QLOCK(9) QLOCK(9)
NAME
qlock, qunlock, canqlock, rlock, runlock, wlock, wunlock -
serial synchronisation
SYNOPSIS
void qlock(QLock *l)
void qunlock(QLock *l)
int canqlock(QLock *l)
void rlock(RWlock *l)
void runlock(RWlock *l)
int canrlock(RWlock *l)
void wlock(RWlock *l)
void wunlock(RWlock *l)
DESCRIPTION
The primitive locking functions described in lock(9) guaran-
tee mutual exclusion, but they implement spin locks, and
should not be used if the process might sleep(9) within a
critical section. The following functions serialise access
to a resource by forming an orderly queue of processes.
Each resource to be controlled is given an associated QLock
structure; it is usually most straightforward to put the
QLock in the structure that represents the resource. It
must be initialised to zero before use (as guaranteed for
global variables and for structures allocated by malloc).
On return from qlock, the process has acquired the lock l,
and can assume exclusive access to the associated resource.
If the lock is not immediately available, the requesting
process is placed on a FIFO queue of processes that have
requested the lock. Processes on this list are blocked in
the `Queueing' state.
Qunlock unlocks l and schedules the first process queued for
it (if any).
Canqlock is a non-blocking form of qlock. It tries to obtain
the lock l and returns true if successful, and 0 otherwise;
it always returns immediately.
RWlock is a form of lock for resources that have distinct
readers and writers. It allows concurrent readers but gives
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QLOCK(9) QLOCK(9)
each writer exclusive access. A caller announces its read
or write intentions by choice of lock (and unlock) function;
the system assumes the caller will not modify a structure
accessed under read lock.
Rlock acquires l for reading. The holder can read but
agrees not to modify the resource. There may be several
concurrent readers. Canrlock is non-blocking: it returns
non-zero if it successfully acquired the lock immediately,
and 0 if the resource was unavailable.
Runlock returns a read lock; the last reader out enables the
first writer waiting (if any).
Wlock acquires a write lock. The holder of such a lock may
assume exclusive access to the resource, and is allowed to
modify it.
Wunlock returns a write lock. The next pending process,
whether reader or writer, is scheduled.
SOURCE
/sys/src/9/port/qlock.c
SEE ALSO
lock(9), splhi(9)
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