code at the expense of safety in the presence of invalid data or
program bugs.
Constraint_Error as required by standard Ada
semantics). These overflow checks correspond to situations in which
the true value of the result of an operation may be outside the base
range of the result type. The following example shows the distinction:
X1 : Integer := Integer'Last; X2 : Integer range 1 .. 5 := 5; X3 : Integer := Integer'Last; X4 : Integer range 1 .. 5 := 5; F : Float := 2.0E+20; ... X1 := X1 + 1; X2 := X2 + 1; X3 := Integer (F); X4 := Integer (F); |
Here the first addition results in a value that is outside the base range
of Integer, and hence requires an overflow check for detection of the
constraint error. Thus the first assignment to X1 raises a
Constraint_Error exception only if `-gnato' is set.
The second increment operation results in a violation
of the explicit range constraint, and such range checks are always
performed (unless specifically suppressed with a pragma suppress
or the use of `-gnatp').
The two conversions of F both result in values that are outside
the base range of type Integer and thus will raise
Constraint_Error exceptions only if `-gnato' is used.
The fact that the result of the second conversion is assigned to
variable X4 with a restricted range is irrelevant, since the problem
is in the conversion, not the assignment.
Basically the rule is that in the default mode (`-gnato' not used), the generated code assures that all integer variables stay within their declared ranges, or within the base range if there is no declared range. This prevents any serious problems like indexes out of range for array operations.
What is not checked in default mode is an overflow that results in
an in-range, but incorrect value. In the above example, the assignments
to X1, X2, X3 all give results that are within the
range of the target variable, but the result is wrong in the sense that
it is too large to be represented correctly. Typically the assignment
to X1 will result in wrap around to the largest negative number.
The conversions of F will result in some Integer value
and if that integer value is out of the X4 range then the
subsequent assignment would generate an exception.
Note that the `-gnato' switch does not affect the code generated
for any floating-point operations; it applies only to integer
semantics).
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).
The reason that we distinguish overflow checking from other kinds of range constraint checking is that a failure of an overflow check can generate an incorrect value, but cannot cause erroneous behavior. This is unlike the situation with a constraint check on an array subscript, where failure to perform the check can result in random memory description, or the range check on a case statement, where failure to perform the check can cause a wild jump.
Note again that `-gnato' is off by default, so overflow checking is
not performed in default mode. This means that out of the box, with the
default settings, GNAT does not do all the checks expected from the
language description in the Ada Reference Manual. If you want all constraint
checks to be performed, as described in this Manual, then you must
explicitly use the -gnato switch either on the gnatmake or
gcc command.
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.
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For most operating systems, gcc does not perform stack overflow
checking by default. This means that if the main environment task or
some other task exceeds the available stack space, then unpredictable
behavior will occur.
To activate stack checking, compile all units with the gcc option `-fstack-check'. For example:
gcc -c -fstack-check package1.adb |
Units compiled with this option will generate extra instructions to check
that any use of the stack (for procedure calls or for declaring local
variables in declare blocks) do not exceed the available stack space.
If the space is exceeded, then a Storage_Error exception is raised.
For declared tasks, the stack size is always controlled by the size
given in an applicable Storage_Size pragma (or is set to
the default size if no pragma is used.
For the environment task, the stack size depends on system defaults and is unknown to the compiler. The stack may even dynamically grow on some systems, precluding the normal Ada semantics for stack overflow. In the worst case, unbounded stack usage, causes unbounded stack expansion resulting in the system running out of virtual memory.
The stack checking may still work correctly if a fixed
size stack is allocated, but this cannot be guaranteed.
To ensure that a clean exception is signalled for stack
overflow, set the environment variable
GNAT_STACK_LIMIT to indicate the maximum
stack area that can be used, as in:
SET GNAT_STACK_LIMIT 1600 |
The limit is given in kilobytes, so the above declaration would set the stack limit of the environment task to 1.6 megabytes. Note that the only purpose of this usage is to limit the amount of stack used by the environment task. If it is necessary to increase the amount of stack for the environment task, then this is an operating systems issue, and must be addressed with the appropriate operating systems commands.
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gcc for Syntax Checking
The s stands for "syntax".
Run GNAT in syntax checking only mode. For example, the command
$ gcc -c -gnats x.adb |
compiles 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.
When the source file is empty or contains only empty lines and/or comments, the output is a warning:
$ gcc -c -gnats -x ada toto.txt toto.txt:1:01: warning: empty file, contains no compilation units $ |
Otherwise, 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 8. Renaming Files Using gnatchop).
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gcc for Semantic Checking
The 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 object code
(no object file is generated).
Because dependent files must be accessed, you must follow the GNAT semantic restrictions on file structuring to operate in this mode:
The output consists of error messages as appropriate. No object file is generated. An `ALI' file is generated for use in the context of cross-reference tools, but this file is marked as not being suitable for binding (since no object file is generated). The checking corresponds exactly to the notion of legality in the Ada 95 Reference Manual.
Any unit can be compiled in semantics-checking-only mode, including units that would not normally be compiled (subunits, and specifications where a separate body is present).
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Although GNAT is primarily an Ada 95 compiler, it accepts this switch to specify that an Ada 83 program is to be compiled in Ada 83 mode. If you specify this switch, GNAT rejects most Ada 95 extensions and applies Ada 83 semantics where this can be done easily. It is not possible to guarantee this switch does a perfect job; for example, some subtle tests, such as are found in earlier ACVC tests (and that have been removed from the ACATS suite for Ada 95), might not compile correctly. Nevertheless, this switch may be useful in some circumstances, for example where, due to contractual reasons, legacy code needs to be maintained using only Ada 83 features.
With few exceptions (most notably the need to use <> on
unconstrained generic formal parameters, the use of the new Ada 95
reserved words, 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.
For further information, please refer to E. Compatibility and Porting Guide.
GNAT is primarily an Ada 95 compiler, and all current releases of GNAT Pro compile in Ada 95 mode by default. Typically, Ada 95 is sufficiently upwards compatible with Ada 83, that legacy Ada 83 programs may be compiled using this default Ada95 mode without problems (see section above describing the use of `-gnat83' to run in Ada 83 mode).
In Ada 95 mode, the use of Ada 2005 features will in general cause error messages or warnings. Some specialized releases of GNAT (notably the GAP academic version) operate in Ada 2005 mode by default (see section below describing the use of `-gnat05' to run in Ada 2005 mode). For such versions the `-gnat95' switch may be used to enforce Ada 95 mode. This option also can be used to cancel the effect of a previous `-gnat83' or `-gnat05' switch earlier in the command line.
Although GNAT is primarily an Ada 95 compiler, it can be set to operate in Ada 2005 mode using this option. Although the new standard has not yet been issued (as of early 2005), many features have been discussed and approved in "Ada Issues" (AI's). For the text of these AI's, see www.ada-auth.org/cgi-bin/cvsweb.cgi/AIs. Included with GNAT releases is a file `features-ada0y' that describes the current set of implemented Ada 2005 features.
If these features are used in Ada 95 mode (which is the normal default), then error messages or warnings may be generated, reflecting the fact that these new features are otherwise unauthorized extensions to Ada 95. The use of the `-gnat05' switch (or an equivalent pragma) causes these messages to be suppressed.
Note that some specialized releases of GNAT (notably the GAP academic version) have Ada 2005 mode on by default, and in such environments, the Ada 2005 features can be used freely without the use of switches.
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Normally GNAT recognizes the Latin-1 character set in source program identifiers, as described in the Ada 95 Reference Manual. This switch causes GNAT to recognize alternate character sets in identifiers. c is a single character indicating the character set, as follows:
1
2
3
4
5
9
p
8
f
n
w
See section 2.2 Foreign Language Representation, for full details on the implementation of these character sets.
h
u
s
e
8
b
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.
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For the source file naming rules, See section 2.3 File Naming Rules.
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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 of subprograms specified by pragma Inline,
you must also specify this switch.
In the absence of this switch, GNAT does not attempt
inlining 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 7.1.5 Inlining of Subprograms.
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gcc when
compiling multiple files indicates whether all source files have
been successfully used to generate object files or not.
When `-pass-exit-codes' is used, gcc exits with an extended
exit status and allows an integrated development environment to better
react to a compilation failure. Those exit status are:
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Debug unit in the compiler source
file `debug.adb'.
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. The following
is a partial list of these special constructions. See the specification
of package Sprint in file `sprint.ads' for a full list.
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]
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. This switch
also suppress generation of cross-reference information (see
`-gnatx') since otherwise the cross-reference information
would refer to the `.dg' file, which would cause
confusion since this is not the original source file.
Note that `-gnatD' actually implies `-gnatG' automatically, so it is not necessary to give both options. In other words `-gnatD' is equivalent to `-gnatDG').
GNAT sources for full details on the format of `-gnatR3'
output. If the switch is followed by an s (e.g. `-gnatR2s'), then
the output is to a file with the name `file.rep' where
file is the name of the corresponding source file.
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.
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GNAT uses two methods for handling exceptions at run-time. The
setjmp/longjmp method saves the context when entering
a frame with an exception handler. Then when an exception is
raised, the context can be restored immediately, without the
need for tracing stack frames. This method provides very fast
exception propagation, but introduces significant overhead for
the use of exception handlers, even if no exception is raised.
The other approach is called "zero cost" exception handling. With this method, the compiler builds static tables to describe the exception ranges. No dynamic code is required when entering a frame containing an exception handler. When an exception is raised, the tables are used to control a back trace of the subprogram invocation stack to locate the required exception handler. This method has considerably poorer performance for the propagation of exceptions, but there is no overhead for exception handlers if no exception is raised. Note that in this mode and in the context of mixed Ada and C/C++ programming, to propagate an exception through a C/C++ code, the C/C++ code must be compiled with the `-funwind-tables' GCC's option.
The following switches can be used to control which of the two exception handling methods is used.
The setjmp/longjmp approach is available on all targets, while
the zero cost approach is available on selected targets.
To determine whether zero cost exceptions can be used for a
particular target, look at the private part of the file system.ads.
Either GCC_ZCX_Support or Front_End_ZCX_Support must
be True to use the zero cost approach. If both of these switches
are set to False, this means that zero cost exception handling
is not yet available for that target. The switch
ZCX_By_Default indicates the default approach. If this
switch is set to True, then the zero cost approach is
used by default.
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The use of mapping files is not required for correct operation of the
compiler, but mapping files can improve efficiency, particularly when
sources are read over a slow network connection. In normal operation,
you need not be concerned with the format or use of mapping files,
and the `-gnatem' switch is not a switch that you would use
explicitly. it is intended only for use by automatic tools such as
gnatmake running under the project file facility. The
description here of the format of mapping files is provided
for completeness and for possible use by other tools.
A mapping file is a sequence of sets of three lines. In each set,
the first line is the unit name, in lower case, with "%s"
appended for
specifications and "%b" appended for bodies; the second line is the
file name; and the third line is the path name.
Example:
main%b main.2.ada /gnat/project1/sources/main.2.ada |
When the switch `-gnatem' is specified, the compiler will create in memory the two mappings from the specified file. If there is any problem (non existent file, truncated file or duplicate entries), no mapping will be created.
Several `-gnatem' switches may be specified; however, only the last one on the command line will be taken into account.
When using a project file, gnatmake create a temporary mapping file
and communicates it to the compiler using this switch.
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GNAT sources may be preprocessed immediately before compilation; the actual text of the source is not the text of the source file, but is derived from it through a process called preprocessing. Integrated preprocessing is specified through switches `-gnatep' and/or `-gnateD'. `-gnatep' indicates, through a text file, the preprocessing data to be used. `-gnateD' specifies or modifies the values of preprocessing symbol.
It is recommended that gnatmake switch -s should be
used when Integrated Preprocessing is used. The reason is that preprocessing
with another Preprocessing Data file without changing the sources will
not trigger recompilation without this switch.
Note that gnatmake switch -m will almost
always trigger recompilation for sources that are preprocessed,
because gnatmake cannot compute the checksum of the source after
preprocessing.
The actual preprocessing function is described in details in section
16. Preprocessing Using gnatprep. This section only describes how integrated
preprocessing is triggered and parameterized.
-gnatep=file
A preprocessing data file is a text file with significant lines indicating how should be preprocessed either a specific source or all sources not mentioned in other lines. A significant line is a non empty, non comment line. Comments are similar to Ada comments.
Each significant line starts with either a literal string or the character '*'. A literal string is the file name (without directory information) of the source to preprocess. A character '*' indicates the preprocessing for all the sources that are not specified explicitly on other lines (order of the lines is not significant). It is an error to have two lines with the same file name or two lines starting with the character '*'.
After the file name or the character '*', another optional literal string indicating the file name of the definition file to be used for preprocessing (see section 16.3 Form of Definitions File). The definition files are found by the compiler in one of the source directories. In some cases, when compiling a source in a directory other than the current directory, if the definition file is in the current directory, it may be necessary to add the current directory as a source directory through switch -I., otherwise the compiler would not find the definition file.
Then, optionally, switches similar to those of gnatprep may
be found. Those switches are:
-b
-c
--! ".
-Dsymbol=value
if,
else, elsif, end, and, or and then.
value is either a literal string, an Ada identifier or any Ada reserved
word. A symbol declared with this switch replaces a symbol with the
same name defined in a definition file.
-s
-u
FALSE
in the context
of a preprocessor test. In the absence of this option, an undefined symbol in
a #if or #elsif test will be treated as an error.
Examples of valid lines in a preprocessor data file:
"toto.adb" "prep.def" -u -- preprocess "toto.adb", using definition file "prep.def", -- undefined symbol are False. * -c -DVERSION=V101 -- preprocess all other sources without a definition file; -- suppressed lined are commented; symbol VERSION has the value V101. "titi.adb" "prep2.def" -s -- preprocess "titi.adb", using definition file "prep2.def"; -- list all symbols with their values. |
-gnateDsymbol[=value]
True.
A symbol is an identifier, following normal Ada (case-insensitive)
rules for its syntax, and value is any sequence (including an empty sequence)
of characters from the set (letters, digits, period, underline).
Ada reserved words may be used as symbols, with the exceptions of if,
else, elsif, end, and, or and then.
A symbol declared with this switch on the command line replaces a symbol with the same name either in a definition file or specified with a switch -D in the preprocessor data file.
This switch is similar to switch `-D' of gnatprep.
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The GCC technology provides a wide range of target dependent `-m' switches for controlling details of code generation with respect to different versions of architectures. This includes variations in instruction sets (e.g. different members of the power pc family), and different requirements for optimal arrangement of instructions (e.g. different members of the x86 family). The list of available `-m' switches may be found in the GCC documentation.
Use of the these `-m' switches may in some cases result in improved code performance.
The GNAT Pro technology is tested and qualified without any `-m' switches, so generally the most reliable approach is to avoid the use of these switches. However, we generally expect most of these switches to work successfully with GNAT Pro, and many customers have reported successful use of these options.
Our general advice is to avoid the use of `-m' switches unless special needs lead to requirements in this area. In particular, there is no point in using `-m' switches to improve performance unless you actually see a performance improvement.
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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:
gcc command line, in the order given.
ADA_PRJ_INCLUDE_FILE environment variable.
ADA_PRJ_INCLUDE_FILE is normally set by gnatmake or by the gnat
driver when project files are used. It should not normally be set
by other means.
ADA_INCLUDE_PATH environment variable.
Construct this value
exactly as the PATH environment variable: a list of directory
names separated by colons (semicolons when working with the NT version).
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 types, subprograms, etc. 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.
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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.
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The following are some typical Ada compilation command line examples:
$ gcc -c xyz.adb
$ gcc -c -O2 -gnata xyz-def.adb
Compile the child unit package in file `xyz-def.adb' with extensive
optimizations, and pragma Assert/Debug statements
enabled.
$ gcc -c -gnatc abc-def.adb
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