sem_post()--Post to Semaphore

 #include <semaphore.h>

 int sem_post(sem_t * sem); 

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

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

See Code disclaimer information for information pertaining to code examples.

The following example initializes an unnamed semaphore and posts to it, incrementing its value by 1.

#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