/* sysV IPC shared memory - read from shared memory
   shrmem2_sysV.c
    
    meant to be used with shrmem1_sysV:
      start shrmem2_sysV in background,
      then start shrmem1_sysV
    */
    #include <unistd.h>
    #include <stdlib.h>
    #include <stdio.h>
    #include <string.h>
    #include <sys/types.h>
    #include <sys/ipc.h>
    #include <sys/shm.h>
    #define MEM_SZ 4096
    
    struct shared_use_st {
      int writ_by_you;
      char some_text[BUFSIZ];
    };
    
    int main() {
      int run = 1;
      void *shared_mem = (void *) 0;
      struct shared_use_st *shared_stuff;
      int shmid;
    
      shmid = shmget( (key_t)1234, MEM_SZ, 0666 | IPC_CREAT);
      if (shmid == -1) {
        perror("shmget in shrmem2_sysV failed");
        exit(EXIT_FAILURE);
      }
      
      shared_mem = shmat(shmid, (void *)0, 0);
      if (shared_mem == (void *)-1) {
        perror("shmat in shrmem2_sysV failed");
        exit(EXIT_FAILURE);
      }
      printf("memory attached at %X\n", (int)shared_mem);
      shared_stuff = (struct shared_use_st *)shared_mem;
      
      shared_stuff->writ_by_you == 0;
      while (run) {
        if (shared_stuff->writ_by_you == 1) {
          printf("You_wrote: %s", shared_stuff->some_text);
          sleep(rand() % 4);
          shared_stuff->writ_by_you = 0;
          if (strncmp(shared_stuff->some_text, "end", 3) == 0) {
            run = 0;
          }
        }
      }
      
      if (shmdt(shared_mem) == -1) {
        perror("shmdt in shrmem2_sysV failed");
        exit(EXIT_FAILURE);
      }
      
      if (shmctl(shmid, IPC_RMID, 0) == -1) {
        perror("shmctl in shrmem2_sysV failed");
        exit(EXIT_FAILURE);
      }
        
      exit(EXIT_SUCCESS);
    }