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.Dd June 8, 2000
.Dt SEM_POST 2
.Os Darwin
.Sh NAME
.Nm sem_post
.Nd unlock a semaphore
.Sh SYNOPSIS
.Fd #include <semaphore.h>
.Ft int
.Fn sem_post "sem_t *sem"
.Sh DESCRIPTION
The the semaphore referenced by
.Fa sem
is unlocked, the value of the semaphore is incremented, and all
threads which are waiting on the semaphore are awakened.
.Pp
.Fn sem_post
is reentrant with respect to signals and may be called from within a
signal hanlder.
.Pp
If successful,
.Fn sem_post
will return 0.  Otherwise, -1 is returned and
.Va errno
is set.
.Sh ERRORS
.Fn sem_post
succeeds unless:
.Bl -tag -width Er
.It Bq Er EINVAL
.Fa sem
is not a valid semaphore descriptor.
.El
.Sh SEE ALSO
.Xr semctl 2 ,
.Xr semget 2 ,
.Xr semop 2 ,
.Xr sem_open 2 ,
.Xr sem_trywait 2 ,
.Xr sem_wait 2
.Sh HISTORY
.Fn sem_post
is specified in the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995).