popen simultaneous read and write [duplicate]

Question

Is it possible to read and write to a file descriptor returned by popen. I have an interactive process I'd like to control through C. If this isn't possible with popen, is there any way around it?

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); }