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
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 sema-
phore 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; semaphorespointed 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.SunOS 5.11 Last change: 5 Feb 2008 1
Standard C Library Functions semaphore(3C) Initializing the semaphore should be doneby only one process. A semaphore initial-
ized 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 isignored. 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 sharedobject USYNC_PROCESS for Solaris threads
and PTHREAD_PROCESS_PRIVATE for POSIX
threads.A semaphore must not be simultaneously initialized by multi-
ple 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, NULL, NULL);
orsema_init(&sp, count, USYNC_THREAD, NULL);
Customized semaphore initialization (inter-process):
sema_t sp;
int count = 1;sema_init(&sp, count, USYNC_PROCESS, NULL);
SunOS 5.11 Last change: 5 Feb 2008 2
Standard C Library Functions semaphore(3C)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 sema-
phore 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 isfor the Solaris threads implementation. For the POSIX-
conforming semaphore interface documentation, seesem_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, anon-zero value indicates an error.
ERRORS
These functions will fail if:EINVAL The sp argument does not refer to a valid sema-
phore.EFAULT Either the sp or arg argument points to an ille-
gal 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:
SunOS 5.11 Last change: 5 Feb 2008 3
Standard C Library Functions semaphore(3C) 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 exceedsSEM_VALUE_MAX.
EXAMPLES
Example 1 The customer waiting-line in a bank is analogous
to the synchronization scheme of a semaphore usingsema_wait() and sema_trywait():
/* cc [ flag ... ] file ... -lthread [ library ... ] */
#include
#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 */ } elsesema_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 */ }SunOS 5.11 Last change: 5 Feb 2008 4
Standard C Library Functions semaphore(3C)ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| MT-Level | Async-Signal-Safe |
|_____________________________|_____________________________|
SEE ALSO
mmap(2), shmop(2), sem_close(3C), sem_destroy(3C),
sem_getvalue(3C), sem_init(3C), sem_open(3C), sem_post(3C),
sem_unlink(3C), sem_wait(3C), attributes(5), standards(5)
NOTES These functions are also available by way of:#include
By default, there is no defined order of unblocking for mul-
tiple threads waiting for a semaphore.SunOS 5.11 Last change: 5 Feb 2008 5