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.\" $FreeBSD: src/lib/libc_r/man/sem_init.3,v 1.1.2.9 2001/12/17 10:08:26 ru Exp $
.\" $DragonFly: src/lib/libc_r/man/sem_init.3,v 1.2 2003/06/17 04:26:48 dillon Exp $
.Dd June 30, 2009
.Dt SEM_INIT 3
.Os
.Sh NAME
.Nm sem_init
.Nd initialize an unnamed semaphore
.Sh LIBRARY
.Lb libpthread
.Sh SYNOPSIS
.In semaphore.h
.Ft int
.Fn sem_init "sem_t *sem" "int pshared" "unsigned int value"
.Sh DESCRIPTION
The
.Fn sem_init
function initializes the unnamed semaphore pointed to by
.Fa sem
to have the value
.Fa value .
A non-zero value for
.Fa pshared
specifies a shared semaphore that can be used by multiple processes
in addition to threads.
Any process able to access the related backing memory, typically via
.Fn fork ,
is allowed to use a semaphore created with a non-zero
.Fa pshared ,
otherwise only threads may access the semaphore.
.Pp
Following a successful call to
.Fn sem_init ,
.Fa sem
can be used as an argument in subsequent calls to
.Fa sem_wait ,
.Fa sem_trywait ,
.Fa sem_post ,
and
.Fa sem_destroy .
.Fa sem
is no longer valid after a successful call to
.Fa sem_destroy .
.Sh IMPLEMENTATION NOTES
Shared sempahores are allocated out of anonymous shared mmapped
pages.
Due to lack of clarity in the standard the mechanics of
destroying such a space are unclear, so
.Fa sem_destroy
is currently unable to reclaim such semaphores.
.Sh RETURN VALUES
.Rv -std sem_init
.Sh ERRORS
.Fn sem_init
will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
.Fa value
exceeds SEM_VALUE_MAX.
.It Bq Er ENOSPC
Memory allocation error.
.El
.Sh SEE ALSO
.Xr umtx 2 ,
.Xr sem_destroy 3 ,
.Xr sem_post 3 ,
.Xr sem_trywait 3 ,
.Xr sem_wait 3
.Sh STANDARDS
.Fn sem_init
conforms to
.St -p1003.1-96 .