sem_post()--Post to Semaphore
Syntax
#include <semaphore.h> int sem_post(sem_t * sem);
Service Program Name: QP0ZPSEM
Default Public Authority: *USE
Threadsafe: Yes
The sem_post() function posts to a semaphore, incrementing its value by one. If the resulting value is greater than zero and if there is a thread waiting on the semaphore, the waiting thread decrements the semaphore value by one and continues running.
Parameters
- sem
- (Input) A pointer to an initialized unnamed semaphore or opened named semaphore.
Authorities
None.
Return Value
0 | sem_post() was successful. |
-1 | sem_post() was not successful. The errno variable is set to indicate the error. |
Error Conditions
If sem_post() 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.
Posting to the semaphore would cause its value to exceed its maximum value. The maximum value is SEM_VALUE_MAX or was set using sem_open_np() or sem_init_np().
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_getvalue()--Get Semaphore Value
- sem_init()--Initialize Unnamed Semaphore
- sem_open()--Open Named Semaphore
- sem_open_np()-Open Named Semaphore with Maximum
Value
- 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 initializes an unnamed semaphore and posts to it, incrementing its value by 1.
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_post(&my_semaphore); sem_getvalue(&my_semaphore, &value); printf("The value of the semaphore after the post is %d\n", value); }
Output:
The initial value of the semaphore is 10 The value of the semaphore after the post is 11
API introduced: V4R4