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'\" t
.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH sem_post 3 2025-05-17 "Linux man-pages (unreleased)"
.SH NAME
sem_post \- unlock a semaphore
.SH LIBRARY
POSIX threads library
.RI ( libpthread ,\~ \-lpthread )
.SH SYNOPSIS
.nf
.B #include <semaphore.h>
.P
.BI "int sem_post(sem_t *" sem );
.fi
.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;
lbx lb lb
l l l.
Interface Attribute Value
T{
.na
.nh
.BR sem_post ()
T} Thread safety MT-Safe
.TE
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001.
.SH NOTES
.BR sem_post ()
is async-signal-safe:
it may be safely called within a signal handler.
.SH EXAMPLES
See
.BR sem_wait (3)
and
.BR shm_open (3).
.SH SEE ALSO
.BR sem_getvalue (3),
.BR sem_wait (3),
.BR sem_overview (7),
.BR signal\-safety (7)
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