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More info about FreeBSD.
"sysctl p1003_1b.sem_nsems_max" gives the maximum number of POSIX semaphores (per process? system wide?).
Since FreeBSD 8.1, "sudo sysctl -w p1003_1b.sem_nsems_max=256" can be used to change this limit at runtime.
Before FreeBSD 8.1, SEM_MAX constant should be changed in the kernel source code, and the kernel have to be recompiled. (p1003_1b.sem_nsems_max is not configurable in /etc/sysctl.conf, it is an hardcoded limit).
Before FreeBSD 8.0, the POSIX semaphores are disabled by default: the kernel have to be compiled using P1003_1B_SEMAPHORES option. Extract of sys/conf/NOTES:
#####################################################################
# POSIX P1003.1B
# Real time extensions added in the 1993 POSIX
# _KPOSIX_PRIORITY_SCHEDULING: Build in _POSIX_PRIORITY_SCHEDULING
options 	_KPOSIX_PRIORITY_SCHEDULING
# p1003_1b_semaphores are very experimental,
# user should be ready to assist in debugging if problems arise.
options 	P1003_1B_SEMAPHORES
# POSIX message queue
options 	P1003_1B_MQUEUE

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