父母与1个孩子之间的IPC。

Question

I'm practicing process communication between parent process and 1 child. What I want to do is that every message that the child sends the parent reads it(some sort of blocking send, parent must read the message first before child continues to send another message). I can't use pipes. I've read on Silberschatz book about blocking send, but I haven't found a good example of it(maybe mailbox too). Any help would be nice! This is a piece of code:

int main(int argc, char** argv) {
printf("This process: ");
printf("%d\n",getpid());
printf("Parent: ");
printf("%d\n",getppid());
pid_t f;
int input;
f = fork();
if (f == 0) {
    for(int i=0;i<5;i++){
        printf("Input a number: ");
        scanf("%d",&input);
        send(getppid(),input);
    }
    printf("\n");
    exit(0);
} else { 
recv(f,input);
printf("%d",input);
}
wait();
exit(0);

}

Answer 1

This is a terrible hack but works :/ You should use semaphores or some kind of signaling to the shared memory. Create the file for ftok (createthisfile.txt). This is doing intensive polling to the shared memory block and uses a "code" (int = 99) to indicate the parent that the child for loop has finished. Please note that 99 cannot be used as a number or the parent will exit prematurely. As a last note, you are studying this, then you should solve your problems by yourself, get tips and hints but do not let others do the work for you :/ otherwise you will major in stackoverflow, not on computer science :) I did it as a quick exercise but regret to post it :/

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

int main(int argc, char** argv) {
   key_t k;
   pid_t f;
   int shmid;
   int input;

   printf("This process: ");
   printf("%d\n",getpid());
   printf("Parent: ");
   printf("%d\n",getppid());

   k = ftok ("createthisfile.txt", 'R');
   shmid = shmget(k, sizeof(int), IPC_CREAT|SHM_R|SHM_W);
   char *addr = shmat(shmid,0,0);

   f = fork();

   if (f == -1) {
      exit(1);
   }

   if (f == 0) {
      for(int i=0;i<5;i++){
         printf("Input a number: ");
         scanf("%d",&input);
         memcpy(addr, &input, sizeof(int));
      }
      input = 99;
      memcpy(addr, &input, sizeof(int));
      printf("\n");
      exit(0);
   } else { 
      while (1) {
         memcpy(&input, addr, sizeof(int));
         if (input != 0) { 
            if (input == 99) {
               exit (0);
            }
            printf("[Parent reads: %d]\n",input);
            input = 0;
            memcpy(addr, &input, sizeof(int));
         }
      }
   }

   exit(0);
}