Releasing Shared Memory

Question

So the program below accesses a piece of shared memory at location "shmid" and attaches it to pointer "total." A child process is then created that accesses and modifies this piece of memory and then once complete, another child process is created and does the same, and then a 3rd child. Once these 3 children have finished modifying the value and finished executing, the parent process releases the shared memory and then the program exits. I was under the impression that this "shmctl" block would have the desired effect, but it doesn't seem to.

if ((shmctl (shmid, IPC_RMID, (struct shmid_ds *) 0)) == -1)
{
  perror ("shmctl");
 exit (-1);
}

I don't believe it does, because following this block I have the following which during execution printed out the same value as before. Doesn't this indicate that the pointer is still accessing the same bit of memory, or am I mistaken?

printf("value after memory release:%d\n", total->value);

Thank you for your input!

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

/* change the key number */
#define SHMKEY ((key_t) 5600) //7890

typedef struct
{
  int value;
} shared_mem;

shared_mem *total;

/*----------------------------------------------------------------------*
 * This function increases the value of shared variable "total"
 *  by one all the way to 100000
 *----------------------------------------------------------------------*/

void process1 ()
{
  int k = 0;

  while (k < 100000)
  {
      k++;
      total->value = total->value + 1;
  }
  printf ("From process1 total = %d\n", total->value); //process 1 prints out total and  returns to main() function
}

/*----------------------------------------------------------------------*
 * This function increases the vlaue of shared memory variable "total"
 *  by one all the way to 170000
 *----------------------------------------------------------------------*/

void process2 ()
{
  int k = 0;

  while (k < 170000)
  {
  k++;
  total->value = total->value + 1;
  }
  printf ("From process2 total = %d\n", total->value); //process 2 prints out total and returns to main() function
}
/*----------------------------------------------------------------------*
 * This function increases the vlaue of shared memory variable "total"
 *  by one all the way to 200000
 *----------------------------------------------------------------------*/

void process3 ()
{
  int k = 0;

  while (k < 200000)
  {
    k++;
    total->value = total->value + 1;
  }
  printf ("From process3 total = %d\n", total->value); //process 3 prints out total and returns to main() function
}

/*----------------------------------------------------------------------*
 * MAIN()
 *----------------------------------------------------------------------*/

int main()
{
  int shmid;
  int pid1;
  int pid2;
  int pid3;
  int ID;
  int status;

  char *shmadd;
  shmadd = (char *) 0;

/* Create and connect to a shared memory segmentt*/

if ((shmid = shmget (SHMKEY, sizeof(int), IPC_CREAT | 0666)) < 0)
{
  perror ("shmget");
  exit (1);
}


if ((total = (shared_mem *) shmat (shmid, shmadd, 0)) == (shared_mem *) -1)
{
  perror ("shmat");
  exit (0);
}

total->value = 0;

if ((pid1 = fork()) == 0) //first child created
{
  process1(); //first child process begins
}

while((ID = wait(&status)) != -1) //parent loops until 1st child is finished
printf("child %d is finished\n", ID); //parent prints out returned value after child is finished


if ((pid1 != 0) && ((pid2 = fork()) == 0)) //second child created
{
  process2(); //second child process begins
}   

while((ID = wait(&status)) != -1) //parent loops until 2nd child is finished
printf("child %d is finished\n", ID); //parent prints out returned value after child is finished

if ((pid1 != 0) && (pid2 != 0) && ((pid3 = fork()) == 0)) //third child created
{
  process3(); //third child process begins
}

while((ID = wait(&status)) != -1) //parent loops until 3rd child is finished
printf("child %d is finished\n", ID); //parent prints out returned value after child is finished



if ((pid1 != 0) && (pid2 != 0) && (pid3 != 0)) 
{
  if ((shmctl (shmid, IPC_RMID, (struct shmid_ds *) 0)) == -1)
  {
 perror ("shmctl");
 exit (-1);
  }

  printf ("\t\t  End of Program.\n"); //prints after all children have finished

  printf("value after memory release:%d\n", total->value);
}
return 0;
} 

/***** Note:  loop for parent to wait for child processes to finish and print ID of each child*****/

Answer 1

From the shmctl man page:

IPC_RMID

Mark the segment to be destroyed. The segment will only actually be destroyed after the last process detaches it (i.e., when the shm_nattch member of the associated structure shmid_ds is zero). The caller must be the owner or creator, or be privileged. If a segment has been marked for destruction, then the (nonstandard) SHM_DEST flag of the shm_perm.mode field in the associated data structure retrieved by IPC_STAT will be set.

The caller must ensure that a segment is eventually destroyed; otherwise its pages that were faulted in will remain in memory or swap.

i.e. your code marks the segment for destruction, but does not detach it until program exit.

If you want to detach it earlier, you can call shmdt(total);. If you do this before that last printf, the printf is likely to cause a seg fault.