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Shared Memory is created using shmget(2) and not with filesystem operations. So what is the specific purpose of /run/shm as opposed to that of /run ?

Neither the Filesystem Hierarchy Standard FHS contains information about it, nor do the Ubuntu version of hier(7) or Wikipedia.

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/run/shm which was previously /dev/shm is temporary world-writable shared-memory. It is strictly intended as storage for programs using the POSIX Shared Memory API. Itmake possible inter-process communication (IPC), where different processes can share and communicate via a common memory area, which in this case is usually a normal file that is stored on a "ramdisk". Of course, it can be and has been used in other creative ways too.

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A search for "POSIX Shared Memory API" reveals man 7 shm_overview. Citation from there: On Linux, shared memory objects are created in a (tmpfs) virtual filesystem, normally mounted under /dev/shm. ... System V shared memory (shmget(2), shmop(2), etc.) is an older shared memory API. POSIX shared memory provides a simpler, and better designed interface; on the other hand POSIX shared memory is somewhat less widely available (especially on older systems) than System V shared memory. – Juergen Feb 5 '16 at 11:32
    
This answer was essentially copied from askubuntu.com/questions/169495/… without attribution. – Mokubai♦ Feb 26 '16 at 19:25

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