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Diffstat (limited to 'doc/info')
| -rw-r--r-- | doc/info/chap/language-facilities.texinfo | 37 |
1 files changed, 37 insertions, 0 deletions
diff --git a/doc/info/chap/language-facilities.texinfo b/doc/info/chap/language-facilities.texinfo index 81908e8..e30352e 100644 --- a/doc/info/chap/language-facilities.texinfo +++ b/doc/info/chap/language-facilities.texinfo @@ -8,6 +8,7 @@ * Booleans:: Proper booleans in the C programming language. * Non-returning functions:: Specifying that functions never return. * Variable alignment:: Aligment of variables and data types. +* Member offsets:: Getting the offset of members of a structure. @end menu @@ -189,3 +190,39 @@ should have the alignment of the type @code{TYPE}. The function call @code{alignof(TYPE)}, returns the alignment of the type @code{TYPE}. + + +@node Member offsets +@section Member offsets + +@hfindex stddef.h +@cpindex Member offsets +@cpindex Structure member offsets +@cpindex Offsets of structure member +@fnindex @code{offsetof} +@sc{ANSI}@tie{}C defines the macro @code{offsetof} +to be declared by the header file @file{<stddef.h>}. +It returns the offset, in bytes, of a direct or +indirect member in a @code{struct} or @code{union}. +The call @code{offsetof(type, member)} returns the +offset of the member @code{member} in the type +@code{type}. + +As en example, consider the structure +@example +struct example +@{ + char offset_is_0[16]; + char offset_is_16[16]; +@} +@end example +@code{offsetof(struct example, offset_is_0)} +evaluates to 0 because it is at the top of the +structure. @code{offsetof(struct example, offset_is_16)} +evaluates to 16 because the member above it in the +structure has offset 0 and size 16. 0 + 16 = 16. + +@code{offsetof} does not support bit field-@code{struct}:s. +@code{offsetof} is also known to be problematic in C++, +because C++ supports redefining operators. + |