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A.5.1 Computing the Width of an Integer Data Type

The most common reason that a program needs to know how many bits are in an integer type is for using an array of long int as a bit vector. You can access the bit at index n with

vector[n / LONGBITS] & (1 << (n % LONGBITS))

provided you define LONGBITS as the number of bits in a long int.

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 CHAR_BIT, defined in the header file limits.h.

CHAR_BIT

This is the number of bits in a char—eight, on most systems. The value has type int.

You can compute the number of bits in any data type type like this:

sizeof (type) * CHAR_BIT

That expression includes padding bits as well as value and sign bits. On all systems supported by the GNU C Library, standard integer types other than _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 #if preprocessor directives, whereas sizeof cannot. The following macros are defined in limits.h.

CHAR_WIDTH
SCHAR_WIDTH
UCHAR_WIDTH
SHRT_WIDTH
USHRT_WIDTH
INT_WIDTH
UINT_WIDTH
LONG_WIDTH
ULONG_WIDTH
LLONG_WIDTH
ULLONG_WIDTH

These are the widths of the types char, signed char, unsigned char, short int, unsigned short int, int, unsigned int, long int, unsigned long int, long long int and unsigned long long int, respectively.

Further such macros are defined in stdint.h. Apart from those for types specified by width (see Integers), the following are defined.

INTPTR_WIDTH
UINTPTR_WIDTH
PTRDIFF_WIDTH
SIG_ATOMIC_WIDTH
SIZE_WIDTH
WCHAR_WIDTH
WINT_WIDTH

These are the widths of the types intptr_t, uintptr_t, ptrdiff_t, sig_atomic_t, size_t, wchar_t and wint_t, respectively.


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