fork后，父子进程是否共享pipe创建的文件描述符？

Question

int main()
{
    int data_processed;
    int file_pipes[2];
    const char some_data[] = "123";
    char buffer[BUFSIZ + 1];
    pid_t fork_result;

    memset(buffer, '\0', sizeof(buffer));

    if (pipe(file_pipes) == 0) {
        fork_result = fork();
        if (fork_result == -1) {
            fprintf(stderr, "Fork failure");
            exit(EXIT_FAILURE);
        }

// We've made sure the fork worked, so if fork_result equals zero, we're in the child process.

        if (fork_result == 0) {
            data_processed = read(file_pipes[0], buffer, BUFSIZ);
            printf("Read %d bytes: %s\n", data_processed, buffer);
            exit(EXIT_SUCCESS);
        }

// Otherwise, we must be the parent process.

        else {
            data_processed = write(file_pipes[1], some_data,
                                   strlen(some_data));
            printf("Wrote %d bytes\n", data_processed);
        }
    }
    exit(EXIT_SUCCESS);
}

Based on my understanding, the child process created by fork doesn't share variables with its parent process. Then, why here the parent can write to one file descriptor and child process can get the data by reading from another file descriptor. Is this because they are controled somehow by the pipe function internally?

Answer 1

File descriptors, including pipes, are duplicated on fork -- the child process ends up with the same file descriptor table, including stdin/out/err and the pipes, as the parent had immediately before the fork.

Based on my understanding, the child process created by fork doesn't share variables with its parent process.

This isn't entirely true -- changes to variables are not shared with the parent, but the values that the parent had immediately prior to the fork are all visible to the child afterwards.

In any case, pipes exist within the operating system, not within the process. As such, data written to one end of the pipe becomes visible to any other process holding a FD for the other end. (If more than one process tries to read the data, the first process to try to read() data gets it, and any other processes miss out.)

Answer 2

The variables are not shared eg if you write file_pipes[0] = 999 in the child, it will not be reflected in the parent. The file descriptors are shared (FD number x in the child refers to the same thing as FD number x in the parent). This is why (for example) you can redirect the output of a shell script which executes other commands (because they share the same standard output file descriptor).

Answer 3

You're right - ordinary variables aren't shared between the parent and the child.

However, pipes are not variables. They're a pseudo-file specifically designed to connect two independent processes together. When you write to a pipe, you're not changing a variable in the current process - you're sending data off to the operating system and asking it to make that data available to the next process to read from the pipe.

It's just like when you write to a real, on-disk file - except that the data isn't written to disk, it's just made available at the other end of the pipe.