sem_getvalue(2) System Calls Manual sem_getvalue(2)
NAME
sem_getvalue - get the value of a POSIX semaphore
SYNOPSIS
DESCRIPTION
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 following 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 semaphore.
SEE ALSO
sem_open(2), <semaphore.h>.
STANDARDS CONFORMANCE
sem_getvalue(2)