Unix shared memory management

We know that Windows shared memory is implemented through memory-mapped files maintained by the system page file and is always controlled by a countable reference (http://msdn.microsoft.com/en-us/library/windows/desktop/aa366537(v= vs .85) .aspx).

I wonder if Unix does a similar thing. Someone (http://stackoverflow.com/q/4278391/939142) says that Unix also manages shm in reference counting. I tried the experiment on Mac OS X and found that it was not.

Using the code provided at the end, compile two programs: host and client. A type

./host 

      

        
        
        
      

    

which creates shm, writes (int) 10 at the first address, and exits. Then enter

./client <shmid>

      

        
        
        
      

    

to check the shm segment that is attached to the generated shm prints the first int and exits.

Note that we used shmat and shmdt to connect / disconnect from shm and shmget to create shm.

To kill shm, we have to do it manually using shmctl.

Conclusion: shm is not managed like reference counting on Unix, at least for Mac OS X

/************************** host.c ****************************/

#include <stdio.h>
#include <sys/shm.h>
#include <mach/vm_param.h>
#include <assert.h>

int
main(int argc, char** argv)
{
int value = 10;
int id;
void* ptr;
    /* create a new shm */
id = shmget(IPC_PRIVATE, PAGE_SIZE, IPC_CREAT | 0666);
assert(id != -1);
    /* attach to the new shm */
ptr = shmat(id, (const void*) NULL, 0);
assert((int*) ptr != -1);
    /* print the id so that client can use */
printf("shm id = %ld\n", id);
printf("address of id = %ld, length = %ld\n", ptr, PAGE_SIZE);
((int*) ptr)[0] = value;
printf("value at address %ld = %ld\n", ptr, *(int*) ptr);
/* detach from the shm and exit */
    shmdt((const void*) ptr);
return 0;
}


/************************** host.c ****************************/

#include <stdio.h>
#include <stdlib.h>
#include <sys/shm.h>
#include "assert.h"
#include <mach/vm_param.h>

int
main(int argc, char** argv)
{
void* ptr;
int id = atoi(argv[1]);
assert(id != -1);
    /* attach to the shm */
ptr = shmat(id, NULL, 0);
assert(ptr != -1);
printf("value at ptr = %ld = %ld\n", ptr, *(int*) ptr);
shmdt((const void*) ptr);
return 0;
}