CONDVAR(9) BSD Kernel Developer's Manual CONDVAR(9)NAME
cv, condvar, cv_init, cv_destroy, cv_wait, cv_wait_sig, cv_timedwait,
cv_timedwait_sig, cv_signal, cv_broadcast, cv_has_waiters — condition
variables
SYNOPSIS
#include <sys/condvar.h>
void
cv_init(kcondvar_t *cv, const char *wmesg);
void
cv_destroy(kcondvar_t *cv);
void
cv_wait(kcondvar_t *cv, kmutex_t *mtx);
int
cv_wait_sig(kcondvar_t *cv, kmutex_t *mtx);
int
cv_timedwait(kcondvar_t *cv, kmutex_t *mtx, int ticks);
int
cv_timedwait_sig(kcondvar_t *cv, kmutex_t *mtx, int ticks);
void
cv_signal(kcondvar_t *cv);
void
cv_broadcast(kcondvar_t *cv);
bool
cv_has_waiters(kcondvar_t *cv);
options DIAGNOSTIC
options LOCKDEBUG
DESCRIPTION
Condition variables (CVs) are used in the kernel to synchronize access to
resources that are limited (for example, memory) and to wait for pending
I/O operations to complete.
The kcondvar_t type provides storage for the CV object. This should be
treated as an opaque object and not examined directly by consumers.
OPTIONS
options DIAGNOSTIC
Kernels compiled with the DIAGNOSTIC option perform basic sanity
checks on CV operations.
options LOCKDEBUG
Kernels compiled with the LOCKDEBUG option perform potentially CPU
intensive sanity checks on CV operations.
FUNCTIONS
cv_init(cv, wmesg)
Initialize a CV for use. No other operations can be performed on
the CV until it has been initialized.
The wmesg argument specifies a string of no more than 8 characters
that describes the resource or condition associated with the CV.
The kernel does not use this argument directly but makes it avail‐
able for utilities such as ps(1) to display.
cv_destroy(cv)
Release resources used by a CV. The CV must not be in use when it
is destroyed, and must not be used afterwards.
cv_wait(cv, mtx)
Cause the current LWP to wait non-interruptably for access to a
resource, or for an I/O operation to complete. The LWP will resume
execution when awoken by another thread using cv_signal() or
cv_broadcast().
mtx specifies a kernel mutex to be used as an interlock, and must
be held by the calling LWP on entry to cv_wait(). It will be
released once the LWP has prepared to sleep, and will be reacquired
before cv_wait() returns.
A small window exists between testing for availability of a
resource and waiting for the resource with cv_wait(), in which the
resource may become available again. The interlock is used to
guarantee that the resource will not be signalled as available
until the calling LWP has begun to wait for it.
Non-interruptable waits have the potential to deadlock the system,
and so must be kept short (typically, under one second).
cv_wait_sig(cv, mtx)
As per cv_wait(), but causes the current LWP to wait interruptably.
If the LWP receives a signal, or is interrupted by another condi‐
tion such as its containing process exiting, the wait is ended
early and an error code returned.
If cv_wait_sig() returns as a result of a signal, the return value
is ERESTART if the signal has the SA_RESTART property. If awoken
normally, the value is zero, and EINTR under all other conditions.
cv_timedwait(cv, mtx, ticks)
As per cv_wait(), but will return early if a timeout specified by
the ticks argument expires.
ticks is an architecture and system dependent value related to the
number of clock interrupts per second. See hz(9) for details. The
mstohz(9) macro can be used to convert a timeout expressed in mil‐
liseconds to one suitable for cv_timedwait(). If the ticks argu‐
ment is zero, cv_timedwait() behaves exactly like cv_wait().
If the timeout expires before the LWP is awoken, the return value
is EWOULDBLOCK. If awoken normally, the return value is zero.
cv_timedwait_sig(cv, mtx, ticks)
As per cv_wait_sig(), but also accepts a timeout value and will
return EWOULDBLOCK if the timeout expires.
cv_signal(cv)
Awaken one LWP (potentially among many) that is waiting on the
specified condition variable. The mutex passed to the wait func‐
tion (mtx) must also be held when calling cv_signal().
(Note that cv_signal() is erroneously named in that it does not
send a signal in the traditional sense to LWPs waiting on a CV.)
cv_broadcast(cv)
Awaken all LWPs waiting on the specified condition variable. The
mutex passed to the wait function (mtx) must also be held when
calling cv_broadcast().
cv_has_waiters(cv)
Return true if one or more LWPs are waiting on the specified condi‐
tion variable.
cv_has_waiters() cannot test reliably for interruptable waits. It
should only be used to test for non-interruptable waits made using
cv_wait().
cv_has_waiters() should only be used when making diagnostic asser‐
tions, and must be called while holding the interlocking mutex
passed to cv_wait().
EXAMPLES
Consuming a resource:
/*
* Lock the resource. Its mutex will also serve as the
* interlock.
*/
mutex_enter(&res->mutex);
/*
* Wait for the resource to become available.
*/
while (res->state == BUSY)
cv_wait(&res->condvar, &res->mutex);
/*
* It's now available to us. Take ownership of the
* resource, and consume it.
*/
res->state = BUSY;
mutex_exit(&res->mutex);
consume(res);
Releasing a resource for the next consumer to use:
mutex_enter(&res->mutex);
res->state = IDLE;
cv_signal(&res->condvar);
mutex_exit(&res->mutex);
CODE REFERENCES
The core of the CV implementation is in sys/kern/kern_condvar.c.
The header file sys/sys/condvar.h describes the public interface.
SEE ALSOsigaction(2), errno(9), mb(9), mstohz(9), mutex(9), rwlock(9)
Jim Mauro and Richard McDougall, Solaris Internals: Core Kernel
Architecture, Prentice Hall, 2001, ISBN 0-13-022496-0.
HISTORY
The CV primitives first appeared in NetBSD 5.0.
BSD June 4, 2008 BSD
