As of svn rev 4807, src/system.c has an experimental implementation of POSIX shared memory. It is
similar to the already existing implementation of MMAP shared memory, but uses POSIX shm_open()
instead of directly mmapping the .xxx.SHM file.
There are several benefits to using POSIX shared memory:
1) on MacOS, the (unchangable?) maximum SYSV shared memory is about 2 Mbytes, too small for most
MIDAS experiments. POSIX shared memory does not seem to have such a limit;
2) on Linux, when using SYSV shared memory, the .xxx.SHM files are tied to the shared memory keys
using ftok(). If the .xxx.SHM files are located on an NFS-mounted filesystem, ftok() has been observed
to malfunction and return the same key for different shared memory buffers, causing mighty confusing
behaviour of MIDAS. (while "man ftok" discusses a theoretical possibility for such collisions, I have
observed ftok() collisions first hand on a running experiment and it cost us several hours to understand
why all the events go into the wrong event buffers). The present POSIX shared memory implementation
does not have such a problem.
This implementation has received limited testing on Linux and MacOS, and it is now the default shared
memory implementation on MacOS. Linux continues to use SYSV shared memory (ipcs & co). Windows
uses it's own implementation of shared memory (same as mmap, the best I can tell).