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(libc.info)Width of Type

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Computing the Width of an Integer Data Type
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   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