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.TH SEM_POST 3 2017-03-13 "Linux" "Linux Programmer's Manual"
.SH NAME
sem_post \- unlock a semaphore
.SH SYNOPSIS
.nf
.B #include <semaphore.h>
.PP
.BI "int sem_post(sem_t *" sem );
.fi
.PP
Link with \fI\-pthread\fP.
.SH DESCRIPTION
.BR sem_post ()
increments (unlocks) the semaphore pointed to by
.IR sem .
If the semaphore's value consequently becomes greater than zero,
then another process or thread blocked in a
.BR sem_wait (3)
call will be woken up and proceed to lock the semaphore.
.SH RETURN VALUE
.BR sem_post ()
returns 0 on success;
on error, the value of the semaphore is left unchanged,
\-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EINVAL
.I sem
is not a valid semaphore.
.TP
.B EOVERFLOW
.\" Added in POSIX.1-2008 TC1 (Austin Interpretation 213)
The maximum allowable value for a semaphore would be exceeded.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lb lb lb
l l l.
Interface	Attribute	Value
T{
.BR sem_post ()
T}	Thread safety	MT-Safe
.TE
.SH CONFORMING TO
POSIX.1-2001.
.SH NOTES
.BR sem_post ()
is async-signal-safe:
it may be safely called within a signal handler.
.SH EXAMPLE
See
.BR sem_wait (3).
.SH SEE ALSO
.BR sem_getvalue (3),
.BR sem_wait (3),
.BR sem_overview (7),
.BR signal-safety (7)