NAME
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qlock, qunlock, canqlock, rlock, runlock, wlock, wunlock – serial
synchronisation |
SYNOPSIS
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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
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The primitive locking functions described in lock(9) guarantee
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 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
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/sys/src/9/port/qlock.c |
SEE ALSO
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lock(9), splhi(9) |