Counting bits with a compiler builtin

popcount.c

#include <stdlib.h>
#include <stdio.h>
int main(int argc, char **argv) {
	unsigned word;
	int count;
	/* check for argument */
	if(argc < 2) { printf("Usage: %s INTEGER\n", argv[0]); abort(); }
	/* convert argument */
	word = (unsigned)atoi(argv[1]);
	/* count number of 1-bits */
	count = __builtin_popcount(word);
	/* print result */
	printf("1-bits: %d\n", count);
	/* this never fails */
	return 0;
}