plan9fox/sys/man/9/qlock

.TH QLOCK 9
.SH NAME
qlock, qunlock, canqlock, rlock, runlock, wlock, wunlock \- serial synchronisation
.SH SYNOPSIS
.de PB
.PP
.ft L
.nf
..
.PB
typedef struct
{
	Lock	use;			/* to access Qlock structure */
	Proc	*head;		/* next process waiting for object */
	Proc	*tail;		/* last process waiting for object */
	int	locked;		/* flag */
} QLock;
.PB
typedef struct
{
	Lock		use;
	Proc		*head;	/* list of waiting processes */
	Proc		*tail;
	uintptr	wpc;		/* pc of writer */
	Proc		*wproc;	/* writing proc */
	int		readers;	/* number of readers */
	int		writer;	/* number of writers */
} RWlock;
.ta \w'\fLvoid 'u
.PP
.B
void	eqlock(QLock *l)
.PP
.B
void	qlock(QLock *l)
.PP
.B
void	qunlock(QLock *l)
.PP
.B
int	canqlock(QLock *l)
.PP
.B
void	rlock(RWlock *l)
.PP
.B
void	runlock(RWlock *l)
.PP
.B
int	canrlock(RWlock *l)
.PP
.B
void	wlock(RWlock *l)
.PP
.B
void	wunlock(RWlock *l)
.SH DESCRIPTION
The primitive locking functions described in
.IR lock (9)
guarantee mutual exclusion, but they implement spin locks,
and should not be used if the process might
.IR sleep (9)
within a critical section.
The following functions serialise access to a resource by forming an orderly
queue of processes.
.PP
Each resource to be controlled is given an associated
.B QLock
structure; it is usually most straightforward to put the
.B 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
.IR malloc ).
.PP
On return from
.IR qlock ,
the process has acquired the lock
.IR 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
.L Queueing
state.
.PP
.I Eqlock
is an interruptible form of
.IR qlock.
.PP
.I Qunlock
unlocks
.I l
and schedules the first process queued for it (if any).
.PP
.I Canqlock
is a non-blocking form of
.IR qlock .
It tries to obtain the lock
.I l
and returns true if successful, and 0 otherwise;
it always returns immediately.
.PP
.B 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.
.PP
.I Rlock
acquires
.I l
for reading.
The holder can read but agrees not to modify the resource.
There may be several concurrent readers.
.I Canrlock
is non-blocking: it returns non-zero if it successfully acquired the lock immediately,
and 0 if the resource was unavailable.
.PP
.I Runlock
returns a read lock;
the last reader out enables the first writer waiting (if any).
.PP
.I Wlock
acquires a write lock.
The holder of such a lock may assume exclusive access to the resource,
and is allowed to modify it.
.PP
.I Wunlock
returns a write lock.
The next pending process, whether reader or writer, is scheduled.
.SH SOURCE
.B /sys/src/9/port/qlock.c
.br
.SH SEE ALSO
.IR lock (9),
.IR malloc (9),
.IR splhi (9)