illumos: manual page: sem

SEM_WAIT(3C) Standard C Library Functions SEM_WAIT(3C)

NAME

sem_wait, sem_trywait - acquire or wait for a semaphore

SYNOPSIS

#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
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.

RETURN VALUES

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.

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 supported
by the system.

The sem_trywait() function will fail if:

EAGAIN
The semaphore was already locked, so it cannot be immediately
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 separate
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
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.

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_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 */
}

ATTRIBUTES