Pragma Directives

PRAGMA DIRECTIVES

The #pragma directive is used to control the actions of the compiler in a particular portion of a program without affecting the program as a whole. You can include the pragma directive in your C program from the point where you want them to take effect. The effect of pragma will be applied from the point where it is included to the end of the compilation unit or until another pragma changes its status.

A #pragma directive is an instruction to the compiler and is usually ignored during preprocessing. The syntax of using a pragma directive can be given as

#pragma string

Here, string can be one of the instructions given to the compiler with any required parameters. Table 10.1 describes some pragma directives.

Pragma Copyright

The syntax of pragma copyright can be given as

#pragma COPYRIGHT "string"

Here, string specifies the set of characters included in the copyright message in the object file.

If no date is specified using pragma COPYRIGHT_DATE, then the current year is used in the copyright message. For example, if we write

#pragma COPYRIGHT "JRT Software Ltd"

Then the following string is placed in the object code (assuming the current year is 2011):

Copyright JRT Software Ltd, 2011. All rights reserved. No part of this program may be photocopied, reproduced, or transmitted without prior written consent of JRT Software Ltd.

Pragma Copyright_date

The syntax of pragma COPYRIGHT_DATE can be given as

#pragma COPYRIGHT_DATE "string"

Here, the string is a date which will be used by the COPYRIGHT pragma.

For example, consider the pragma given below

#pragma COPYRIGHT DATE "1999-2011"

#pragma COPYRIGHT "JRT Software Ltd."

The above pragma will place the following string in the object code:

© Copyright JRT Software Ltd, 1999-2011. All rights reserved. No part of this program may be photocopied, reproduced, or transmitted without prior written consent of JRT Software Ltd.

Pragma Optimize

The syntax of using the pragma optimize can be given as

#pragma OPTIMIZE ON

#pragma OPTIMIZE OFF

The pragma optimize is basically used to turn on/off optimization in sections of a source program. However, when using this pragma you must specify one of the optimization options on the acc command (while giving the command to compile the program), otherwise this pragma is ignored. Also remember that the pragma optimize cannot be used within a function. For example,

aCC +02 Prog.C // Set optimization to level 2 for Prog.C

#pragma OPTIMIZE OFF

void Func1 ()

{ // Turn off optimization for this function

………

}

#pragma OPTIMIZE ON

void Func2 ()

{ // Restore optimization to level 2

…………

}

Pragma OPT_LEVEL

The syntax for pragma OPT_LEVEL which is used to set the optimization level to 1,2,3, or 4 can be given as

#pragma OPT_LEVEL 1

#pragma OPT_LEVEL 2

#pragma OPT_LEVEL 3

#pragma OPT_LEVEL 4

Like the optimization pragma, even this pragma cannot be used within a function. Finally, OPT_LEVEL 3 and 4 are allowed only at the beginning of a file.

For example,

aCC -O prog.C

#pragma OPT_LEVEL 1

void Func1 ()

{ // Optimize Funcl() at level 1.

………..

}

#pragma OPT_LEVEL 2

void Func2()

{ // Optimize Func2() at level 2.

………….

}

Note

The kind of optimization done by the operating system at each level is beyond the scope of this book.

Pragma HP_SHLIB_VERSION

The syntax for HP_SHLIB_VERSION pragma which is used to create different versions of a shared library routine can be given as

#pragma HP_SHLIB_VERSION ["]date ["]

Here, the date argument is of the form month/year, optionally enclosed in quotes. The month must be specified using any number from 1 to 12. The year can be specified as either the last two digits of the year (99 for 1999) or a full year specification (1999). Here the two-digit year codes from 00 to 40 are used to represent the years from 2000 to 2040, respectively.

Note

The version number applies to all global symbols defined in the module's source file.

This pragma should be used only if incompatible changes are made to a source file.

Pragma LOCALITY

The syntax of pragma locality which is used to specify a name to be associated with the code that is written to a re-locatable object module can be given as

#pragma LOCALITY "string"

Here, string specifies a name to be used for a code subspace. After this directive, all codes following the directive are associated with the name specified in string. The smallest scope of a unique LOCALITY pragma is a function.

 Pragma VERSIONID

The syntax of pragma VERSIONID can be given as

 #pragma VERSIONID "string"

Here, string is a string of characters that is placed in the object file. For example, if we write

#pragma VERSIONID "JRT Software Ltd.,

Version 12345.A.01.10"

Then this pragma places the characters JRT Software Ltd., Version 12345.A.01.10 into the object file.

Pragma once

The pragma once specifies that the file, in which this pragma directive is specified will be included (opened) only once by the compiler in a building of a particular file. Its syntax can be given as

#pragma once

Conclusion The pragma preprocessor directive is mainly used where each implementation of C supports some features unique to its host machine or operating system. For example, some programs may need to exercise precise control over the memory areas where data is placed or to control the way certain functions receive parameters. In such cases, #pragma directive provides machine- and operating-system-specific features for each compiler while retaining overall compatibility with the C language.