CONDVAR(9F) Kernel Functions for Drivers CONDVAR(9F)


condvar, cv_init, cv_destroy, cv_wait, cv_signal, cv_broadcast,
cv_wait_sig, cv_timedwait, cv_timedwait_sig - condition variable routines


#include <sys/ksynch.h>

void cv_init(kcondvar_t *cvp, char *name, kcv_type_t type, void *arg);

void cv_destroy(kcondvar_t *cvp);

void cv_wait(kcondvar_t *cvp, kmutex_t *mp);

void cv_signal(kcondvar_t *cvp);

void cv_broadcast(kcondvar_t *cvp);

int cv_wait_sig(kcondvar_t *cvp, kmutex_t *mp);

clock_t cv_timedwait(kcondvar_t *cvp, kmutex_t *mp, clock_t timeout);

clock_t cv_timedwait_sig(kcondvar_t *cvp, kmutex_t *mp, clock_t timeout);

clock_t cv_reltimedwait(kcondvar_t *cvp, kmutex_t *mp, clock_t delta,
time_res_t res);

clock_t cv_reltimedwait_sig(kcondvar_t *cvp, kmutex_t *mp, clock_t delta,
time_res_t res);


illumos DDI specific (illumos DDI).


A pointer to an abstract data type kcondvar_t.

A pointer to a mutual exclusion lock (kmutex_t), initialized
by mutex_init(9F) and held by the caller.

Descriptive string. This is obsolete and should be NULL.
(Non-NULL strings are legal, but they're a waste of kernel

The constant CV_DRIVER.

A type-specific argument, drivers should pass arg as NULL.

A time, in absolute ticks since boot, when cv_timedwait() or
cv_timedwait_sig() should return.

A time, in ticks, to indicate how long cv_reltimedwait() or
cv_reltimedwait_sig() should wait before returning. Drivers
should use drv_usectohz(9F) to determine this value.

A value which describes the requested accuracy of the timeout
argument. The system may anticipate or defer the timeout
based on the requested accuracy to prevent unnecessary
wakeups. The following identifiers are valid arguments for
this type:

Nanosecond granularity
Microsecond granularity
Millisecond granularity
Second granularity
Granularity of a kernel clock tick,
commonly 100 or 1000 Hz.

Note that the granularity of these functions is in clock
ticks, therefore the only values which will have an effect


Condition variables are a standard form of thread synchronization. They
are designed to be used with mutual exclusion locks (mutexes). The
associated mutex is used to ensure that a condition can be checked
atomically and that the thread can block on the associated condition
variable without missing either a change to the condition or a signal
that the condition has changed. Condition variables must be initialized
by calling cv_init(), and must be deallocated by calling cv_destroy().

The usual use of condition variables is to check a condition (for
example, device state, data structure reference count, etc.) while
holding a mutex which keeps other threads from changing the condition. If
the condition is such that the thread should block, cv_wait() is called
with a related condition variable and the mutex. At some later point in
time, another thread would acquire the mutex, set the condition such that
the previous thread can be unblocked, unblock the previous thread with
cv_signal() or cv_broadcast(), and then release the mutex.

cv_wait() suspends the calling thread and exits the mutex atomically so
that another thread which holds the mutex cannot signal on the condition
variable until the blocking thread is blocked. Before returning, the
mutex is reacquired.

cv_signal() signals the condition and wakes one blocked thread. All
blocked threads can be unblocked by calling cv_broadcast(). cv_signal()
and cv_broadcast() can be called by a thread even if it does not hold the
mutex passed into cv_wait(), though holding the mutex is necessary to
ensure predictable scheduling.

The function cv_wait_sig() is similar to cv_wait() but returns 0 if a
signal (for example, by kill(2)) is sent to the thread. In any case, the
mutex is reacquired before returning.

The function cv_timedwait() is similar to cv_wait(), except that it
returns -1 without the condition being signaled after the timeout time
has been reached.

The function cv_timedwait_sig() is similar to cv_timedwait() and
cv_wait_sig(), except that it returns -1 without the condition being
signaled after the timeout time has been reached, or 0 if a signal (for
example, by kill(2)) is sent to the thread.

For both cv_timedwait() and cv_timedwait_sig(), time is in absolute clock
ticks since the last system reboot. The current time may be found by
calling ddi_get_lbolt(9F).

The functions cv_reltimedwait() and cv_reltimedwait_sig() behave
similarly to cv_timedwait() and cv_timedwait_sig() respectively, except
instead of taking a time in absolute clock ticks, they take a relative
number of clock ticks to wait. In addition, both functions take an
additional argument res, which specifies the desired granularity that the
system should default to, generally the value TR_CLOCK_TICK .


For cv_wait_sig(), cv_timedwait_sig(), and cv_reltimedwait_sig()
indicates that the condition was not necessarily signaled and
the function returned because a signal (as in kill(2)) was

For cv_timedwait(), cv_timedwait_sig(), cv_reltimedwait(), and
cv_reltimedwait_sig(), indicates that the condition was not
necessarily signaled and the function returned because the
timeout time was reached.

For cv_wait_sig(), cv_timedwait(), cv_timedwait_sig(),
cv_reltimedwait(), and cv_reltimedwait_sig(), indicates that the
condition was met and the function returned due to a call to
cv_signal() or cv_broadcast(), or due to a premature wakeup (see


These functions can be called from user, kernel or interrupt context. In
most cases, however, cv_wait(), cv_timedwait(), cv_wait_sig(),
cv_timedwait_sig(), cv_reltimedwait(), and cv_reltimedwait_sig(), should
not be called from interrupt context, and cannot be called from a high-
level interrupt context.

If cv_wait(), cv_timedwait(), cv_wait_sig(), cv_timedwait_sig(),
cv_reltimedwait(), or cv_reltimedwait_sig(), are used from interrupt
context, lower-priority interrupts will not be serviced during the wait.
This means that if the thread that will eventually perform the wakeup
becomes blocked on anything that requires the lower-priority interrupt,
the system will hang.

For example, the thread that will perform the wakeup may need to first
allocate memory. This memory allocation may require waiting for paging
I/O to complete, which may require a lower-priority disk or network
interrupt to be serviced. In general, situations like this are hard to
predict, so it is advisable to avoid waiting on condition variables or
semaphores in an interrupt context.


Example 1: Waiting for a Flag Value in a Driver's Unit

Here the condition being waited for is a flag value in a driver's unit
structure. The condition variable is also in the unit structure, and the
flag word is protected by a mutex in the unit structure.

while (un->un_flag & UNIT_BUSY)
cv_wait(&un->un_cv, &un->un_lock);
un->un_flag |= UNIT_BUSY;

Example 2: Unblocking Threads Blocked by the Code in Example 1

At some later point in time, another thread would execute the following
to unblock any threads blocked by the above code.

un->un_flag &= ~UNIT_BUSY;


It is possible for cv_wait(), cv_wait_sig(), cv_timedwait(),
cv_timedwait_sig(), cv_reltimedwait(), and cv_reltimedwait_sig() to
return prematurely, that is, not due to a call to cv_signal() or
cv_broadcast(). This occurs most commonly in the case of cv_wait_sig()
and cv_timedwait_sig() when the thread is stopped and restarted by job
control signals or by a debugger, but can happen in other cases as well,
even for cv_wait(). Code that calls these functions must always recheck
the reason for blocking and call again if the reason for blocking is
still true.

If your driver needs to wait on behalf of processes that have real-time
constraints, use cv_timedwait() rather than delay(9F). The delay()
function calls timeout(9F), which can be subject to priority inversions.

Not all threads can receive signals from user level processes. In cases
where such reception is impossible (such as during execution of close(9E)
due to exit(2)), cv_wait_sig() behaves as cv_wait(), cv_timedwait_sig()
behaves as cv_timedwait(), and cv_reltimedwait_sig() behaves as
cv_reltimedwait(). To avoid unkillable processes, users of these
functions may need to protect against waiting indefinitely for events
that might not occur. The ddi_can_receive_sig(9F) function is provided to
detect when signal reception is possible.


kill(2), ddi_can_receive_sig(9F), ddi_get_lbolt(9F), drv_usectohz(9F)
mutex(9F), mutex_init(9F),

Writing Device Drivers

illumos March 29, 2016 CONDVAR(9F)