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| -rw-r--r-- | ChangeLog | 9 | ||||
| -rw-r--r-- | manual/lang.texi | 79 |
2 files changed, 48 insertions, 40 deletions
diff --git a/ChangeLog b/ChangeLog index 421e82afa0..9e7dd959da 100644 --- a/ChangeLog +++ b/ChangeLog @@ -1,5 +1,14 @@ 2017-08-10 Rical Jasan <ricaljasan@pacific.net> + * manual/lang.texi + (Computing the Width of an Integer Data Type): Rename section to + "Width of an Integer Type". Remove inaccurate statement regarding + lack of C language facilities for determining width of integer + types, and reorder content to improve flow and context of + discussion. + +2017-08-10 Rical Jasan <ricaljasan@pacific.net> + * lang.texi (va_copy): Change standard from ISO to C99. (__va_copy): Add standard and header annotation. Update description for clarity of origins and current use. diff --git a/manual/lang.texi b/manual/lang.texi index d8c4bd7f69..ca90a59f8f 100644 --- a/manual/lang.texi +++ b/manual/lang.texi @@ -609,48 +609,17 @@ which give you this information in full detail. @end menu @node Width of Type -@subsection Computing the Width of an Integer Data Type +@subsection Width of an Integer Type @cindex integer type width @cindex width of integer type @cindex type measurements, integer - -The most common reason that a program needs to know how many bits are in -an integer type is for using an array of @code{long int} as a bit vector. -You can access the bit at index @var{n} with - -@smallexample -vector[@var{n} / LONGBITS] & (1 << (@var{n} % LONGBITS)) -@end smallexample - -@noindent -provided you define @code{LONGBITS} as the number of bits in a -@code{long int}. - @pindex limits.h -There is no operator in the C language that can give you the number of -bits in an integer data type. But you can compute it from the macro -@code{CHAR_BIT}, defined in the header file @file{limits.h}. - -@deftypevr Macro int CHAR_BIT -@standards{C90, limits.h} -This is the number of bits in a @code{char}. POSIX.1-2001 requires -this to be 8. -You can compute the number of bits in any data type @var{type} like -this: - -@smallexample -sizeof (@var{type}) * CHAR_BIT -@end smallexample -@end deftypevr - -That expression includes padding bits as well as value and sign bits. -On all systems supported by @theglibc{}, standard integer types other -than @code{_Bool} do not have any padding bits. TS 18661-1:2014 -defines additional macros for the width of integer types (the number -of value and sign bits); these macros can also be used in @code{#if} -preprocessor directives, whereas @code{sizeof} cannot. The following -macros are defined in @file{limits.h}. +TS 18661-1:2014 defines macros for the width of integer types (the +number of value and sign bits). One benefit of these macros is they +can be used in @code{#if} preprocessor directives, whereas +@code{sizeof} cannot. The following macros are defined in +@file{limits.h}. @vtable @code @item CHAR_WIDTH @@ -665,7 +634,6 @@ macros are defined in @file{limits.h}. @itemx LLONG_WIDTH @itemx ULLONG_WIDTH @standards{ISO, limits.h} - These are the widths of the types @code{char}, @code{signed char}, @code{unsigned char}, @code{short int}, @code{unsigned short int}, @code{int}, @code{unsigned int}, @code{long int}, @code{unsigned long @@ -675,7 +643,7 @@ respectively. Further such macros are defined in @file{stdint.h}. Apart from those for types specified by width (@pxref{Integers}), the following are -defined. +defined: @vtable @code @item INTPTR_WIDTH @@ -686,12 +654,43 @@ defined. @itemx WCHAR_WIDTH @itemx WINT_WIDTH @standards{ISO, stdint.h} - These are the widths of the types @code{intptr_t}, @code{uintptr_t}, @code{ptrdiff_t}, @code{sig_atomic_t}, @code{size_t}, @code{wchar_t} and @code{wint_t}, respectively. @end vtable +A common reason that a program needs to know how many bits are in an +integer type is for using an array of @code{unsigned long int} as a +bit vector. You can access the bit at index @var{n} with: + +@smallexample +vector[@var{n} / ULONG_WIDTH] & (1UL << (@var{n} % ULONG_WIDTH)) +@end smallexample + +Before @code{ULONG_WIDTH} was a part of the C language, +@code{CHAR_BIT} was used to compute the number of bits in an integer +data type. + +@deftypevr Macro int CHAR_BIT +@standards{C90, limits.h} +This is the number of bits in a @code{char}. POSIX.1-2001 requires +this to be 8. +@end deftypevr + +The number of bits in any data type @var{type} can be computed like +this: + +@smallexample +sizeof (@var{type}) * CHAR_BIT +@end smallexample + +That expression includes padding bits as well as value and sign bits. +On all systems supported by @theglibc{}, standard integer types other +than @code{_Bool} do not have any padding bits. + +@strong{Portability Note:} One cannot actually easily compute the +number of usable bits in a portable manner. + @node Range of Type @subsection Range of an Integer Type @cindex integer type range |