sem_getvalue()--Get Semaphore Value
Syntax
#include <semaphore.h> int sem_getvalue(sem_t * sem, int * value);
Service Program Name: QP0ZPSEM
Default Public Authority: *USE
Threadsafe: Yes
The sem_getvalue() function retrieves the value of a named or unnamed semaphore. If the current value of the semaphore is zero and there are threads waiting on the semaphore, a negative value is returned. The absolute value of this negative value is the number of threads waiting on the semaphore.
Parameters
- sem
- (Input) A pointer to an initialized unnamed semaphore or an opened named
semaphore.
- value
- (Output) A pointer to the integer that contains the value of the semaphore.
Authorities
None.
Return Value
0 | sem_getvalue() was successful. |
-1 | sem_getvalue() was not successful. The errno variable is set to indicate the error. |
Error Conditions
If sem_getvalue() is not successful, errno usually indicates one of the following errors. Under some conditions, errno could indicate an error other than those listed here.
- [EINVAL]
- The value specified for the argument is not correct.
A function was passed incorrect argument values, or an operation was attempted on an object and the operation specified is not supported for that type of object.
An argument value is not valid, out of range, or NULL.
Error Messages
None.
Related Information
- The <semaphore.h> file (see
Header Files for UNIX®-Type Functions)
- sem_close()--Close Named Semaphore
- sem_destroy()--Destroy Unnamed Semaphore
- sem_init()--Initialize Unnamed Semaphore
- sem_init_np()--Initialize Unnamed Semaphore with
Maximum Value
- sem_open()--Open Named Semaphore
- sem_open_np()--Open Named Semaphore with Maximum
Value
- sem_post()--Post to Semaphore
- sem_post_np()--Post Value to Semaphore
- sem_trywait()--Try to Decrement Semaphore
- sem_unlink()--Unlink Named Semaphore
- sem_wait()--Wait for Semaphore
- sem_wait_np()--Wait for Semaphore with Timeout
Example
The following example retrieves the value of a semaphore before and after it is decremented by sem_wait().
Note: By using the code examples, you agree to the terms of the Code license and disclaimer information.
#include <stdio.h> #include <semaphore.h> main() { sem_t my_semaphore; int value; sem_init(&my_semaphore, 0, 10); sem_getvalue(&my_semaphore, &value); printf("The initial value of the semaphore is %d\n", value); sem_wait(&my_semaphore); sem_getvalue(&my_semaphore, &value); printf("The value of the semaphore after the wait is %d\n", value); }
Output:
The initial value of the semaphore is 10 The value of the semaphore after the wait is 9
API introduced: V4R4