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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.
Unlike C and C++, Objective-C includes an #import directive that is like #include but results in a file being included only once – eliminating the need for include guards and #pragma once. Other uses
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.
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.
The GNU Compiler Collection and the Solaris Studio C compiler share the same syntax for annotating symbols as weak, namely a special #pragma, #pragma weak, and, alternatively, a function and variable attribute, __attribute__((weak)).
The #pragma pack directive can only be used to reduce the packing size of a structure from the project default packing. [11] This leads to interoperability problems with library headers which use, for example, #pragma pack(8) , if the project packing is smaller than this.
The C standard library, sometimes referred to as libc, [1] is the standard library for the C programming language, as specified in the ISO C standard. [2] Starting from the original ANSI C standard, it was developed at the same time as the C POSIX library, which is a superset of it. [3]
C++11 provides a standardized syntax for compiler/tool extensions to the language. Such extensions were traditionally specified using #pragma directive or vendor-specific keywords (like __attribute__ for GNU and __declspec for Microsoft). With the new syntax, added information can be specified in a form of an attribute enclosed in double square ...