Working with shared memory on OS X

April 5, 2010 One minute read

If you’re working with Qt’s QSharedMemory, on Mac OS X you’re working with System V shared memory subsystem. If your data is sizable, the first limit you’ll hit in the maximum segment size, which is for some reason around 4 megs. To increase it invoke this magic incantation:

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sudo sysctl -w kern.sysv.shmmax=33554432

and to see other shared mem related kernel variables:

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sysctl -A|grep shm

If you’d like these settings to remain after you reboot your machine follow the instruction here.

The other problem you’re likely to hit is that shared memory persists between processes (as it probably should) unless you detach and then destroy the QSharedMemory object. This makes testing somewhat inconvenient. You can monitor and remove shared segments with ipcs and ipcrm commands. The first command lists shared segments, the second deletes them.

Again, I’ve only verified this on Mac OS X 10.5, but it should work on other Unix-like systems that support System V IPC.