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If applicable, add the following below this CDDL HEADER, with the fields enclosed by brackets "[]" replaced with your own identifying information: Portions Copyright [yyyy] [name of copyright owner] .TH RWLOCK 3C "May 14, 1998" .SH NAME rwlock, rwlock_init, rwlock_destroy, rw_rdlock, rw_wrlock, rw_tryrdlock, rw_trywrlock, rw_unlock \- multiple readers, single writer locks .SH SYNOPSIS .LP .nf cc -mt [ \fIflag\fR... ] \fIfile\fR...[ \fIlibrary\fR... ] #include \fBint\fR \fBrwlock_init\fR(\fBrwlock_t *\fR\fIrwlp\fR, \fBint\fR \fItype\fR, \fBvoid *\fR \fIarg\fR); .fi .LP .nf \fBint\fR \fBrwlock_destroy\fR(\fBrwlock_t *\fR\fIrwlp\fR); .fi .LP .nf \fBint\fR \fBrw_rdlock\fR(\fBrwlock_t *\fR\fIrwlp\fR); .fi .LP .nf \fBint\fR \fBrw_wrlock\fR(\fBrwlock_t *\fR\fIrwlp\fR); .fi .LP .nf \fBint\fR \fBrw_unlock\fR(\fBrwlock_t *\fR\fIrwlp\fR); .fi .LP .nf \fBint\fR \fBrw_tryrdlock\fR(\fBrwlock_t *\fR\fIrwlp\fR); .fi .LP .nf \fBint\fR \fBrw_trywrlock\fR(\fBrwlock_t *\fR\fIrwlp\fR); .fi .SH DESCRIPTION .sp .LP Many threads can have simultaneous read-only access to data, while only one thread can have write access at any given time. Multiple read access with single write access is controlled by locks, which are generally used to protect data that is frequently searched. .sp .LP Readers/writer locks can synchronize threads in this process and other processes if they are allocated in writable memory and shared among cooperating processes (see \fBmmap\fR(2)), and are initialized for this purpose. .sp .LP Additionally, readers/writer locks must be initialized prior to use. \fBrwlock_init()\fR The readers/writer lock pointed to by \fIrwlp\fR is initialized by \fBrwlock_init()\fR. A readers/writer lock is capable of having several types of behavior, which is specified by \fBtype\fR. \fIarg\fR is currently not used, although a future type may define new behavior parameters by way of \fIarg\fR. .sp .LP The \fItype\fR argument can be one of the following: .sp .ne 2 .na \fB\fBUSYNC_PROCESS\fR \fR .ad .RS 18n The readers/writer lock can synchronize threads in this process and other processes. The readers/writer lock should be initialized by only one process. \fIarg\fR is ignored. A readers/writer lock initialized with this type, must be allocated in memory shared between processses, i.e. 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 to not allocate it in such shared memory. .RE .sp .ne 2 .na \fB\fBUSYNC_THREAD\fR \fR .ad .RS 18n The readers/writer lock can synchronize threads in this process, only. \fIarg\fR is ignored. .RE .sp .LP Additionally, readers/writer locks can be initialized by allocation in zeroed memory. A \fBtype\fR of \fBUSYNC_THREAD\fR is assumed in this case. Multiple threads must not simultaneously initialize the same readers/writer lock. And a readers/writer lock must not be re-initialized while in use by other threads. .sp .LP The following are default readers/writer lock initialization (intra-process): .sp .in +2 .nf rwlock_t rwlp; rwlock_init(&rwlp, NULL, NULL); .fi .in -2 .sp .LP or .sp .in +2 .nf rwlock_init(&rwlp, USYNC_THREAD, NULL); .fi .in -2 .sp .LP or .sp .in +2 .nf rwlock_t rwlp = DEFAULTRWLOCK; .fi .in -2 .sp .LP The following is a customized readers/writer lock initialization (inter-process): .sp .in +2 .nf rwlock_init(&rwlp, USYNC_PROCESS, NULL); .fi .in -2 .sp .LP Any state associated with the readers/writer lock pointed to by \fIrwlp\fR are destroyed by \fBrwlock_destroy()\fR and the readers/writer lock storage space is not released. .sp .LP \fBrw_rdlock()\fR gets a read lock on the readers/writer lock pointed to by \fIrwlp\fR. If the readers/writer lock is currently locked for writing, the calling thread blocks until the write lock is freed. Multiple threads may simultaneously hold a read lock on a readers/writer lock. .sp .LP \fBrw_tryrdlock()\fR trys to get a read lock on the readers/writer lock pointed to by \fIrwlp\fR. If the readers/writer lock is locked for writing, it returns an error; otherwise, the read lock is acquired. .sp .LP \fBrw_wrlock()\fR gets a write lock on the readers/writer lock pointed to by \fIrwlp\fR. If the readers/writer lock is currently locked for reading or writing, the calling thread blocks until all the read and write locks are freed. At any given time, only one thread may have a write lock on a readers/writer lock. .sp .LP \fBrw_trywrlock()\fR trys to get a write lock on the readers/writer lock pointed to by \fIrwlp\fR. If the readers/writer lock is currently locked for reading or writing, it returns an error. .sp .LP \fBrw_unlock()\fR unlocks a readers/writer lock pointed to by \fIrwlp\fR, if the readers/writer lock is locked and the calling thread holds the lock for either reading or writing. One of the other threads that is waiting for the readers/writer lock to be freed will be unblocked, provided there is other waiting threads. If the calling thread does not hold the lock for either reading or writing, no error status is returned, and the program's behavior is unknown. .SH RETURN VALUES .sp .LP If successful, these functions return \fB0\fR. Otherwise, a non-zero value is returned to indicate the error. .SH ERRORS .sp .LP The \fBrwlock_init()\fR function will fail if: .sp .ne 2 .na \fB\fBEINVAL\fR \fR .ad .RS 11n \fBtype\fR is invalid. .RE .sp .LP The \fBrw_tryrdlock()\fR or \fBrw_trywrlock()\fR functions will fail if: .sp .ne 2 .na \fB\fBEBUSY\fR \fR .ad .RS 10n The reader or writer lock pointed to by \fIrwlp\fR was already locked. .RE .sp .LP These functions may fail if: .sp .ne 2 .na \fB\fBEFAULT\fR \fR .ad .RS 11n \fIrwlp\fR or \fIarg\fR points to an illegal address. .RE .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 MT-Safe .TE .SH SEE ALSO .sp .LP \fBmmap\fR(2), \fBattributes\fR(5) .SH NOTES .sp .LP These interfaces also available by way of: .sp .LP \fB#include\fR \fB\fR .sp .LP If multiple threads are waiting for a readers/writer lock, the acquisition order is random by default. However, some implementations may bias acquisition order to avoid depriving writers. The current implementation favors writers over readers.