#include <semaphore.h>

int sem_wait(sem_t *sem);

int sem_trywait(sem_t *sem);

The sem_wait() function locks the semaphore referenced by sem by performing a semaphore lock operation on that semaphore. If the semaphore value is currently zero, then the calling thread will not return from the call to sem_wait() until it either locks the semaphore or the call is interrupted by a signal. The sem_trywait() function locks the semaphore referenced by sem only if the semaphore is currently not locked; that is, if the semaphore value is currently positive. Otherwise, it does not lock the semaphore.

Upon successful return, the state of the semaphore is locked and remains locked until the sem_post(3C) function is executed and returns successfully.

The sem_wait() function is interruptible by the delivery of a signal.

The sem_wait() and sem_trywait() functions return 0 if the calling process successfully performed the semaphore lock operation on the semaphore designated by sem. If the call was unsuccessful, the state of the semaphore is unchanged, and the function returns -1 and sets errno to indicate the error.

The sem_wait() and sem_trywait() functions will fail if:

EINVAL

ENOSYS

The sem_trywait() function will fail if:

EAGAIN

The sem_wait() and sem_trywait() functions may fail if:

EDEADLK

EINTR

Realtime applications may encounter priority inversion when using semaphores. The problem occurs when a high priority thread "locks" (that is, waits on) a semaphore that is about to be "unlocked" (that is, posted) by a low priority thread, but the low priority thread is preempted by a medium priority thread. This scenario leads to priority inversion; a high priority thread is blocked by lower priority threads for an unlimited period of time. During system design, realtime programmers must take into account the possibility of this kind of priority inversion. They can deal with it in a number of ways, such as by having critical sections that are guarded by semaphores execute at a high priority, so that a thread cannot be preempted while executing in its critical section.

Example 1 The customer waiting-line in a bank may be analogous to the synchronization scheme of a semaphore utilizing sem_wait() and sem_trywait():

#include <errno.h>
#define TELLERS 10
sem_t bank_line; /* semaphore */
int banking_hours(), deposit_withdrawal;
void *customer(), do_business(), skip_banking_today();
thread_t tid;
...
 
sem_init(&bank_line,TRUE,TELLERS); /* 10 tellers 
 available */
while(banking_hours())
 thr_create(NULL, NULL, customer, 
 (void *)deposit_withdrawal, THREAD_NEW_LWP, &tid);
...
 
void *
customer(deposit_withdrawal)
void *deposit_withdrawal;
{
 int this_customer, in_a_hurry = 50;
 this_customer = rand() % 100;
 if (this_customer == in_a_hurry) {
 if (sem_trywait(&bank_line) != 0)
 if (errno == EAGAIN) { /* no teller available */
 skip_banking_today(this_customer);
 return;
 } /*else go immediately to available teller 
 & decrement bank_line*/
 }
 else
 sem_wait(&bank_line); /* wait for next teller, 
 then proceed, and decrement bank_line */
 do_business((int *)deposit_withdrawal);
 sem_getvalue(&bank_line,&num_tellers);
 sem_post(&bank_line); /* increment bank_line; 
 this_customer's teller is now available */
}

See attributes(5) for descriptions of the following attributes:

sem_post(3C), attributes(5), standards(5)