管道在C UNIX shell中

Question

I am not very sure how to create a pipe between two child processes. This is the way I did it:

pipe(&fd[0]);                               //Create a pipe
proc1 = fork();

//Child process 1
if (proc1 ==  0)
{
    close(fd[0]);                           //process1 doenst need to read from pipe
    close(STD_INPUT);                       //prepare for output
    dup(fd[1]);                             //Standard output = fd[1]
    close(fd[1]);
    execvp(parameter[0], parameter);        //Execute the process
}

else
{
     proc2 = fork();
    if (proc2 == 0)
    {
        close(fd[1]);
        close(STD_OUTPUT);
        dup(fd[0]);
        close(fd[0]);
        execvp(parameter2[0], parameter2);
    }
        //Parent process
    else
    {
    waitpid(-1, &status, 0);            //Wait for the child to be done
    }
}

I am trying to redirect the output of one child process to another, but I think I am making a mistake with the pipe in the second child process because when I run the program with the 2nd child process I get incorrect results (like a simple execution like "ls" is executed improperly), however, if I remove the second child process, the program runs fine (not including pipe, just simple fork: ls, ls -l, ps, etc.).

Answer 1

The best answer I can give is Beej's Guide to Unix IPC .

(Peek at section 4.3, where he gives a very similar example to the question you asked...)

Answer 2

In the child process 1 setup, close(STD_INPUT); dup(fd[1]); close(STD_INPUT); dup(fd[1]); will duplicate fd[1] to the lowest available descriptor, which is 0 ( STD_INPUT ) rather than 1 ( STD_OUTPUT ). Don't you want to close STD_OUTPUT instead? Also, I'd recommend dup2(fd[1], STD_OUTPUT) in place of this sequence; it will do what you want and is more clear anyway.

Likewise child process 2 uses STD_OUTPUT where you probably want STD_INPUT .