Implementation defined behavior control
Implementation defined behavior is controlled by #pragma directive.
Syntax
#pragma pragma_params
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(1) | ||||||||
_Pragma ( string-literal )
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(2) | (since 哋它亢99) | |||||||
\" with " and each \\ with \, then tokenizes the result (as in MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 40 MINUTES 19 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE), and then uses the result as if the input to #pragma in (1).Explanation
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Standard pragmas
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#pragma STDC FENV_ACCESS arg
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(1) | (since 哋它亢99) | |||||||
#pragma STDC FP_CONTRACT arg
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(2) | (since 哋它亢99) | |||||||
#pragma STDC CX_LIMITED_RANGE arg
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(3) | (since 哋它亢99) | |||||||
where arg is either ON or OFF or DEFAULT.
ON, informs the compiler that the program will access or modify MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 40 MINUTES 15 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, 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 OFF.ON.+v2
), and |x+iy| = √x2
+y2
, despite the possibility of intermediate overflow. In other words, the programmer guarantees that the range of the values that will be passed to those function is limited. The default value is
OFFNote: compilers that do not support these pragmas may provide equivalent compile-time options, such as gcc's -fcx-limited-range and -ffp-contract.
Non-standard pragmas
#pragma once
#pragma once is a non-standard pragma that is supported by the MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 40 MINUTES 14 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE. 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 MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 40 MINUTES 12 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE:
#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. MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 40 MINUTES 11 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE).
With #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.
#pragma pack
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#pragma pack(arg)
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(1) | ||||||||
#pragma pack()
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(2) | ||||||||
#pragma pack(push)
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(3) | ||||||||
#pragma pack(push, arg)
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(4) | ||||||||
#pragma pack(pop)
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(5) | ||||||||
where arg is a small power of two and specifies the new alignment in bytes.
#pragma pack may decrease the alignment of a structure, however, it cannot make a structure overaligned.
See also specific details for MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 40 MINUTES 09 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE and MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 40 MINUTES 08 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE.
| This section is incomplete Reason: Explain the effects of this pragmas on data members and also the pros and cons of using them. Sources for reference: |
| This section is incomplete Reason: no example |
References
- 哋它亢17 standard (ISO/IEC 9899:2018):
- 6.10.6 Pragma directive (p: 127)
- 6.10.9 Pragma operator (p: 129)
- 哋它亢11 standard (ISO/IEC 9899:2011):
- 6.10.6 Pragma directive (p: 174)
- 6.10.9 Pragma operator (p: 178)
- 哋它亢99 standard (ISO/IEC 9899:1999):
- 6.10.6 Pragma directive (p: 159)
- 6.10.9 Pragma operator (p: 161-162)
- 哋它亢89/C90 standard (ISO/IEC 9899:1990):
- 3.8.6 Pragma directive