LOCK(2) LOCK(2)
NAME
lock, canlock, unlock, qlock, canqlock, qunlock, rlock,
runlock, wlock, wunlock, incref, decref - shared memory spin
locks, rendez-vous locks, reader-writer locks, and atomic
increment and decrement
SYNOPSIS
#include <u.h>
#include <libc.h>
void lock(Lock*)
int canlock(Lock*)
void unlock(Lock*)
void qlock(QLock*)
void qunlock(QLock*)
int canqlock(QLock*)
void rlock(RWLock*)
void runlock(RWLock*)
void wlock(RWLock*)
void wunlock(RWLock*)
#include <thread.h>
typedef struct Ref {
long ref;
} Ref;
void incref(Ref*)
long decref(Ref*)
DESCRIPTION
These routines are used to synchronize processes sharing
memory.
The first group (lock, canlock, unlock) uses spin locks in
shared memory. The second group (qlock, canqlock, qunlock),
uses rendezvous locks in shared memory. The third group
(rlock, runlock, wlock, wunlock), also uses rendezvous locks
but has slightly different semantics.
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LOCK(2) LOCK(2)
Locks work in regular programs as well as programs that use
the thread library (see thread(2)). The thread library
replaces the rendezvous system call (see rendezvous(2)) with
its own implementation, threadrendezvous, so that threads as
well as processes may be synchronized by locking calls in
threaded programs.
Used carelessly, spin locks can be expensive and can easily
generate deadlocks. Their use is discouraged, especially in
programs that use the thread library because they prevent
context switches between threads.
Lock blocks until the lock has been obtained. Canlock is
non-blocking. It tries to obtain a lock and returns a non-
zero value if it was successful, 0 otherwise. Unlock
releases a lock.
QLocks have the same interface but are not spin locks;
instead if the lock is taken qlock will suspend execution of
the calling task until it is released.
Although Locks are the more primitive lock, they have limi-
tations; for example, they cannot synchronize between tasks
in the same proc. Use QLocks instead.
RWLocks manage access to a data structure that has distinct
readers and writers. Rlock grants read access; runlock
releases it. Wlock grants write access; wunlock releases
it. There may be any number of simultaneous readers, but
only one writer. Moreover, if write access is granted no
one may have read access until write access is released.
All types of lock should be initialized to all zeros before
use; this puts them in the unlocked state.
A Ref contains a long that can be incremented and decre-
mented atomically: Incref increments the Ref in one atomic
operation. Decref atomically decrements the Ref and returns
zero if the resulting value is zero, non-zero otherwise.
SOURCE
/sys/src/libc/port/lock.c
/sys/src/libc/9sys/qlock.c
/sys/src/libthread/ref.c
SEE ALSO
rfork in fork(2)
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