Ignoring some details there are two low-level SHM APIs available for in Linux.
We have the older (e.g System V IPC vs POSIX IPC) SysV interface using:
and the newer Posix interface (though Posix seems to standardize the SysV one as well):
It is possible and safe to share memory such that one program uses shm_open() while the other uses shmget() ?
I think the answer is no, though someone wiser may know better.
shm_open(path,...) maps one file to a shared memory segment whereas ftok(path,id,...) maps a named placeholder file to one or more segments. See this related question - Relationship between shared memory and files
So on the one hand you have a one to one mapping between filenames and segments and on the other a one to many - as in the linked question.
Also the path used by shmget() is just a placeholder. For shm_open() the map might be the actual file (though this is implementation defined).
I'm not sure there is anyway to make shm_open() and shmat() refer to the same memory location. Even if you could mix them somehow it would probably be undefined behaviour.
If you look the glibc implementation of shm_open it is simply a wrapper to opening a file. The implementation of shmget and shmat are internal system calls.
It may be that they share an implementation further down in the Linux kernal but this is not a detail that should be exposed or relied upon.