QLOCK(10.2) QLOCK(10.2)
NAME
qlock, qunlock, canqlock, rlock, runlock, wlock, wunlock - serial syn‐
chronisation
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(10.2) guarantee
mutual exclusion, but they implement spin locks, and should not be used
if the process might sleep(10.2) within a critical section. The fol‐
lowing 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 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 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 mod‐
ify 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. Canr‐
lock 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
/os/port/qlock.c
/os/emu/port/lock.c
SEE ALSO
lock(10.2), splhi(10.2)
QLOCK(10.2)
