带有UNIX进程和管道的“乒乓”游戏

Question

Can someone please help me fix this problem?

I wrote the complete problem below.

Ping-pong. Two processes will play the ping-pong game. The first process will generate a random number between 5000 and 15000 that will be send to the other process. This process will subtract a random value (between 50 and 1000) and will send the number back, The chat between the processes will be implemented using pipe channels. The game ends when the value is below zero. Each process will print the received value.

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <time.h>

int main ()
{

  int  p2c[2], c2p[2], number, n;

  pipe(p2c);    //parent to child pipe
  pipe(c2p);    //child to parent pipe

  number = rand() %10000 + 5000;
  printf("Generated number: %d\n", number);

  if (fork () == 0 ) 
  { //child process
      while(number > 0)
      {
         printf("in child process\n");
         read(p2c[0], &number, sizeof(int));
         printf("(F).received number: %d\n", number);
         n = rand() %1000 + 50;
         number = number - n;
         write(c2p[1], &number, sizeof(int));
       } 
      close(p2c[0]); close(p2c[1]);
      close(c2p[0]); close(c2p[1]);
      exit(0);

  } else {
    while(number > 0) 
    {
      printf("in parent process\n");
      read(c2p[0], &number, sizeof(int));
      printf("(P).received number: %d\n", number);
      number = number - n;
      write(p2c[1], &number, sizeof(int));
    }
   close(p2c[0]); close(p2c[1]);
   close(c2p[0]); close(c2p[1]);
   wait();
 }
printf ("The final number is: %d\n", number);
}

It does not work and I don't understand why. All that it prints is:

Generated number: (a random number...)
in parent process
in child process

Also I don't understand why it first goes to the parent process and no to the child process. Can someone please help me fix this?

Answer 1

First of all, there is no order to fork ed processes. Child and parent execute more-or-less simultaneously, despite the fact that child is higher in source code than parent. Here, parent just happened to slap their hand on the standard output a bit faster than child.

The main problem you are having is a deadlock. The parent is waiting for the child to say something ( read(c2p[0]...) ); the child is waiting for the parent to break the silence ( read(p2c[0], ...) ). One of them has to say something first, or they will just die of old age before they move.

Start one of them talking before the loop by telling the other the initial state. Then it should work okay later, as they alternate between talking and listening.