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Using #pragma once allows the C preprocessor to include a header file when it is needed and to ignore an #include directive otherwise. This has the effect of altering the behavior of the C preprocessor itself, and allows programmers to express file dependencies in a simple fashion, obviating the need for manual management.
In computer programming, a directive or pragma (from "pragmatic") is a language construct that specifies how a compiler (or other translator) should process its input. Depending on the programming language , directives may or may not be part of the grammar of the language and may vary from compiler to compiler.
While there is no standard way of defining the alignment of structure members (while C and C++ allow using the alignas specifier for this purpose it can be used only for specifying a stricter alignment), some compilers use #pragma directives to specify packing inside source files. Here is an example:
The C preprocessor (CPP) is a text file processor that is used with C, C++ and other programming tools.The preprocessor provides for file inclusion (often header files), macro expansion, conditional compilation, and line control.
For #include guards to work properly, each guard must test and conditionally set a different preprocessor macro. Therefore, a project using #include guards must work out a coherent naming scheme for its include guards, and make sure its scheme doesn't conflict with that of any third-party headers it uses, or with the names of any globally visible macros.
Auto-linking is a mechanism for automatically determining which libraries to link to while building a C, C++ or Obj-C program. It is activated by means of #pragma comment(lib, <name>) statements in the header files of the library, or @import <name>, depending on the compiler.
The Watcom C/C++ compiler also uses the #pragma aux [20] directive that allows the user to specify their own calling convention. As its manual states, "Very few users are likely to need this method, but if it is needed, it can be a lifesaver".
OpenMP (Open Multi-Processing) is an application programming interface (API) that supports multi-platform shared-memory multiprocessing programming in C, C++, and Fortran, [3] on many platforms, instruction-set architectures and operating systems, including Solaris, AIX, FreeBSD, HP-UX, Linux, macOS, and Windows.