shm_unlink from the shell?

Question

I have a program that creates a shared memory object with shm_open that I'm working on. It tries to release the object with shm_unlink, but sometimes a programming error will cause it to crash before it can make that call. In that case, I need to unlink the shared memory object "by hand," and I'd like to be able to do it in a normal shell, without writing any C code -- i.e. with normal Linux utilities.

Can it be done? This question seems to say that using unlink(1) on /dev/shm/path_passed_to_shm_open, but the manpages aren't clear.

Answer 1

Unlinking the file in /dev/shm will delete the shared memory object if no other process has the object mapped.

Unlike SYSV shared memory, which is implemented in the Kernel, POSIX shared memory objects are simply "files in disguise".

When you call shm_open and mmap, you can see the following in the process process map (using pmap -X):

 Address Perm   Offset Device   Inode Mapping
b7737000 r--s 00000000  00:0d 2267945 test_object

The device major and minor number correspond to the tmpfs mounted at /dev/shm (some systems mount this at /run, and then symlink /dev/shm to /run/shm).

A listing of the folder will show the same inode number:

$ ls -li /dev/shm/
2267945 -rw------- 1 mikel mikel 1 Apr 14 13:36 test_object

Like any other inode, the space will be freed when all references are removed. If we close the only program referencing this we see:

$ cat /proc/meminfo | grep Shmem
Shmem:               700 kB

Once we remove the last reference (created in /dev/shm), the space will be freed:

$ rm /dev/shm/test_object
$ cat /proc/meminfo | grep Shmem
Shmem:               696 kB

If you're curious, you can look at the corresponding files in the glibc sources. shm_directory.c and shm_directory.h generate the filename as /dev/your_shm_name. The implementations shm_open and shm_unlink simply open and unlink this file. So it should be easy to see that rm /dev/shm/your_shm_name performs the same operation,