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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 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 sem_open 2 ,
.Xr sem_trywait 2 ,
.Xr sem_wait 2 ,
.Xr semctl 2 ,
.Xr semget 2 ,
.Xr semop 2
.Sh HISTORY
.Fn sem_post
is specified in the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995).