Can popen() make bidirectional pipes like pipe() + fork()?

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I'm implementing piping on a simulated file system in C++ (with mostly C). It needs to run commands in the host shell but perform the piping itself on the simulated file system.

I could achieve this with the pipe(), fork(), and system() system calls, but I'd prefer to use popen() (which handles creating a pipe, forking a process, and passing a command to the shell). This may not be possible because (I think) I need to be able to write from the parent process of the pipe, read on the child process end, write the output back from the child, and finally read that output from the parent. The man page for popen() on my system says a bidirectional pipe is possible, but my code needs to run on a system with an older version supporting only unidirectional pipes.

With the separate calls above, I can open/close pipes to achieve this. Is that possible with popen()?

For a trivial example, to run ls -l | grep .txt | grep cmds I need to:

  • Open a pipe and process to run ls -l on the host; read its output back
  • Pipe the output of ls -l back to my simulator
  • Open a pipe and process to run grep .txt on the host on the piped output of ls -l
  • Pipe the output of this back to the simulator (stuck here)
  • Open a pipe and process to run grep cmds on the host on the piped output of grep .txt
  • Pipe the output of this back to the simulator and print it

man popen

From Mac OS X:

The popen() function 'opens' a process by creating a bidirectional pipe, forking, and invoking the shell. Any streams opened by previous popen() calls in the parent process are closed in the new child process. Historically, popen() was implemented with a unidirectional pipe; hence, many implementations of popen() only allow the mode argument to specify reading or writing, not both. Because popen() is now implemented using a bidirectional pipe, the mode argument may request a bidirectional data flow. The mode argument is a pointer to a null-terminated string which must be 'r' for reading, 'w' for writing, or 'r+' for reading and writing.

dup2 popen pipe fork c
6 Answers

Here's the code (C++, but can be easily converted to C):

#include <unistd.h>

#include <cstdlib>
#include <cstdio>
#include <cstring>
#include <utility>

// Like popen(), but returns two FILE*: child's stdin and stdout, respectively.
std::pair<FILE *, FILE *> popen2(const char *__command)
{
    // pipes[0]: parent writes, child reads (child's stdin)
    // pipes[1]: child writes, parent reads (child's stdout)
    int pipes[2][2];

    pipe(pipes[0]);
    pipe(pipes[1]);

    if (fork() > 0)
    {
        // parent
        close(pipes[0][0]);
        close(pipes[1][1]);

        return {fdopen(pipes[0][1], "w"), fdopen(pipes[1][0], "r")};
    }
    else
    {
        // child
        close(pipes[0][1]);
        close(pipes[1][0]);

        dup2(pipes[0][0], STDIN_FILENO);
        dup2(pipes[1][1], STDOUT_FILENO);

        execl("/bin/sh", "/bin/sh", "-c", __command, NULL);

        exit(1);
    }
}

Usage:

int main()
{
    auto [p_stdin, p_stdout] = popen2("cat -n");

    if (p_stdin == NULL || p_stdout == NULL)
    {
        printf("popen2() failed\n");
        return 1;
    }

    const char msg[] = "Hello there!";
    char buf[32];

    printf("I say \"%s\"\n", msg);

    fwrite(msg, 1, sizeof(msg), p_stdin);
    fclose(p_stdin);

    fread(buf, 1, sizeof(buf), p_stdout);
    fclose(p_stdout);

    printf("child says \"%s\"\n", buf);

    return 0;
}

Possible Output:

I say "Hello there!"
child says "     1      Hello there!"
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