Open a cmd program with full functionality (i/o)

问题:

I tried popen() and it is working well for output with "r" passed as a second argument; I know you can use "w" as writing mode and it worked for me (the program was just one scanf()). My question is how to use the append ("a") mode. You can both write and read, how do you know when the program is outputting something and when it's requesting for user input?

回答1:

popen uses a pipe (that's the "p" in "popen") and pipes are unidirectional. You can either read or write from one end of a pipe, not both. To get both read/write access you should use a socketpair instead. I use this in my programs when I want something like popen, but for read/write:

    #include <sys/types.h>
    #include <sys/socket.h>
    #include <stdio.h>
    #include <unistd.h>

    FILE *sopen(const char *program)
    {
        int fds[2];
        pid_t pid;

        if (socketpair(AF_UNIX, SOCK_STREAM, 0, fds) < 0)
            return NULL;

        switch(pid=vfork()) {
        case -1:    /* Error */
            close(fds[0]);
            close(fds[1]);
            return NULL;
        case 0:     /* child */
            close(fds[0]);
            dup2(fds[1], 0);
            dup2(fds[1], 1);
            close(fds[1]);
            execl("/bin/sh", "sh", "-c", program, NULL);
            _exit(127);
        }
        /* parent */
        close(fds[1]);
        return fdopen(fds[0], "r+");
    }

Note that since it doesn't return the child's pid, you'll have a a zombie process after the child program exits. (Unless you set up SIGCHLD...)