子进程如何从管道读取stdout，而父进程如何将stdin写入管道？

Question

// This code is pasted from
// http://linux.die.net/man/2/pipe

#include <sys/wait.h>
#include <unistd.h>
int main(int argc, char *argv[])
{
    int pipefd[2];
    pid_t cpid;
    char buf;
    if (argc != 2) {
    fprintf(stderr, "Usage: %s <string>\n", argv[0]);
    exit(EXIT_FAILURE);
    }
    if (pipe(pipefd) == -1) {
        perror("pipe");
        exit(EXIT_FAILURE);
    }
    cpid = fork();
    if (cpid == -1) {
        perror("fork");
        exit(EXIT_FAILURE);
    }
    if (cpid == 0) {    /* Child reads from pipe */                   <----- Point A
        close(pipefd[1]);          /* Close unused write end */
        while (read(pipefd[0], &buf, 1) > 0)
            write(STDOUT_FILENO, &buf, 1);
        write(STDOUT_FILENO, "\n", 1);
        close(pipefd[0]);
        _exit(EXIT_SUCCESS);
    } else {            /* Parent writes argv[1] to pipe */           <----- Point B
        close(pipefd[0]);          /* Close unused read end */
        write(pipefd[1], argv[1], strlen(argv[1]));
        close(pipefd[1]);          /* Reader will see EOF */
        wait(NULL);                /* Wait for child */
        exit(EXIT_SUCCESS);
    }
}

As what I understood,

if (...)
  ............;   ---+
else                 |---> " Only ONE of them can be reached! "
  ............;   ---+

So, how can the child process read from the pipe AND the parent process write to the pipe in this code?

Answer 1

The result of fork() is that one process becomes two (by asexual reproduction). So while it is still the case that exactly one branch of the if/else block will be taken in a process, there are two processes, and one path will be taken by each.

More specifically, look at what fork() returns: a PID to the parent, and 0 to the new child. Apart from that the two processes are almost identical. So the if (cpid == 0) check is a common pattern after fork() so that you can proceed with distinct logic in each process. In your case, that's reading in one process and writing in the other.

Answer 2

The system call fork() returns twice. Both in the parent process and the child process. The moment you call fork() , two exact copies of your program are running. The SINGLE difference is the return value of fork() .

So your "if else only one" rule is still valid when you consider each process in isolation.

Answer 3

Check this resource for a description of the fork call return value:

On success, the PID of the child process is returned in the parent, and 0 is returned in the child. On failure, -1 is returned in the parent, no child process is created, and errno is set appropriately.

So the line that contains cpid = fork(); is executed by both process after the fork, where the parent receives the new process' PID and the child receives 0 as PID. Hence the distinction between parent and child.