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The GNU C Library

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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


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