C - pipe without using popen

Question

how can I transform this:

FILE *f;
char in_buffer[80];
f=popen("command","r");
fgets(in_buffer,sizeof(in_buffer),f)

without using popen(), but only pipe() or other instruction?

Answer 1

Here's my simple implementation, with comments explaining what's being done.

#include <unistd.h>
#include <stdio.h>

FILE *
my_popen (const char *cmd)
{
    int fd[2];
    int read_fd, write_fd;
    int pid;               

    /* First, create a pipe and a pair of file descriptors for its both ends */
    pipe(fd);
    read_fd = fd[0];
    write_fd = fd[1];

    /* Now fork in order to create process from we'll read from */
    pid = fork();
    if (pid == 0) {
        /* Child process */

        /* Close "read" endpoint - child will only use write end */
        close(read_fd);

        /* Now "bind" fd 1 (standard output) to our "write" end of pipe */
        dup2(write_fd,1);

        /* Close original descriptor we got from pipe() */
        close(write_fd);

        /* Execute command via shell - this will replace current process */
        execl("/bin/sh", "sh", "-c", cmd, NULL);

        /* Don't let compiler be angry with us */
        return NULL;
    } else {
        /* Parent */

        /* Close "write" end, not needed in this process */
        close(write_fd);

        /* Parent process is simpler - just create FILE* from file descriptor,
           for compatibility with popen() */
        return fdopen(read_fd, "r");
    }
}

int main ()
{
    FILE *p = my_popen ("ls -l");
    char buffer[1024];
    while (fgets(buffer, 1024, p)) {
        printf (" => %s", buffer);
    }
    fclose(p);
}

Notes:

1. Thir code supports only "r" mode of popen. Implementing other modes, namely "w" mode is left as an exercise for the reader.
2. System functions used in this example may fail - error handling is left as an exercise for the reader.
3. Implementation of pclose is left as an exercise for the reader - see close, waiptid, and fclose.

If you want to look at real impementations, you can look into sources of OSX, GNU glibc and OpenSolaris, among others.

Hope this helps!