chained pipes in linux using pipe()

问题:

I know how to create one pipe in Linux with C that looks like cat /tmp/txt |grep foo, but I have problems implementing multiple chained pipes like this cat /tmp/1.txt | uniq -c | sort. How to do it with C using pipe() in Linux?

UPDATE: I've figured it out. Here's the code if anybody ever have the same question:

enum PIPES {
    READ, WRITE
};


 int filedes[2];
    int filedes2[2];
    pipe(filedes);
    pipe(filedes2); 

    pid_t pid = fork();
    if (pid == 0) {
        dup2(filedes[WRITE], 1);
    char *argv[] = {"cat", "/tmp/1.txt", NULL};
        execv("/bin/cat", argv); 
        exit(0);
    } 
    else {
        close(filedes[1]);
    }

    pid = fork();
    if (pid == 0) {
    dup2(filedes[READ], 0);
    dup2(filedes2[WRITE], 1);
        char *argv[] = {"uniq", "-c", NULL};
        execv("/usr/bin/uniq", argv);       
    }
    else {
        close(filedes2[1]);
    }

    pid = fork();
    if (pid == 0) {
        dup2(filedes2[READ], 0);
        char *argv1[] = {"sort", NULL};
            execv("/usr/bin/sort", argv1);
    }

    waitpid(pid);

回答1:

A pipe has two ends (read and write) and pipe() accordingly puts two file descriptors in the array you specify. The first one is the read end, and the second one the write end.

So, in your example, you would create two pipes and:

• connect the standard output of cat to the write end of the first pipe,
• connect the standard input of uniq to the read end of the first pipe,
• connect the standard output of uniq to the write end of the second pipe,
• connect the standard input of sort to the read end of the second pipe.

回答2:

You get a pair of file descriptors - let's call them a and b. A subsequent call to pipe gives you file descriptors c and d.

So you connect b to c in order to chain those two processes. Create yet another pair of file descriptors and connect d to e, for example, and so on.