ANSI-C: maximum number of characters printing a decimal int

Question

I'd like to know if it is an easy way of determining the maximum number of characters to print a decimal int.

I know <limits.h> contains definitions like INT_MAX that say the maximum value an int can assume, but it is not what I want.

I'd like to be able to do something like:

int get_int( void )
{
    char draft[ MAX_CHAR_OF_A_DECIMAL_INT ];

    fgets( draft, sizeof( draft ), stdin );
    return strtol( draft, NULL, 10 );
}

But how to find the value of MAX_CHAR_OF_A_DECIMAL_INT in a portable and low overheaded way?

Thanks!

Answer 1

If you assume CHAR_BIT is 8 (required on POSIX, so a safe assumption for any code targetting POSIX systems as well as any other mainstream system like Windows), a cheap safe formula is 3*sizeof(int)+2. If not, you can make it 3*sizeof(int)*CHAR_BIT/8+2, or there's a slightly simpler version.

In case you're interested in the reason this works, sizeof(int) is essentially a logarithm of INT_MAX (roughly log base 2^CHAR_BIT), and conversion between logarithms of different bases (e.g. to base 10) is just multiplication. In particular, 3 is an integer approximation/upper bound on log base 10 of 256.

The +2 is to account for a possible sign and null termination.