c - 使用管道在进程之间交换信息

Question

所以我试图让程序将信息从一个进程传递到另一个进程。它应该从 child 开始并减去 5 然后转到 parent 并除以 5。我似乎遇到的问题是输出出现在 parent 具有正确值但 child 没有正确值的地方并且它来了以某种奇怪的顺序出来。

预期输出:

x = 19530
迭代 1
child :x = 19525
父级:x = 3905
迭代 2
child :x = 3900
父级:x = 780
迭代 3
child :x = 775
parent :x = 155
迭代 4
child :x = 150
parent :x = 30
迭代 5
child :x = 25
父级:x = 5

这是我的代码:

#include <errno.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <string.h>
#include <stdlib.h>

int main()
{
   int x = 19530;
   pid_t childpid;
   int child[2], parent[2];
   int i, j;

   if (pipe(parent) == -1 || pipe(child) == -1)
   {
       perror("failed to pipe");
       exit(-1);
   }

   if ((childpid = fork()) <= -1)
   {
       perror("failed to fork");
       exit(-1);
   }

   if (childpid == 0)
   {
        //close(parent[1]);
        //close(child[0]);
       for (j = 0; j < 5; j++)
       {
          x -= 5;

          if (write(child[1], &x, sizeof(x)) <= 0)
          {
            perror("Child: failed to write to the pipe");
            exit(-1);
          }
          printf("Child : x = %d\n", x);
        }
        //close(parent[0]);
        //close(child[1]);

        exit(-1);
    }

    else
    {
        //close(parent[0]);
        //close(child[1]);
        for (i = 0; i < 5; i++)
        {
            sleep(1);
            x /= 5;
            x -= 1;
            if (write(parent[1], &x, sizeof(x)) <= 0)
            {
                 perror("parent: failed");
                 exit(-1);
            }
            printf("Parent : x = %d\n", x);
         }
         //close(parent[1]);
         //close(child[0]);

         exit(-1);
     }

     return 0;

我得到的输出: child :x = 19525
child :x = 19520
child :x = 19515
child :x = 19510
child :x = 19505
父级:x = 3905
父级:x = 780
parent :x = 155
parent :x = 30
父级:x = 5

不太确定我哪里出错了。 parent 给出了正确的值(value)，但 child 却没有。并且不确定如何让它们以正确的顺序打印。

Answer 1

您需要使用read 从其他进程获取更新后的值。 以下是每次迭代中发生的情况:

 Child:
   1. read x from parent pipe
   2. x = x - 5
   3. write x to child pipe
 Parent:
   1. write x to the parent pipe
   2. read x from child pipe
   3. x = x / 5

下面的代码提供了正确的输出:

 #include <errno.h>
 #include <stdio.h>
 #include <unistd.h>
 #include <sys/types.h>
 #include <string.h>
 #include <stdlib.h>

 int main() {
   int x = 19530;
   pid_t childpid;
   int child[2], parent[2];
   int i, j;

   if (pipe(parent) == -1 || pipe(child) == -1) {
     perror("failed to pipe");
     exit(-1);
   }

   if ((childpid = fork()) <= -1) {
     perror("failed to fork");
     exit(-1);
   }
   if (childpid == 0){  // child
     close(parent[1]);
     close(child[0]);
   }else {             // parent
     close(parent[0]);
     close(child[1]); 
   }
   for (int i = 0; i < 5; i ++){
      if (childpid == 0) {  // child
        sleep(1);
        printf("ITERATION %d\n", i+1);
        // 1. read 
        if (read(parent[0], &x, sizeof(x)) <= 0) {
          perror("Child: failed to read from the pipe");
          exit(-1);
        }
        // 2. subtract 
        x -= 5;

        // 3. write
        if (write(child[1], &x, sizeof(x)) <= 0) {
          perror("Child: failed to write to the pipe");
          exit(-1);
        }
        printf("Child : x = %d\n", x);
      } else {
        // 1. write 
        if (write(parent[1], &x, sizeof(x)) <= 0) {
          perror("parent: failed");
          exit(-1);
        }
        // 2. read
        if (read(child[0], &x, sizeof(x)) <= 0) {
          perror("Parent: failed to read from the pipe");
          exit(-1);
        }
        // 3. divide
        x /= 5;
        printf("Parent : x = %d\n", x);
      }
    }
   return 0;
 }