c函数中执行shell 命令

在c函数中执行shell 命令 并获得命令的返回值

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

int main(void)
{
int ret = -1;
FILE* pf;
/* Can not get the output of the cmd */
ret = system("ls /dev/loop* | wc -l");

printf("ret = %d\n", ret);

/* This mothed can get the output of the cmd */
pf  = popen("ls /dev/loop* | wc -l", "r");

fscanf(pf, "%d", &ret);
printf("ret = %d\n", ret);

pclose(pf);
return 0;
}