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
The type provides storage for the CV object. This should be treated as an opaque object and not examined directly by consumers.
Kernels compiled with the
DIAGNOSTIC
option perform basic sanity checks on CV operations.
Kernels compiled with the
LOCKDEBUG
option perform potentially CPU intensive sanity checks
on CV operations.
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 available for
utilities such as
ps(1)
to display.
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
, 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
, mtx
)
As per
cv_wait(),
but causes the current LWP to wait interruptably.
If the LWP receives a signal, or is interrupted by another condition 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
, 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 milliseconds to
one suitable for
cv_timedwait().
If the
ticks
argument 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
, mtx
, ticks
)
As per
cv_wait_sig(),
but also accepts a timeout value and will return
EWOULDBLOCK
if the timeout expires.
cv
)
Awaken one LWP (potentially among many) that is waiting on the specified
condition variable.
The mutex passed to the wait function
mtx(.blm Pp
)
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
)
Awaken all LWPs waiting on the specified condition variable.
The mutex passed to the wait function
mtx(.blm Pp
)
must also be held when calling
cv_broadcast().
cv
)
Return
true
if one or more LWPs are waiting on the specified condition 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 assertions, and must
be called while holding the interlocking mutex passed to
cv_wait(
).
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);
/usr/src
.
The core of the CV implementation is in
sys/kern/kern_condvar.c
.
The header file
sys/sys/condvar.h
describes the public interface.