sem_trywait()--Try to Decrement Semaphore

 #include <semaphore.h>

 int sem_trywait(sem_t * sem); 

The sem_trywait() function attempts to decrement the value of the semaphore. The semaphore will be decremented if its value is greater than zero. If the value of the semaphore is zero, then sem_trywait() will return -1 and set errno to EAGAIN.

Parameters

sem

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

Authorities

None

Return Value

Error Conditions

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

[EAGAIN]

Operation would have caused the process to be suspended.

The value of the semaphore is currently zero and cannot be decremented.

[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_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 attempts to decrement a semaphore with a current value of zero.

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

  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);
  rc = sem_trywait(&my_semaphore);
  if ((rc == -1) && (errno == EAGAIN)) {
   printf("sem_trywait did not decrement the semaphore\n");
  }

}

Output:

The initial value of the semaphore is 1
The value of the semaphore after the wait is 0
sem_trywait did not decrement the semaphore