~/tmp$ cat popeneg.c
#define _XOPEN_SOURCE 700
#include <stdio.h>

int main(void) {
    char t[10000], cmd[999];
    sprintf(cmd, "export BC_LINE_LENGTH=0 && echo '"
                     "n = 2^252 + 27742317777372353535851937790883648493;"
                     "a = 2^252 + 89238542387652589439587203958490854373;"
                     "b = 2^252 + 90734573867697548675497907437639208375;"
                     "c = 2^252 + 47653267546254725635642536544366523666;"
                     "(a*b + c) %% n"
                 "' | bc -q");
    FILE *p = popen(cmd, "r");
    while (fgets(t, 10000, p)) puts(t);
    pclose(p);
}
~/tmp$ gcc popeneg.c
~/tmp$ ./a.out
3873785929238931340063544470626197248000322972572793973933979209930525381333

~/tmp$