Capture stdin, stdout, stderr of child process using pipes

Question

I have to write program in C (Unix) which captures stdin, stdout and stderr of command passed in command line arguments.

For example,

./capture echo Hi
== stdout ===
Hi
== end of stdout ==
./capture bc -q
== stdin ==
1+2
== end of stdin ==
== stdout ==
3
== end of stdout ==

I've already implemented such behaviour for stdout and stderr:

int pipe_out[2];
int pipe_in[2];
int finished = 0;

void sig_handler(int signo)
{
    if (signo == SIGCHLD)
    {
        finished = 1;
        wait(NULL);
    }
}

void ChildProcess (char * argv[], char * env[])
{

    dup2(pipe_out[1], 1);
    dup2(pipe_out[1], 2);
    dup2(pipe_in[0], 0);

    close(pipe_out[0]);
    close(pipe_out[1]);

    close(pipe_in[0]);
    close(pipe_in[1]);

    int return_code = execvpe(argv[0], argv, env);
    printf("This should not appear: %d\n", return_code);
    printf("Error: %s\n\n", strerror(errno));
}   

void ParentProcess(pid_t pid)
{
    int status;
    char buf[10000];

    fd_set out_fds;
    fd_set in_fds;

    signal(SIGCHLD, sig_handler);

    do
    {
        FD_ZERO(&out_fds);
        FD_ZERO(&in_fds);
        FD_SET(pipe_in[1], &in_fds); 
        FD_SET(pipe_out[0], &out_fds);
        int cnt = select(max(pipe_out[0], pipe_out[0]) + 1, &out_fds, NULL, NULL, NULL);
        if (cnt > 0 && FD_ISSET(pipe_out[0], &out_fds))
        {
            puts("== stdout ==");
            read(pipe_out[0], buf, sizeof(buf));
            printf("%s", buf);
            puts("== end of stdout == ");
            continue;
        }
        if (0 && FD_ISSET(pipe_in[1], &in_fds))
        {
            puts("== stdin ==");
            write(pipe_in[1], "1+2\n", sizeof("1+2\n"));
            puts("== end of stdin ==");
            continue;
        }
    } while (!finished);
}

As far as I understand it works in this way: parent process call select() and wait for output of child process. After that parent process prints additional information (== stdout ==) and child's output.

When I try implement same behaviour for stdin I see that in_fds always ready independent on what child process need. So capture always reads from stdin. Even when child process need output something.

So my question is how to implement such behaviour?

I think there must be a special signal which indicates when child process needs some input, but I can't find anything.

Stdin goes the opposite direction: Your program should read and capture its own stdin, and write it to the pipe connected to the child process's stdin. — Barmar, Apr 08 '13 at 18:40
Yes, it should. But how should it find out when it needs to output some data and when it needs to read input from stdin? — vralex, Apr 08 '13 at 18:43
You can't tell when it needs input, you just keep writing to the pipe and it will consume what it needs. If you write faster than it reads, the pipe buffer might eventually fill up, and you can use `select()` on `out_fds` to detect this (it will block if you can't write safely). — Barmar, Apr 08 '13 at 18:56

Capture stdin, stdout, stderr of child process using pipes

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