lecture 10 -- Unix shared memory APIs

* shmget to create or locate a chunk of memory
* shmat to attach it
* you specify how many bytes it should be, and permissions
* key should be unique across the system (pick one that other people
aren't using)
* it persists until all processes that are using it detach it, and one
of them removes it
* don't go crazy and make lots of them with huge amounts of memory, or
we'll have to reboot mercury



#include <stdio.h>
#include <sys/shm.h>

#define SHMKEY  (key_t)10398

int main () {

  int shmid;

  int *shared_int;

  if((shmid = shmget (SHMKEY, sizeof(int),
		      IPC_CREAT | 0666)) == -1){
    perror("shared get failed: ");
    exit(1);
  }

  printf("shmget is happy\n");

  printf ("created shared mem with ID = %d\n", shmid);

  if ((shared_int = (int *)
       shmat(shmid, (const void *)0, 0)) == -1) {
    perror("shared attach failed: ");
    exit(1);
  }

  printf ("attached shared mem\n");

  *shared_int = *shared_int + 1;

  printf("shared integer is %d\n", *shared_int);



}