condvar, cv_init, cv_destroy, cv_wait, cv_signal, cv_broadcast,
cv_wait_sig, cv_timedwait, cv_timedwait_sig - condition variable rou‐
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);
Solaris DDI specific (Solaris 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 This is obsolete and should be NULL.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.
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 exam‐
ple, 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 condi‐
tion 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_sig‐
nal() 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 nec‐
essary 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).
For cv_wait_sig() and cv_timedwait_sig() indicates that the
condition was not necessarily signaled and the function
returned because a signal (as in kill(2)) was pending.
For cv_timedwait() and cv_timedwait_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() or cv_timedwait_sig() indi‐
cates that the condition was met and the function returned due
to a call to cv_signal() or cv_broadcast(), or due to a prema‐
ture wakeup (see NOTES).
These functions can be called from user, kernel or interrupt context.
In most cases, however, cv_wait(), cv_timedwait(), cv_wait_sig(), and
cv_timedwait_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(), or cv_timedwait_sig() are
used from interrupt context, lower-priority interrupts will not be ser‐
viced during the wait. This means that if the thread that will eventu‐
ally 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)
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(), and
cv_timedwait_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 func‐
tions 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 inver‐
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(), and
cv_timedwait_sig() behaves as cv_timedwait(). To avoid unkillable pro‐
cesses, 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.
SEE ALSOkill(2), ddi_can_receive_sig(9F), ddi_get_lbolt(9F), mutex(9F),
Writing Device Drivers
Dec 15, 2003 CONDVAR(9F)