Implementation defined behavior is controlled by
| ||(2)||(since C99)|
\, then tokenizes the result (as in translation stage 3), and then uses the result as if the input to
The pragma directive controls implementation-specific behavior of the compiler, such as disabling compiler warnings or changing alignment requirements. Any pragma that is not recognized is ignored.
The following three pragmas are defined by the language standard:
where arg is either
ON, informs the compiler that the program will access or modify floating-point environment, which means that optimizations that could subvert flag tests and mode changes (e.g., global common subexpression elimination, code motion, and constant folding) are prohibited. The default value is implementation-defined, usually
(x*y) + zwith a single fused multiply-add CPU instruction. The default value is implementation-defined, usually
Note: compilers that do not support these pragmas may provide equivalent compile-time options, such as gcc's
#pragma once is a non-standard pragma that is supported by the vast majority of modern compilers. If it appears in a header file, it indicates that it is only to be parsed once, even if it is (directly or indirectly) included multiple times in the same source file.
Standard approach to preventing multiple inclusion of the same header is by using include guards:
#ifndef LIBRARY_FILENAME_H #define LIBRARY_FILENAME_H // contents of the header #endif /* LIBRARY_FILENAME_H */
So that all but the first inclusion of the header in any translation unit are excluded from compilation. All modern compilers record the fact that a header file uses an include guard and do not re-parse the file if it is encountered again, as long as the guard is still defined. (see e.g. gcc).
#pragma once, the same header appears as.
#pragma once // contents of the header
Unlike header guards, this pragma makes it impossible to erroneously use the same macro name in more than one file. On the other hand, since with
#pragma once files are excluded based on their filesystem-level identity, this can't protect against including a header twice if it exists in more than one location in a project.
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