popen simultaneous read and write [duplicate]

Answer 1

As already answered, popen works in one direction. If you need to read and write, You can create a pipe with pipe(), span a new process by fork() and exec functions and then redirect its input and outputs with dup2(). Anyway I prefer exec over popen, as it gives you better control over the process (e.g. you know its pid)

EDITED:

As comments suggested, a pipe can be used in one direction only. Therefore you have to create separate pipes for reading and writing. Since the example posted before was wrong, I deleted it and created a new, correct one:

#include<unistd.h>
#include<sys/wait.h>
#include<sys/prctl.h>
#include<signal.h>
#include<stdlib.h>
#include<string.h>
#include<stdio.h>

int main(int argc, char** argv)
{
  pid_t pid = 0;
  int inpipefd[2];
  int outpipefd[2];
  char buf[256];
  char msg[256];
  int status;

  pipe(inpipefd);
  pipe(outpipefd);
  pid = fork();
  if (pid == 0)
  {
    // Child
    dup2(outpipefd[0], STDIN_FILENO);
    dup2(inpipefd[1], STDOUT_FILENO);
    dup2(inpipefd[1], STDERR_FILENO);

    //ask kernel to deliver SIGTERM in case the parent dies
    prctl(PR_SET_PDEATHSIG, SIGTERM);

    //replace tee with your process
    execl("/usr/bin/tee", "tee", (char*) NULL);
    // Nothing below this line should be executed by child process. If so, 
    // it means that the execl function wasn't successfull, so lets exit:
    exit(1);
  }
  // The code below will be executed only by parent. You can write and read
  // from the child using pipefd descriptors, and you can send signals to 
  // the process using its pid by kill() function. If the child process will
  // exit unexpectedly, the parent process will obtain SIGCHLD signal that
  // can be handled (e.g. you can respawn the child process).

  //close unused pipe ends
  close(outpipefd[0]);
  close(inpipefd[1]);

  // Now, you can write to outpipefd[1] and read from inpipefd[0] :  
  while(1)
  {
    printf("Enter message to send\n");
    scanf("%s", msg);
    if(strcmp(msg, "exit") == 0) break;

    write(outpipefd[1], msg, strlen(msg));
    read(inpipefd[0], buf, 256);

    printf("Received answer: %s\n", buf);
  }

  kill(pid, SIGKILL); //send SIGKILL signal to the child process
  waitpid(pid, &status, 0);
}

Answer 2

In one of netresolve backends I'm talking to a script and therefore I need to write to its stdin and read from its stdout. The following function executes a command with stdin and stdout redirected to a pipe. You can use it and adapt it to your liking.

static bool
start_subprocess(char *const command[], int *pid, int *infd, int *outfd)
{
    int p1[2], p2[2];

    if (!pid || !infd || !outfd)
        return false;

    if (pipe(p1) == -1)
        goto err_pipe1;
    if (pipe(p2) == -1)
        goto err_pipe2;
    if ((*pid = fork()) == -1)
        goto err_fork;

    if (*pid) {
        /* Parent process. */
        *infd = p1[1];
        *outfd = p2[0];
        close(p1[0]);
        close(p2[1]);
        return true;
    } else {
        /* Child process. */
        dup2(p1[0], 0);
        dup2(p2[1], 1);
        close(p1[0]);
        close(p1[1]);
        close(p2[0]);
        close(p2[1]);
        execvp(*command, command);
        /* Error occured. */
        fprintf(stderr, "error running %s: %s", *command, strerror(errno));
        abort();
    }

err_fork:
    close(p2[1]);
    close(p2[0]);
err_pipe2:
    close(p1[1]);
    close(p1[0]);
err_pipe1:
    return false;
}

https://github.com/crossdistro/netresolve/blob/master/backends/exec.c#L46

(I used the same code in Can popen() make bidirectional pipes like pipe() + fork()?)