popen simultaneous read and write [duplicate]

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);
}