Posix shared memory vs mapped files

If you have a parent/child relationship, it's perfectly fine to use mmap.

sysv_shm is the original unix implementation that allows related and unrelated processes to share memory. posix_shm standardized shared memory.

If you're on posix system without mmap, you'd use posix_shm. If you're on a unix without posix_shm you'd use sysv_shm. If you only need to share memory vs a parent/child you'd use mmap if available.

POSIX is a family of standards, specified by the IEEE, to clarify and make uniform the application programming interfaces (and ancillary issues, such as commandline shell utilities) provided by Unix-y operating systems.

When you write your programs to rely on POSIX standards, you can be pretty sure to be able to port them easily among a large family of Unix derivatives (including Linux, but not limited to it!); if and when you use some Linux API that's not standardized as part of Posix, you will have a harder time if and when you want to port that program or library to other Unix-y systems (e.g., MacOSX) in the future.