c - How to use `strtoul` to parse string where zero may be valid?

Question

According to the documentation for strtoul, regarding its return value...

This function returns the converted integral number as a long int value. If no valid conversion could be performed, a zero value is returned.

What if I'm parsing a user-supplied string of "0" where, for my application, "0" may be a valid entry? In that case it seems that I have no way to determine from using strtoul if a valid conversion was performed. Is there another way to handle this?

Answer 1

Read further the man page:

Since strtoul() can legitimately return 0 or ULONG_MAX (ULLONG_MAX for strtoull()) on both success and failure, the calling program should set errno to 0 before the call, and then determine if an error occurred by checking whether errno has a nonzero value after the call.

Also, to handle another scenario, where no digits were read in the input. If this happens, strtol() sets the value of *endptr to that of the nptr. So, you should also check that the pointer values compare equal or not.