54.6. Summary

A POSIX shared memory object is used to share a region of memory between unrelated processes without creating an underlying disk file. To do this, we replace the call to open() that normally precedes mmap() with a call to shm_open(). The shm_open() call creates a file in a memory-based file system, and we can employ traditional file descriptor system calls to perform various operations on this virtual file. In particular, ftruncate() must be used to set the size of the shared memory object, since initially it has a length of zero.

We have now described three techniques for sharing memory regions between unrelated processes: System V shared memory, shared file mappings, and POSIX shared memory objects. There are several similarities between the three techniques. There are also some important differences, and, except for the issue of portability, these differences favor shared file mappings and POSIX shared memory objects.