NAME
sem_getvalue — get the value of a POSIX semaphore
SYNOPSIS
#include <sys/semaphore.h>
int sem_getvalue(sem_t *sem, int *sval);
DESCRIPTION
sem_getvalue()
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 semaphore
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
sem_open()
and the process must have read permission on this semaphore.
To use this function, link in the realtime library by specifying
-lrt
on the compiler or linker command line.
EXAMPLES
The following call to
sem_getvalue()
will read the value of the semaphore
sem
and store it in
sval.
RETURN VALUE
A successful call to
sem_getvalue()
will return 0.
Otherwise, the call to
sem_getvalue()
will return -1 with errno set to the appropriate value of the error condition.
ERRORS
sem_getvalue()
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.
STANDARDS CONFORMANCE
sem_getvalue(): POSIX
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