how to Execute external program from c/c++

Execute external program from c/c++ 

#include <stdio.h>

void main(void) {

    FILE *in;

    extern FILE *popen();

    char buff[512];

    if(!(in = popen("ls -sail", "r"))){

        exit(0);

    }

    while(fgets(buff, sizeof(buff), in)!=NULL){

        printf("%s", buff);

    }

    pclose(in);

}