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Pragma Directives

Each implementation of C and C++ supports some features unique to its host machine or operating system. Some programs, for instance, need to exercise precise control over the memory areas where data is placed or to control the way certain functions receive parameters. The #pragma directives offer a way for each compiler to offer machine- and operating-system-specific features while retaining overall compatibility with the C and C++ languages. Pragmas are machine- or operating-system-specific by definition, and are usually different for every compiler.

#pragma token-string

The token-string is a series of characters that gives a specific compiler instruction and arguments, if any. The number sign (#) must be the first non-white-space character on the line containing the pragma; white-space characters can separate the number sign and the word pragma. Following #pragma, write any text that the translator can parse as preprocessing tokens. The argument to #pragma is subject to macro expansion.

If the compiler finds a pragma it does not recognize, it issues a warning, but compilation continues.

Some pragmas provide the same functionality as compiler options. When a pragma is encountered in the code, it overrides the behavior specified by the compiler option. For example, if you specified /Zp8, you can override this compiler setting for specific portions of the code with pack:

cl /Zp8 ...

<file> - packing is 8
// ...
#pragma pack(push, 1) - packing is now 1
// ...
#pragma pack(pop) - packing is 8
</file>

Pragmas can be used in conditional statements, to provide new preprocessor functionality, or to provide implementation-defined information to the compiler. The C and C++ compilers recognize the following pragmas:

alloc_text conform1 inline_depth pack
auto_inline const_seg inline_recursion pointers_to_members1
bss_seg data_seg intrinsic runtime_checks
check_stack function keyword1 section
code_seg hdrstop message setlocale
comment include_alias once vtordisp1
component init_seg1 optimize warning

1. Supported only by the C++ compiler.