How to convert blocking file io to non-blocking in C

Question

I am writing a code send the output of a terminal command over a socket in C. I have tried using select for asynchronous reading and avoid blocking the event-loop, but I wasn't successful.

How can I change this code to make the file stream IO non-blocking?

int maxfdpl;
fd_set rset;
char sendline[100], recvline[100], my_msg[100];
FILE *in;
char str[30]="ping 192.168.26.219";
if(!(in = popen(str, "r"))){
   return EXIT_FAILURE;
}
FD_ZERO(&rset);
FD_SET(fileno(in), &rset);
maxfdpl =fileno(in) + 1;
select(maxfdpl, &rset, NULL, NULL, NULL);
while(1) {
      if (FD_ISSET(fileno(in), &rset)) {
         if (fgets(sendline, 100, in)) {
            send_over_socket(sendline);
         }
      }
}

How can I remove the while loop (which is blocking the event-loop) and replace the code with a non-blocking IO operation?

Answer 1

int blockFD(int fd, int blocking)
{
    /* Save the current flags */
    int flags = fcntl(fd, F_GETFL, 0);
    if (flags == -1)
        return 0;

    if (blocking)
        flags &= ~O_NONBLOCK;
    else
        flags |= O_NONBLOCK;

    return fcntl(fd, F_SETFL, flags) != -1;
}

Returns 0 if failed.