sem_wait(3RT) Realtime Library Functions sem_wait(3RT)NAME
sem_wait, sem_trywait - acquire or wait for a semaphore
SYNOPSIS
cc [ flag... ] file... -lrt [ library... ]
#include <semaphore.h>
int sem_wait(sem_t *sem);
int sem_trywait(sem_t *sem);
DESCRIPTION
The sem_wait() function locks the semaphore referenced by sem by per‐
forming 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 sema‐
phore 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(3RT) function is executed and returns
successfully.
The sem_wait() function is interruptible by the delivery of a signal.
RETURN VALUES
The sem_wait() and sem_trywait() functions return 0 if the calling
process successfully performed the semaphore lock operation on the sem‐
aphore 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.
ERRORS
The sem_wait() and sem_trywait() functions will fail if:
EINVAL The sem function does not refer to a valid semaphore.
ENOSYS The sem_wait() and sem_trywait() functions are not sup‐
ported by the system.
The sem_trywait() function will fail if:
EAGAIN The semaphore was already locked, so it cannot be imme‐
diately locked by the sem_trywait() operation.
The sem_wait() and sem_trywait() functions may fail if:
EDEADLK A deadlock condition was detected; that is, two sepa‐
rate processes are waiting for an available resource to
be released via a semaphore "held" by the other
process.
EINTR A signal interrupted this function.
USAGE
Realtime applications may encounter priority inversion when using sema‐
phores. 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 pre‐
empted 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 pro‐
grammers must take into account the possibility of this kind of prior‐
ity 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.
EXAMPLES
Example 1: The customer waiting-line in a bank may be analogous to the
synchronization scheme of a semaphore utilizing sem_wait() and sem_try‐
wait():
#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 */
}
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌─────────────────────────────┬─────────────────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├─────────────────────────────┼─────────────────────────────┤
│Interface Stability │Standard │
├─────────────────────────────┼─────────────────────────────┤
│MT-Level │MT-Safe │
└─────────────────────────────┴─────────────────────────────┘
SEE ALSOsem_post(3RT), attributes(5), standards(5)SunOS 5.10 28 Jun 2002 sem_wait(3RT)
