Is max size of shared memory segments limited on OS X?

[leofang]

On Mac OS X the default maximum size for each shared memory segment is 4MB:

leofang@Leos-MacBook-Pro:~/$ sysctl -A | grep shm
kern.sysv.shmmax: 4194304
kern.sysv.shmmin: 1
kern.sysv.shmmni: 32
kern.sysv.shmseg: 8
kern.sysv.shmall: 1024
machdep.pmap.hashmax: 21
security.mac.posixshm_enforce: 1
security.mac.sysvshm_enforce: 1

leofang@Leos-MacBook-Pro:~/$ ipcs -M
IPC status from <running system> as of Wed May 15 11:09:08 EDT 2019
shminfo:
	shmmax: 4194304	(max shared memory segment size)
	shmmin:       1	(min shared memory segment size)
	shmmni:      32	(max number of shared memory identifiers)
	shmseg:       8	(max shared memory segments per process)
	shmall:    1024	(max amount of shared memory in pages)

This will have to be considered if the reconstructed arrays are too large.

That being said, so far I don't find this causes any problem:

import posix_ipc
s1 = posix_ipc.SharedMemory('/xxx', flags=posix_ipc.O_CREAT, size=4194304+1) # 1 byte larger than maximum

Refs:
http://www.spy-hill.com/help/apple/SharedMemory.html
https://stackoverflow.com/questions/36595754/shared-memory-folder-in-mac-error-shm-invalid-argument