Linux C中子与父之间的管道示例

Question

这看起来很简单，一个管道，父级将在管道的写入端写入管道，子级将在while循环中从管道读取，并打印读取的内容，但我的程序未能停止...

#include "kernel/types.h"
#include "kernel/stat.h"
#include "user/user.h"

int
main(int argc, char *argv[]){
  int fd[2];
  int number;
  int write_number[1];
  pipe(fd);
  int pid;
  if ((pid = fork()) >0){
     close(fd[0]);
     for (int i = 2; i < 36; i++){
         *write_number = i;
         write(fd[1], write_number, 4);
     }
     printf("write finished from parent process:%d\n", pid);
     close(fd[1]);
  }else{// child process
     close(fd[1]);
     while(read(fd[0], &number, 4))
        printf("The number is %d from child %d\n", number, getpid());
     close(fd[0]);
  }
  exit(0);
}

顺便说一句，这是来自 xv6 类 Unix 系统，但我认为这与在 Linux 中几乎相同，输出：

$ primes
The numbwerite finished from parent prorc eis ss:24
 from $ child 4
The number is 3 from child 4
The number is 4 from child 4
The number is 5 from child 4
The number is 6 from child 4
The number is 7 from child 4
The number is 8 from child 4
The number is 9 from child 4
The number is 10 from child 4
The number is 11 from child 4
The number is 12 from child 4
The number is 13 from child 4
The number is 14 from child 4
The number is 15 from child 4
The number is 16 from child 4
The number is 17 from child 4
The number is 18 from child 4
The number is 19 from child 4
The number is 20 from child 4
The number is 21 from child 4
The number is 22 from child 4
The number is 23 from child 4
The number is 24 from child 4
The number is 25 from child 4
The number is 26 from child 4
The number is 27 from child 4
The number is 28 from child 4
The number is 29 from child 4
The number is 30 from child 4
The number is 31 from child 4
The number is 32 from child 4
The number is 33 from child 4
The number is 34 from child 4
The number is 35 from child 4

它没有停止，所以它挂起，但是我已经关闭了我应该关闭的文件描述符，所以如果没有读取任何内容，read 系统调用应该返回 0，为什么它仍然挂起，所以任何人都可以帮我一把？谢谢！

Answer 1

它确实停止了——在孩子打印“孩子 4 的数字是 3”之前，父母退出并在混乱的输出中从 shell 产生提示：

$ primes
The numbwerite finished from parent prorc eis ss:24
 from $ child 4
The number is 3 from child 4

您可能应该使用wait() 或waitpid() 函数让父进程在循环中等待，以便子进程在父进程退出并且shell 重新提示之前完成。

#include <sys/wait.h>

…

int status;
int corpse;
while ((corpse = wait(&status)) > 0)
    print("Child %d died with exit status 0x%.4X\n", corpse, status);

虽然不太可能，但进程可能会产生当前程序没有派生的子进程，并且循环也可以在这种情况下正常工作。

切线注释：

1. 由于忽略命令行参数，最好使用int main(void)。

2. exit(0); 不是必需的。您可以省略它，如果您在 C99 或更高版本的模式下编译，该过程将以零退出。这只是main() 函数的特例规则。就个人而言，我更喜欢在main() 的末尾使用显式的return 0;。

3. 你有：

    for (int i = 2; i < 36; i++){
        *write_number = i;
        write(fd[1], write_number, 4);
    }
   

你可以使用：

    for (int i = 2; i < 36; i++)
        write(fd[1], &i, sizeof(i));

使用sizeof() 是惯用的；使用4 容易出错。并且不需要write_number 数组。

4. 标题的选择是常规的。您应该使用<stdio.h>、<unistd.h> 和<sys/wait.h>。你实际上并不需要<sys/types.h>。内核头文件只应在编译代码以在内核中运行时使用——该程序不是内核代码。

---

你说：

我添加了一些类似printf("child process is finished")的代码，在child条件下，它打印，但是exit(0)之前没有wait(0)，这意味着子进程没有问题，它会等待从管道关闭，但最后程序没有完成，只是挂在那里似乎在等待输入，我认为这是父母退出但不等待孩子退出（）的情况，但孩子确实在结束，但他的程序没有完成....最后的混乱

IMO，您的代码应如下所示：

#include <stdio.h>
#include <sys/wait.h>
#include <unistd.h>

int main(void)
{
    int fd[2];
    pipe(fd);
    int pid;
    if ((pid = fork()) > 0)
    {
        close(fd[0]);
        for (int i = 2; i < 36; i++)
            write(fd[1], &i, sizeof(i));
        close(fd[1]);
        printf("write finished from parent process:%d\n", pid);
        int status;
        int corpse;
        while ((corpse = wait(&status)) > 0)
        {
            printf("%d: child %d exited with status 0x%.4X\n",
                   (int)getpid(), corpse, status);
        }
    }
    else if (pid < 0)
    {
        fprintf(stderr, "failed to fork\n");
        return 1;
    }
    else
    {
        int number;
        close(fd[1]);
        while (read(fd[0], &number, sizeof(number)) > 0)
            printf("The number is %d from child %d\n", number, getpid());
        close(fd[0]);
    }
    return 0;
}

我得到的一个示例输出（在运行 macOS Big Sur 11.2.3 的 MacBook Pro 上运行）是：

write finished from parent process:53072
The number is 2 from child 53072
The number is 3 from child 53072
The number is 4 from child 53072
The number is 5 from child 53072
The number is 6 from child 53072
The number is 7 from child 53072
The number is 8 from child 53072
The number is 9 from child 53072
The number is 10 from child 53072
The number is 11 from child 53072
The number is 12 from child 53072
The number is 13 from child 53072
The number is 14 from child 53072
The number is 15 from child 53072
The number is 16 from child 53072
The number is 17 from child 53072
The number is 18 from child 53072
The number is 19 from child 53072
The number is 20 from child 53072
The number is 21 from child 53072
The number is 22 from child 53072
The number is 23 from child 53072
The number is 24 from child 53072
The number is 25 from child 53072
The number is 26 from child 53072
The number is 27 from child 53072
The number is 28 from child 53072
The number is 29 from child 53072
The number is 30 from child 53072
The number is 31 from child 53072
The number is 32 from child 53072
The number is 33 from child 53072
The number is 34 from child 53072
The number is 35 from child 53072
53069: child 53072 exited with status 0x0000

如果输出通过管道传输到另一个程序，“write finished from parent process:53072”行会出现在“退出”打印之前的末尾。