sem_getvalue(2)sem_getvalue(2)NAMEsem_getvalue - get the value of a POSIX semaphore
SYNOPSISDESCRIPTION
is used to read the value of the semaphore. The value of the semaphore
specified by sem is read, at some unspecified time during the call, and
then stored into sval. If the semaphore value is <= 0, at that time,
the semaphore is considered unavailable. If the semaphore value is >
0, at that time, the semaphore is considered available.
If sval is positive, it is equal to the number of locks available on
the semaphore, at the time the semaphore value was read. If sval is
negative, its absolute value is equal to the number of blocked threads
waiting for the semaphore to become available, at the time the sema‐
phore value was read.
If the specified semaphore referred to by sem is a named semaphore,
then this semaphore must have been opened by the calling process with
and the process must have read permission on this semaphore.
To use this function, link in the realtime library by specifying on the
compiler or linker command line.
EXAMPLES
The following call to will read the value of the semaphore sem and
store it in sval.
RETURN VALUE
A successful call to will return 0. Otherwise, the call to will return
-1 with errno set to the appropriate value of the error condition.
ERRORS
fails and does not perform the requested operation if any of the fol‐
lowing conditions are encountered:
[EPERM] The calling process does not have the privileges
necessary to read the semaphore.
[EINVAL] The argument sem does not refer to a valid sema‐
phore.
SEE ALSOsem_open(2), <semaphore.h>.
STANDARDS CONFORMANCEsem_getvalue(2)