pipe system call

Question

can someone give me simple example in c, of using pipe() system call to and use ssh to connect to a remote server and execute a simple ls command and parse the reply. thanks in advance,..

Answer 1

int main()
{
    const char host[] = "foo.example.com";  // assume same username on remote
    enum { READ = 0, WRITE = 1 };
    int c, fd[2];
    FILE *childstdout;

    if (pipe(fd) == -1
     || (childstdout = fdopen(fd[READ], "r")) == NULL) {
        perror("pipe() or fdopen() failed");
        return 1;
    }
    switch (fork()) {
      case 0:  // child
        close(fd[READ]);
        if (dup2(fd[WRITE], STDOUT_FILENO) != -1)
            execlp("ssh", "ssh", host, "ls", NULL);
        _exit(1);
      case -1: // error
        perror("fork() failed");
        return 1;
    }

    close(fd[WRITE]);
    // write remote ls output to stdout;
    while ((c = getc(childstdout)) != EOF)
        putchar(c);
    if (ferror(childstdout)) {
        perror("I/O error");
        return 1;
    }
}

Note: the example doesn't parse the output from ls, since no program should ever do that. It's unreliable when filenames contain whitespace.

Answer 2

pipe(2) creates a pair of file descriptors, one for reading, the other for writing, that are connected to each other. Then you can fork(2) to split your process into two and have them talk to each other via these descriptors.

You cannot "connect" to pre-existing process using pipe(2).