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manual: Rewrite the section on widths of integer types.

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The manual contradicted itself by saying the number of bits in an
integer type needed to be computed, and then listing a number of
macros that later standards provided for exactly that.  The entire
section has been reworked to provide those macros first, while
preserving the documentation of CHAR_BIT and the associated examples
within that context.

	* 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.
This commit is contained in:
Rical Jasan 2017-07-27 05:04:48 -07:00
parent 4fcdfbfcf9
commit 925733a913
2 changed files with 48 additions and 40 deletions
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@ -1,3 +1,12 @@
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.

79
manual/lang.texi
View File

@ -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