how to store output from "bc" into a variable?

Question

What this program is supposed to do is ask user a simple arithmetic question, e.g. 5 + 7 and then check the answer with "bc" (whether it's correct).

I have the following code, but I don't understand how to edit it to to store the result from "5 + 7" into a variable (currently the result goes into STDOUT).

Any help is welcome.

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/wait.h>

int main( int argc, char *argv[], char *env[] )
{

    char *expr = "5 + 7\n";
    int answer;

    printf("%s = ", expr);
    scanf("%d", &answer);


    int pfds[2];
    pipe(pfds);

    if (!fork()) {
        close(1);       /* close normal stdout */
        dup(pfds[1]);   /* make stdout same as pfds[1] */
        close(pfds[0]); /* we don't need this */
        printf("%s\n", expr);

        /***********************/
        /* How to store printf()'s output into a variable? */

        exit(0);
    } else {
        close(0);       /* close normal stdin */
        dup(pfds[0]);   /* make stdin same as pfds[0] */
        close(pfds[1]); /* we don't need this */
        execlp("bc", "bc", NULL);
    }


   return 0;
}

Answer 1

You need to create a second pipe and redirect stdout to it in the child process.