Tribblix: manual page: semaphore.3c

SEMAPHORE(3C) Standard C Library Functions SEMAPHORE(3C)

NAME

semaphore, sema_init, sema_destroy, sema_wait, sema_trywait,
sema_post - semaphores

SYNOPSIS

cc [ flag... ] file... -lthread -lc [ library... ]
#include <synch.h>

int sema_init(sema_t *sp, unsigned int count, int type,
void * arg);

int sema_destroy(sema_t *sp);

int sema_wait(sema_t *sp);

int sema_trywait(sema_t *sp);

int sema_post(sema_t *sp);

DESCRIPTION

A semaphore is a non-negative integer count and is generally used to
coordinate access to resources. The initial semaphore count is set to
the number of free resources, then threads slowly increment and
decrement the count as resources are added and removed. If the
semaphore count drops to 0, which means no available resources,
threads attempting to decrement the semaphore will block until the
count is greater than 0.

Semaphores can synchronize threads in this process and other
processes if they are allocated in writable memory and shared among
the cooperating processes (see mmap(2)), and have been initialized
for this purpose.

Semaphores must be initialized before use; semaphores pointed to by
sp to count are initialized by sema_init(). The type argument can
assign several different types of behavior to a semaphore. No current
type uses arg, although it may be used in the future.

The type argument may be one of the following:

USYNC_PROCESS
The semaphore can synchronize threads in this
process and other processes. Initializing the
semaphore should be done by only one process. A
semaphore initialized with this type must be
allocated in memory shared between processes,
either in Sys V shared memory (see shmop(2)), or in
memory mapped to a file (see mmap(2)). It is
illegal to initialize the object this way and not
allocate it in such shared memory. arg is ignored.

USYNC_THREAD
The semaphore can synchronize threads only in this
process. The arg argument is ignored. USYNC_THREAD
does not support multiple mappings to the same
logical synch object. If you need to mmap() a synch
object to different locations within the same
address space, then the synch object should be
initialized as a shared object USYNC_PROCESS for
Solaris threads and PTHREAD_PROCESS_PRIVATE for
POSIX threads.

A semaphore must not be simultaneously initialized by multiple
threads, nor re-initialized while in use by other threads.

Default semaphore initialization (intra-process):

sema_t sp;
int count = 1;
sema_init(&sp, count, 0, NULL);

or

sema_init(&sp, count, USYNC_THREAD, NULL);

Customized semaphore initialization (inter-process):

sema_t sp;
int count = 1;
sema_init(&sp, count, USYNC_PROCESS, NULL);

The sema_destroy() function destroys any state related to the
semaphore pointed to by sp. The semaphore storage space is not
released.

The sema_wait() function blocks the calling thread until the
semaphore count pointed to by sp is greater than 0, and then it
atomically decrements the count.

The sema_trywait() function atomically decrements the semaphore count
pointed to by sp, if the count is greater than 0; otherwise, it
returns an error.

The sema_post() function atomically increments the semaphore count
pointed to by sp. If there are any threads blocked on the semaphore,
one will be unblocked.

The semaphore functionality described on this man page is for the
Solaris threads implementation. For the POSIX-conforming semaphore
interface documentation, see sem_close(3C), sem_destroy(3C),
sem_getvalue(3C), sem_init(3C), sem_open(3C), sem_post(3C),
sem_unlink(3C), and sem_wait(3C).

RETURN VALUES

Upon successful completion, 0 is returned; otherwise, a non-zero
value indicates an error.

ERRORS

These functions will fail if:

EINVAL
The sp argument does not refer to a valid semaphore.

EFAULT
Either the sp or arg argument points to an illegal
address.

The sema_wait() function will fail if:

EINTR
The wait was interrupted by a signal or fork().

The sema_trywait() function will fail if:

EBUSY
The semaphore pointed to by sp has a 0 count.

The sema_post() function will fail if:

EOVERFLOW
The semaphore value pointed to by sp exceeds
SEM_VALUE_MAX.

EXAMPLES

Example 1: The customer waiting-line in a bank is analogous to the

synchronization scheme of a semaphore using sema_wait() and
sema_trywait():

/* cc [ flag ... ] file ... -lthread [ library ... ] */
#include <errno.h>
#define TELLERS 10
sema_t tellers; /* semaphore */
int banking_hours(), deposit_withdrawal;
void*customer(), do_business(), skip_banking_today();
...

sema_init(&tellers, TELLERS, USYNC_THREAD, NULL);
/* 10 tellers available */
while(banking_hours())
pthread_create(NULL, NULL, customer, deposit_withdrawal);
...

void *
customer(int deposit_withdrawal)
{
int this_customer, in_a_hurry = 50;
this_customer = rand() % 100;

if (this_customer == in_a_hurry) {
if (sema_trywait(&tellers) != 0)
if (errno == EBUSY){ /* no teller available */
skip_banking_today(this_customer);
return;
} /* else go immediately to available teller and
decrement tellers */
}
else
sema_wait(&tellers); /* wait for next teller, then
proceed, and decrement tellers */

do_business(deposit_withdrawal);
sema_post(&tellers); /* increment tellers; this_customer's
teller is now available */
}

ATTRIBUTES

NOTES

These functions are also available by way of:

#include <thread.h>

By default, there is no defined order of unblocking for multiple
threads waiting for a semaphore.

February 5, 2008 SEMAPHORE(3C)