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.Dd June 8, 2000
.Dt SEM_WAIT 2
.Os Darwin
.Sh NAME
.Nm sem_wait, sem_trywait
.Nd lock a semaphore
.Sh SYNOPSIS
.Fd #include <semaphore.h>
.Ft int
.Fn sem_wait "sem_t *sem"
.Ft int
.Fn sem_trywait "sem_t *sem"
.Sh DESCRIPTION
The semaphore referenced by
.Fa sem
is locked.  When calling
.Fn sem_wait ,
if the semaphore's value is zero, the calling thread will block until
the lock is aquired or until the call is interrupted by a
signal. Alternatively, the
.Fn sem_trywait
function will fail if the semaphore is already locked, rather than
blocking on the semaphore.
.Pp
If successful (the lock was aquired),
.Fn sem_wait
and
.Fn sem_trywait
will return 0.  Otherwise, -1 is returned and
.Va errno
is set, and the state of the semaphore is unchanged.
.Sh ERRORS
.Fn sem_wait
and
.Fn sem_trywait
succeed unless:
.Bl -tag -width Er
.It Bq Er EAGAIN
The semaphore is already locked.
.It Bq Er EINVAL
.Fa sem
is not a valid semaphore descriptor.
.It Bq Er EDEADLK
A deadlock was detected.
.It Bq Er EINTR
The call was interrupted by a signal.
.El
.Sh NOTES
Applications may encounter a priority inversion while using
semaphores.  When a thread is waiting on a semaphore which is about to
be posted by a lower-priority thread and the lower-priority thread is
preempted by another thread (of medium priority), a priority inversion
has occured, and the higher-priority thread will be blocked for an
unlimited time period.  Programmers using the realtime functionality
of the system should take care to avoid priority inversions.
.Sh SEE ALSO
.Xr semctl 2 ,
.Xr semget 2 ,
.Xr semop 2 ,
.Xr sem_open 2 ,
.Xr sem_post 2
.Sh HISTORY
.Fn sem_wait
and
.Fn sem_trywait
are specified in the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995).