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.TH SEM_GETVALUE 3 2017-09-15 "Linux" "Linux Programmer's Manual"
.SH NAME
sem_getvalue \- get the value of a semaphore
.SH SYNOPSIS
.nf
.B #include <semaphore.h>
.PP
.BI "int sem_getvalue(sem_t *" sem ", int *" sval );
.fi
.PP
Link with \fI\-pthread\fP.
.SH DESCRIPTION
.BR sem_getvalue ()
places the current value of the semaphore pointed to
.I sem
into the integer pointed to by
.IR sval .
.PP
If one or more processes or threads are blocked
waiting to lock the semaphore with
.BR sem_wait (3),
POSIX.1 permits two possibilities for the value returned in
.IR sval :
either 0 is returned;
or a negative number whose absolute value is the count
of the number of processes and threads currently blocked in
.BR sem_wait (3).
Linux adopts the former behavior.
.SH RETURN VALUE
.BR sem_getvalue ()
returns 0 on success;
on error, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EINVAL
.I sem
is not a valid semaphore.
.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_getvalue ()
T}	Thread safety	MT-Safe
.TE
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008.
.SH NOTES
The value of the semaphore may already have changed by the time
.BR sem_getvalue ()
returns.
.SH SEE ALSO
.BR sem_post (3),
.BR sem_wait (3),
.BR sem_overview (7)