The Linux Programming Interface

1114 Chapter 54

From the output, we can see that the program resized the shared memory object so

that it is large enough to hold the specified string.

Finally, we use the program in Listing 54-3 to display the string in the shared

memory object:

$ ./pshm_read /demo_shm

hello

Applications must typically use some synchronization technique to allow processes

to coordinate their access to shared memory. In the example shell session shown

here, the coordination was provided by the user running the programs one after

the other. Typically, applications would instead use a synchronization primitive

(e.g., semaphores) to coordinate access to a shared memory object.

54.4 Removing Shared Memory Objects

SUSv3 requires that POSIX shared memory objects have at least kernel persistence;

that is, they continue to exist until they are explicitly removed or the system is

rebooted. When a shared memory object is no longer required, it should be

removed using shm_unlink().

The shm_unlink() function removes the shared memory object specified by name.

Removing a shared memory object doesn’t affect existing mappings of the object

(which will remain in effect until the corresponding processes call munmap() or ter-

minate), but prevents further shm_open() calls from opening the object. Once all pro-

cesses have unmapped the object, the object is removed, and its contents are lost.

The program in Listing 54-4 uses shm_unlink() to remove the shared memory

object specified in the program’s command-line argument.

Listing 54-4: Using shm_unlink() to unlink a POSIX shared memory object

––––––––––––––––––––––––––––––––––––––––––––––––––––––– pshm/pshm_unlink.c

#include <fcntl.h>

#include <sys/mman.h>

#include "tlpi_hdr.h"

int

main(int argc, char *argv[])

{

if (argc != 2 || strcmp(argv[1], "--help") == 0)

usageErr("%s shm-name\n", argv[0]);

if (shm_unlink(argv[1]) == -1)

errExit("shm_unlink");

exit(EXIT_SUCCESS);

}

––––––––––––––––––––––––––––––––––––––––––––––––––––––– pshm/pshm_unlink.c

#include <sys/mman.h>

int shm_unlink(const char *name);

Returns 0 on success, or –1 on error