Non-iterative approach to finding the number of digits in an integer. Also works for both positive and negative numbers.

NumDigits.c

#include <stdio.h>
#include <math.h>
#include <stdlib.h>

int main(int argc, char** argv)
{
    int digits;

	digits = (atoi(argv[1]) == 0) ? 1 : (int)log10(abs(atoi(argv[1]))) + 1;

	printf("Number of digits in %s is %d\n", argv[1], digits);
	
	return 0;
}