Thread: fork with shared memory

  1. #1
    Registered User
    Join Date
    Nov 2011

    fork with shared memory


    I created two processes thanks fork. Now I would like to send 10 int values from the child process to the father process.

    Something like that:
    son writes int in shared memory
    father reads int from shared memory
    son writes int in shared memory
    father reads int from shared memory

    and so on.

    #include <sys/types.h>
    #include <sys/ipc.h>
    #include <sys/shm.h>
    #include <stdlib.h>
    #include <unistd.h>
    #include <stdio.h>
    int main(int argc, char **argv)
      int shmid, shmflg, permflg, res, son;
      volatile int*data;
      void *map_adr;
      permflg = (SHM_R|SHM_W) | ((SHM_R|SHM_W)>>3) | ((SHM_R|SHM_W)>>6);
      shmflg = IPC_CREAT | IPC_EXCL | permflg;
      shmid = shmget(IPC_PRIVATE, 8*1024, shmflg);
      map_adr = shmat(shmid, NULL, 0);
      data = (int *)map_adr;
      *data = 1;
      if((son = (fork() == 0))) { 
        map_adr = shmat(shmid, NULL, 0);
        while(*data != 0);
        *data = *data + 1;
         //printf("value is %d\n", *data);
        //*data = 1;
      else {
        while(*data != 10) {
        printf("value is %d\n", *data);
      res = shmdt(map_adr);
        res = shmctl(shmid, IPC_RMID, NULL);
      return 0;
    I think my problem is my while-loop because it stops after data reaches 1. I don't know how to fix it.

    Thanks for your help!

  2. #2
    spurious conceit MK27's Avatar
    Join Date
    Jul 2008
    segmentation fault

     while(*data != 0);
    is an infinite busy loop. There is no block associated with it.

    Fixing that will move you on to the next set of problems:

    1) that the two forks do not synchronously alternate; ie, the child loop could happen several times in a row while the parent loop happens only once.
    2) that as a consequence of #1, the parent while() loop condition may very likely skip ever being met, and so become endless.
    3) that the child fork is certainly endless.
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

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