WAIT_EVENT(9) Driver Basics WAIT_EVENT(9)NAME
wait_event - sleep until a condition gets true
SYNOPSIS
wait_event(wq, condition);
ARGUMENTS
wq
the waitqueue to wait on
condition
a C expression for the event to wait for
DESCRIPTION
The process is put to sleep (TASK_UNINTERRUPTIBLE) until the condition evaluates to true. The condition is checked each time the waitqueue
wq is woken up.
wake_up has to be called after changing any variable that could change the result of the wait condition.
COPYRIGHT Kernel Hackers Manual 3.10 June 2014 WAIT_EVENT(9)
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pthread_cond_wait(3T)pthread_cond_wait(3T)NAME
pthread_cond_wait(), pthread_cond_timedwait() - wait or timed wait on a condition variable
SYNOPSIS
Parameters
cond Pointer to the condition variable to be waited on.
mutex Pointer to the mutex associated with the condition variable cond.
abstime Absolute time at which the wait expires, even if the condition has not been signaled or broadcast.
DESCRIPTION
The function is used to wait for the occurrence of a condition associated with the condition variable cond.
The function is used to wait a limited amount of time for the occurrence of a condition associated with the condition variable cond. The
abstime parameter specifies the time at which this function should time out. If the absolute time specified by abstime passes and the
indicated condition has not been signaled, the function returns an error to the caller.
Note: abstime is the time at which the wait expires, not the length of time the thread will wait.
The condition variabled denoted by cond must have been dynamically initialized by a call to or statically initialized with the macro
Both functions should be called with mutex locked by the calling thread. If mutex is not locked by the calling thread, undefined behavior
will result. These functions atomically release mutex and cause the calling thread to block on the condition variable cond. If another
thread is able to acquire the mutex after the about-to-block thread has released it but before it has actually blocked, a subsequent call
to or by the other thread will behave as if it were issued after the about-to-block thread has blocked.
When the condition is signaled or the timed wait expires, the caller is unblocked and will reacquire mutex before returning. Whether these
functions succeed or fail, mutex will always be reacquired before returning to the caller.
Using different mutexes for concurrent calls to these functions on the same condition variable results in undefined behavior.
When using condition variables, there is a predicate associated with the condition wait. If this predicate is false, the thread should
perform a condition wait. Spurious wakeups may occur when waiting on a condition variable. A spurious wakeup occurs when a thread returns
from a condition wait when it should really continue waiting. A normal signal being delivered to a thread may cause a spurious wakeup dur-
ing a condition wait. Since the return values from and do not imply anything about the value of the predicate, the predicate should be re-
evaluated.
A condition wait is a When the calling thread has deferred cancellation enabled, cancellation requested will be acted upon. If a cancella-
tion request is acted upon while a thread is blocked in one of these functions, mutex is reacquired before calling the cancellation cleanup
handlers. The cancellation cleanup handlers should release mutex so that application deadlock does not occur. If the condition signal and
the cancellation request both occur, the canceled thread will not consume the condition signal (i.e., a different thread will be unblocked
due to the condition signal).
If a signal is delivered to a thread waiting for a condition variable, upon return from the signal handler, the thread may return zero due
to a spurious wakeup or continue waiting for the condition.
RETURN VALUE
and return the following values:
Successful completion.
Failure.
An error number is returned to indicate the error. (The variable is not set.)
ERRORS
The following error value is returned by if the corresponding condition is detected.
abstime has passed and a condition signal has not been received.
One of the following error values is returned by and if the corresponding condition is detected.
A cond, mutex, or abstime parameter points to an illegal address.
The value specified by
cond, mutex, or abstime is invalid.
mutex is not owned by the calling thread. This error is not returned for a or mutex on HP-UX.
Different mutexes are being used for
cond. This error is not detected on HP-UX.
WARNINGS
It is important to note that when or return without error, the associated predicate may still be false. When returns with the timeout
error, the associated predicate may be true. It is recommended that a condition wait be enclosed in the equivalent of a "while loop,"
which checks the predicate.
Undefined behavior results if these functions are called with a mutex.
EXAMPLES
is recommended to be used in a loop testing the predicate associated with it. This will take care of any spurious wakeups that may occur.
is also recommended to be used in a loop. This function can return success even if the predicate is not true. It should be called in a
loop while checking the predicate. If the function times out, the predicate may still have become true. The predicate should be checked
before processing the timeout case. The example given below does not do any other error checking.
AUTHOR
and were derived from the IEEE POSIX P1003.1c standard.
SEE ALSO pthread_cond_init(3T), pthread_cond_signal(3T).
STANDARDS CONFORMANCE
Pthread Library pthread_cond_wait(3T)