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SEM_GETVALUE(3) Linux Programmer's Manual SEM_GETVALUE(3)

NAME

     sem_getvalue - get the value of a semaphore

SYNOPSIS

     #include <semaphore.h>

     int sem_getvalue(sem_t *sem, int *sval);

     Link with -pthread.

DESCRIPTION

     sem_getvalue() places the current value of the semaphore pointed to sem
     into the integer pointed to by sval.

     If one or more processes or threads are blocked  waiting  to  lock  the
     semaphore  with  sem_wait(3), POSIX.1 permits two possibilities for the
     value returned in 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  sem_wait(3).   Linux  adopts  the  former
     behavior.

RETURN VALUE

     sem_getvalue() returns 0 on success; on error, -1 is returned and errno
     is set to indicate the error.

ERRORS

     EINVAL sem is not a valid semaphore.

ATTRIBUTES

     For  an  explanation  of  the  terms  used   in   this   section,   see
     attributes(7).

     +---------------+---------------+---------+
     |Interface      | Attribute     | Value   |
     +---------------+---------------+---------+
     |sem_getvalue() | Thread safety | MT-Safe |
     +---------------+---------------+---------+

CONFORMING TO

     POSIX.1-2001, POSIX.1-2008.

NOTES

     The  value  of  the  semaphore  may  already  have  changed by the time
     sem_getvalue() returns.

SEE ALSO

     sem_post(3), sem_wait(3), sem_overview(7)

COLOPHON

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     description  of  the project, information about reporting bugs, and the
     latest    version    of    this    page,    can     be     found     at
     https://www.kernel.org/doc/man-pages/.

Linux 2017-09-15 SEM_GETVALUE(3)