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All Rights Reserved. .\" Copyright (c) 2001, The IEEE and The Open Group. All Rights Reserved. .\" Portions Copyright (c) 2008 Sun Microsystems, Inc. All Rights Reserved. .\" .TH SEMAPHORE 3C "Feb 5, 2008" .SH NAME semaphore, sema_init, sema_destroy, sema_wait, sema_trywait, sema_post \- semaphores .SH SYNOPSIS .LP .nf cc [ \fIflag\fR... ] \fIfile\fR... -lthread -lc [ \fIlibrary\fR... ] #include \fBint\fR \fBsema_init\fR(\fBsema_t *\fR\fIsp\fR, \fBunsigned int\fR \fIcount\fR, \fBint\fR \fItype\fR, \fBvoid *\fR \fIarg\fR); .fi .LP .nf \fBint\fR \fBsema_destroy\fR(\fBsema_t *\fR\fIsp\fR); .fi .LP .nf \fBint\fR \fBsema_wait\fR(\fBsema_t *\fR\fIsp\fR); .fi .LP .nf \fBint\fR \fBsema_trywait\fR(\fBsema_t *\fR\fIsp\fR); .fi .LP .nf \fBint\fR \fBsema_post\fR(\fBsema_t *\fR\fIsp\fR); .fi .SH DESCRIPTION .sp .LP 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. .sp .LP Semaphores can synchronize threads in this process and other processes if they are allocated in writable memory and shared among the cooperating processes (see \fBmmap\fR(2)), and have been initialized for this purpose. .sp .LP Semaphores must be initialized before use; semaphores pointed to by \fIsp\fR to \fIcount\fR are initialized by \fBsema_init()\fR. The \fItype\fR argument can assign several different types of behavior to a semaphore. No current type uses \fIarg\fR, although it may be used in the future. .sp .LP The \fItype\fR argument may be one of the following: .sp .ne 2 .na \fB\fBUSYNC_PROCESS\fR \fR .ad .RS 18n 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 \fBshmop\fR(2)), or in memory mapped to a file (see \fBmmap\fR(2)). It is illegal to initialize the object this way and not allocate it in such shared memory. \fIarg\fR is ignored. .RE .sp .ne 2 .na \fB\fBUSYNC_THREAD\fR \fR .ad .RS 18n The semaphore can synchronize threads only in this process. The \fIarg\fR argument is ignored. \fBUSYNC_THREAD\fR does not support multiple mappings to the same logical synch object. If you need to \fBmmap()\fR a synch object to different locations within the same address space, then the synch object should be initialized as a shared object \fBUSYNC_PROCESS\fR for Solaris threads and \fBPTHREAD_PROCESS_PRIVATE\fR for POSIX threads. .RE .sp .LP A semaphore must not be simultaneously initialized by multiple threads, nor re-initialized while in use by other threads. .sp .LP Default semaphore initialization (intra-process): .sp .in +2 .nf sema_t sp; int count = 1; sema_init(&sp, count, 0, NULL); .fi .in -2 .sp .LP or .sp .in +2 .nf sema_init(&sp, count, USYNC_THREAD, NULL); .fi .in -2 .sp .LP Customized semaphore initialization (inter-process): .sp .in +2 .nf \fBsema_t sp; int count = 1; sema_init(&sp, count, USYNC_PROCESS, NULL);\fR .fi .in -2 .sp .LP The \fBsema_destroy()\fR function destroys any state related to the semaphore pointed to by \fIsp\fR. The semaphore storage space is not released. .sp .LP The \fBsema_wait()\fR function blocks the calling thread until the semaphore count pointed to by \fIsp\fR is greater than 0, and then it atomically decrements the count. .sp .LP The \fBsema_trywait()\fR function atomically decrements the semaphore count pointed to by \fIsp\fR, if the count is greater than 0; otherwise, it returns an error. .sp .LP The \fBsema_post()\fR function atomically increments the semaphore count pointed to by \fIsp\fR. If there are any threads blocked on the semaphore, one will be unblocked. .sp .LP The semaphore functionality described on this man page is for the Solaris threads implementation. For the POSIX-conforming semaphore interface documentation, see \fBsem_close\fR(3C), \fBsem_destroy\fR(3C), \fBsem_getvalue\fR(3C), \fBsem_init\fR(3C), \fBsem_open\fR(3C), \fBsem_post\fR(3C), \fBsem_unlink\fR(3C), and \fBsem_wait\fR(3C). .SH RETURN VALUES .sp .LP Upon successful completion, \fB0\fR is returned; otherwise, a non-zero value indicates an error. .SH ERRORS .sp .LP These functions will fail if: .sp .ne 2 .na \fB\fBEINVAL\fR \fR .ad .RS 11n The \fIsp\fR argument does not refer to a valid semaphore. .RE .sp .ne 2 .na \fB\fBEFAULT\fR \fR .ad .RS 11n Either the \fIsp\fR or \fIarg\fR argument points to an illegal address. .RE .sp .LP The \fBsema_wait()\fR function will fail if: .sp .ne 2 .na \fB\fBEINTR\fR \fR .ad .RS 10n The wait was interrupted by a signal or \fBfork()\fR. .RE .sp .LP The \fBsema_trywait()\fR function will fail if: .sp .ne 2 .na \fB\fBEBUSY\fR \fR .ad .RS 10n The semaphore pointed to by \fIsp\fR has a 0 count. .RE .sp .LP The \fBsema_post()\fR function will fail if: .sp .ne 2 .na \fB\fBEOVERFLOW\fR \fR .ad .RS 14n The semaphore value pointed to by \fIsp\fR exceeds \fBSEM_VALUE_MAX\fR. .RE .SH EXAMPLES .LP \fBExample 1 \fRThe customer waiting-line in a bank is analogous to the synchronization scheme of a semaphore using \fBsema_wait()\fR and \fBsema_trywait()\fR: .sp .in +2 .nf /* 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 */ } 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 */ } .fi .in -2 .SH ATTRIBUTES .sp .LP See \fBattributes\fR(5) for descriptions of the following attributes: .sp .sp .TS box; c | c l | l . ATTRIBUTE TYPE ATTRIBUTE VALUE _ MT-Level Async-Signal-Safe .TE .SH SEE ALSO .sp .LP \fBmmap\fR(2), \fBshmop\fR(2), \fBsem_close\fR(3C), \fBsem_destroy\fR(3C), \fBsem_getvalue\fR(3C), \fBsem_init\fR(3C), \fBsem_open\fR(3C), \fBsem_post\fR(3C), \fBsem_unlink\fR(3C), \fBsem_wait\fR(3C), \fBattributes\fR(5), \fBstandards\fR(5) .SH NOTES .sp .LP These functions are also available by way of: .sp .in +2 .nf #include .fi .in -2 .sp .LP By default, there is no defined order of unblocking for multiple threads waiting for a semaphore.