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I'd go for a non standard function, because C sucks with bit operations:

#include <stdio.h>
#ifdef __GNUC__
int popcount(int x) {
 return __builtin_popcount(x);
}
#else
#error Unimplemented popcount
#endif
int main(void)
{
 int x;
 scanf("%d", &x);
 printf("%d\n", popcount(x));
 return 0;
}

This will be translated to efficient processor instructions where available, or an efficient library implementation where it's not.

References: http://en.wikipedia.org/wiki/Hamming_weight

http://stackoverflow.com/questions/15736602/fastest-way-to-count-number-of-1s-in-a-register-arm-assembly

http://wm.ite.pl/articles/sse-popcount.html

http://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/Other-Builtins.html#Other-Builtins

I'd go for a non standard function, because C sucks with bit operations:

#include <stdio.h>
#ifdef __GNUC__
int popcount(int x) {
 return __builtin_popcount(x);
}
#else
#error Unimplemented popcount
#endif
int main(void)
{
 int x;
 scanf("%d", &x);
 printf("%d\n", popcount(x));
 return 0;
}

This will be translated to efficient processor instructions where available, or an efficient library implementation where it's not.

References: http://en.wikipedia.org/wiki/Hamming_weight

https://stackoverflow.com/questions/15736602/fastest-way-to-count-number-of-1s-in-a-register-arm-assembly

http://wm.ite.pl/articles/sse-popcount.html

http://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/Other-Builtins.html#Other-Builtins