NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | EXAMPLES | ATTRIBUTES | SEE ALSO | NOTES
cc [ flag... ] file...- lthread - lc [ library... ] #include <synch.h>int sema_init(sema_t *sp, unsigned int count, int type, void * arg);
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 zero, which means no available resources, threads attempting to decrement the semaphore will block until the count is greater than zero.
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(). type can assign several different types of behavior to a semaphore. No current type uses arg although it may be used in the future.
type may be one of the following:
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, i.e. either in Sys V shard memory (see shmop(2)), or in memory mapped to a file (see mmap(2)). It is illegal to initialize the object this way and to not allocate it in such shared memory. arg is ignored.
The semaphore can synchronize threads only in this process. arg is ignored.
A semaphore must not be simultaneously initialized by multiple threads, nor re-initialized while in use by other threads.
sema_t sp; int count = 1; sema_init(&sp, count, NULL, 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);
sema_destroy() destroys any state related to the semaphore pointed to by sp. The semaphore storage space is not released.
sema_wait() blocks the calling thread until the semaphore count pointed to by sp is greater than zero, and then it atomically decrements the count.
sema_trywait() atomically decrements the semaphore count pointed to by sp, if the count is greater than zero; otherwise, it returns an error.
sema_post() 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-compliant semaphore interface documentation, see sem_open(3R), sem_init(3R), sem_wait(3R), sem_post(3R), sem_getvalue(3R), sem_unlink(3R), sem_close(3R), sem_destroy(3R)).
Upon successful completion, 0 is returned; otherwise, a non-zero value indicates an error.
These functions fail and return the corresponding value if any of the following conditions are detected:
Invalid argument.
sp or arg points to an illegal address.
The sema_wait() function fails and returns the corresponding value if any of the following conditions are detected:
The wait was interrupted by a signal or fork().
The sema_trywait() function fails and returns the corresponding value if any of the following conditions are detected:
The semaphore pointed to by sp has a zero count.
The sema_post() function fails and returns the corresponding value if any of the following conditions are detected:
The semaphore value pointed to by sp exceeds SEM_VALUE_MAX.
/* 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 == EAGAIN){ /* 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 */ }
See attributes(5) for descriptions of the following attributes:
ATTRIBUTE TYPE | ATTRIBUTE VALUE |
---|---|
MT-Level | Async-Signal-Safe |
mmap(2), shmop(2), sem_close(3R), sem_destroy(3R), sem_getvalue(3R), sem_init(3R), sem_open(3R), sem_post(3R), sem_unlink(3R), sem_wait(3R), attributes(5), standards(5)
These interfaces are also available by way of:
#include <thread.h>
If multiple threads are waiting for a semaphore, by default, there is no defined order of unblocking.
USYNC_THREAD does not support multiple mapplings 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, and PTHREAD_PROCESS_PRIVATE for POSIX.
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | EXAMPLES | ATTRIBUTES | SEE ALSO | NOTES