sum the bits of an integer

bit_sum.c

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

unsigned long bit_sum(unsigned long n) {
  return n ? (1 + bit_sum(n & (n - 1))) : 0;
}

void main(int argc, char ** argv) {
  char * p;
  long n;
  if(argc < 2) {exit(EXIT_FAILURE);}
  n = strtol(argv[1], &p, 0); // see strotol manpage for error checking
  if(*argv[1] == '\0' || *p != '\0') {exit(EXIT_FAILURE);}
  printf("%ld\n", bit_sum(n));
  exit(EXIT_SUCCESS);
}