Question

according documentation:

On success, the function returns the converted integral number as a
long int value. If no valid conversion could be performed, a zero
value is returned. If the correct value is out of the range of
representable values, LONG_MAX or LONG_MIN is returned, and the global
variable errno is set to ERANGE.

Consider strtol(str, (char**)NULL, 10); if str is "0\0" how to know if the function failed or only has converted the string with "0" number?

Answer 1

You need to pass a real pointer address if you want error checking, so you can distinguish 0 values arising from "0" and similar from 0 values arising from "pqr":

char *endptr;
errno = 0;
long result = strtol(str, &endptr, 10);
if (endptr == str)
{
    // nothing parsed from the string, handle errors or exit
}
if ((result == LONG_MAX || result == LONG_MIN) && errno == ERANGE)
{
    // out of range, handle or exit
}
// all went fine, go on