sem_wait()--Wait for Semaphore

 #include <semaphore.h>

 int sem_wait(sem_t * sem);  

The sem_wait() function decrements by one the value of the semaphore. The semaphore will be decremented when its value is greater than zero. If the value of the semaphore is zero, then the current thread will block until the semaphore's value becomes greater than zero.

Parameters

sem

(Input) A pointer to an initialized unnamed semaphore or opened named semaphore.

Authorities

None

Return Value

Error Conditions

If sem_wait() is not successful, errno usually indicates one of the following errors. Under some conditions, errno could indicate an error other than those listed here.

[EINTR]

Interrupted function call.

[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_getvalue()--Get Semaphore Value
  
  
• 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_np()--Wait for Semaphore with Timeout

Example

See Code disclaimer information for information pertaining to code examples.

The following example creates a semaphore with an initial value of 10. The value is decremented by calling sem_wait().

#include <stdio.h>
#include <semaphore.h>
main() {
  sem_t my_semaphore;
  int value;

  sem_init(&my_semaphore, 0, 1);
  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 1
The value of the semaphore after the wait is 0