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PTHREAD_COND_TIMEDWAIT(3P)POSIX Programmer's ManualTHREAD_COND_TIMEDWAIT(3P)
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
pthread_cond_timedwait, pthread_cond_wait — wait on a condition
#include <pthread.h>
int pthread_cond_timedwait(pthread_cond_t *restrict cond,
pthread_mutex_t *restrict mutex,
const struct timespec *restrict abstime);
int pthread_cond_wait(pthread_cond_t *restrict cond,
pthread_mutex_t *restrict mutex);
The pthread_cond_timedwait() and pthread_cond_wait() functions shall
block on a condition variable. The application shall ensure that
these functions are called with mutex locked by the calling thread;
otherwise, an error (for PTHREAD_MUTEX_ERRORCHECK and robust mutexes)
or undefined behavior (for other mutexes) results.
These functions atomically release mutex and cause the calling thread
to block on the condition variable cond; atomically here means
``atomically with respect to access by another thread to the mutex
and then the condition variable''. That is, if another thread is able
to acquire the mutex after the about-to-block thread has released it,
then a subsequent call to pthread_cond_broadcast() or
pthread_cond_signal() in that thread shall behave as if it were
issued after the about-to-block thread has blocked.
Upon successful return, the mutex shall have been locked and shall be
owned by the calling thread. If mutex is a robust mutex where an
owner terminated while holding the lock and the state is recoverable,
the mutex shall be acquired even though the function returns an error
code.
When using condition variables there is always a Boolean predicate
involving shared variables associated with each condition wait that
is true if the thread should proceed. Spurious wakeups from the
pthread_cond_timedwait() or pthread_cond_wait() functions may occur.
Since the return from pthread_cond_timedwait() or pthread_cond_wait()
does not imply anything about the value of this predicate, the
predicate should be re-evaluated upon such return.
When a thread waits on a condition variable, having specified a
particular mutex to either the pthread_cond_timedwait() or the
pthread_cond_wait() operation, a dynamic binding is formed between
that mutex and condition variable that remains in effect as long as
at least one thread is blocked on the condition variable. During this
time, the effect of an attempt by any thread to wait on that
condition variable using a different mutex is undefined. Once all
waiting threads have been unblocked (as by the
pthread_cond_broadcast() operation), the next wait operation on that
condition variable shall form a new dynamic binding with the mutex
specified by that wait operation. Even though the dynamic binding
between condition variable and mutex may be removed or replaced
between the time a thread is unblocked from a wait on the condition
variable and the time that it returns to the caller or begins
cancellation cleanup, the unblocked thread shall always re-acquire
the mutex specified in the condition wait operation call from which
it is returning.
A condition wait (whether timed or not) is a cancellation point. When
the cancelability type of a thread is set to PTHREAD_CANCEL_DEFERRED,
a side-effect of acting upon a cancellation request while in a
condition wait is that the mutex is (in effect) re-acquired before
calling the first cancellation cleanup handler. The effect is as if
the thread were unblocked, allowed to execute up to the point of
returning from the call to pthread_cond_timedwait() or
pthread_cond_wait(), but at that point notices the cancellation
request and instead of returning to the caller of
pthread_cond_timedwait() or pthread_cond_wait(), starts the thread
cancellation activities, which includes calling cancellation cleanup
handlers.
A thread that has been unblocked because it has been canceled while
blocked in a call to pthread_cond_timedwait() or pthread_cond_wait()
shall not consume any condition signal that may be directed
concurrently at the condition variable if there are other threads
blocked on the condition variable.
The pthread_cond_timedwait() function shall be equivalent to
pthread_cond_wait(), except that an error is returned if the absolute
time specified by abstime passes (that is, system time equals or
exceeds abstime) before the condition cond is signaled or
broadcasted, or if the absolute time specified by abstime has already
been passed at the time of the call. When such timeouts occur,
pthread_cond_timedwait() shall nonetheless release and re-acquire the
mutex referenced by mutex, and may consume a condition signal
directed concurrently at the condition variable.
The condition variable shall have a clock attribute which specifies
the clock that shall be used to measure the time specified by the
abstime argument. The pthread_cond_timedwait() function is also a
cancellation point.
If a signal is delivered to a thread waiting for a condition
variable, upon return from the signal handler the thread resumes
waiting for the condition variable as if it was not interrupted, or
it shall return zero due to spurious wakeup.
The behavior is undefined if the value specified by the cond or mutex
argument to these functions does not refer to an initialized
condition variable or an initialized mutex object, respectively.
Except in the case of [ETIMEDOUT], all these error checks shall act
as if they were performed immediately at the beginning of processing
for the function and shall cause an error return, in effect, prior to
modifying the state of the mutex specified by mutex or the condition
variable specified by cond.
Upon successful completion, a value of zero shall be returned;
otherwise, an error number shall be returned to indicate the error.
These functions shall fail if:
ENOTRECOVERABLE
The state protected by the mutex is not recoverable.
EOWNERDEAD
The mutex is a robust mutex and the process containing the
previous owning thread terminated while holding the mutex
lock. The mutex lock shall be acquired by the calling thread
and it is up to the new owner to make the state consistent.
EPERM The mutex type is PTHREAD_MUTEX_ERRORCHECK or the mutex is a
robust mutex, and the current thread does not own the mutex.
The pthread_cond_timedwait() function shall fail if:
ETIMEDOUT
The time specified by abstime to pthread_cond_timedwait() has
passed.
EINVAL The abstime argument specified a nanosecond value less than
zero or greater than or equal to 1000 million.
These functions may fail if:
EOWNERDEAD
The mutex is a robust mutex and the previous owning thread
terminated while holding the mutex lock. The mutex lock shall
be acquired by the calling thread and it is up to the new
owner to make the state consistent.
These functions shall not return an error code of [EINTR].
The following sections are informative.
None.
Applications that have assumed that non-zero return values are errors
will need updating for use with robust mutexes, since a valid return
for a thread acquiring a mutex which is protecting a currently
inconsistent state is [EOWNERDEAD]. Applications that do not check
the error returns, due to ruling out the possibility of such errors
arising, should not use robust mutexes. If an application is supposed
to work with normal and robust mutexes, it should check all return
values for error conditions and if necessary take appropriate action.
If an implementation detects that the value specified by the cond
argument to pthread_cond_timedwait() or pthread_cond_wait() does not
refer to an initialized condition variable, or detects that the value
specified by the mutex argument to pthread_cond_timedwait() or
pthread_cond_wait() does not refer to an initialized mutex object, it
is recommended that the function should fail and report an [EINVAL]
error.
Condition Wait Semantics
It is important to note that when pthread_cond_wait() and
pthread_cond_timedwait() return without error, the associated
predicate may still be false. Similarly, when
pthread_cond_timedwait() returns with the timeout error, the
associated predicate may be true due to an unavoidable race between
the expiration of the timeout and the predicate state change.
The application needs to recheck the predicate on any return because
it cannot be sure there is another thread waiting on the thread to
handle the signal, and if there is not then the signal is lost. The
burden is on the application to check the predicate.
Some implementations, particularly on a multi-processor, may
sometimes cause multiple threads to wake up when the condition
variable is signaled simultaneously on different processors.
In general, whenever a condition wait returns, the thread has to re-
evaluate the predicate associated with the condition wait to
determine whether it can safely proceed, should wait again, or should
declare a timeout. A return from the wait does not imply that the
associated predicate is either true or false.
It is thus recommended that a condition wait be enclosed in the
equivalent of a ``while loop'' that checks the predicate.
Timed Wait Semantics
An absolute time measure was chosen for specifying the timeout
parameter for two reasons. First, a relative time measure can be
easily implemented on top of a function that specifies absolute time,
but there is a race condition associated with specifying an absolute
timeout on top of a function that specifies relative timeouts. For
example, assume that clock_gettime() returns the current time and
cond_relative_timed_wait() uses relative timeouts:
clock_gettime(CLOCK_REALTIME, &now)
reltime = sleep_til_this_absolute_time -now;
cond_relative_timed_wait(c, m, &reltime);
If the thread is preempted between the first statement and the last
statement, the thread blocks for too long. Blocking, however, is
irrelevant if an absolute timeout is used. An absolute timeout also
need not be recomputed if it is used multiple times in a loop, such
as that enclosing a condition wait.
For cases when the system clock is advanced discontinuously by an
operator, it is expected that implementations process any timed wait
expiring at an intervening time as if that time had actually
occurred.
Cancellation and Condition Wait
A condition wait, whether timed or not, is a cancellation point. That
is, the functions pthread_cond_wait() or pthread_cond_timedwait() are
points where a pending (or concurrent) cancellation request is
noticed. The reason for this is that an indefinite wait is possible
at these points—whatever event is being waited for, even if the
program is totally correct, might never occur; for example, some
input data being awaited might never be sent. By making condition
wait a cancellation point, the thread can be canceled and perform its
cancellation cleanup handler even though it may be stuck in some
indefinite wait.
A side-effect of acting on a cancellation request while a thread is
blocked on a condition variable is to re-acquire the mutex before
calling any of the cancellation cleanup handlers. This is done in
order to ensure that the cancellation cleanup handler is executed in
the same state as the critical code that lies both before and after
the call to the condition wait function. This rule is also required
when interfacing to POSIX threads from languages, such as Ada or C++,
which may choose to map cancellation onto a language exception; this
rule ensures that each exception handler guarding a critical section
can always safely depend upon the fact that the associated mutex has
already been locked regardless of exactly where within the critical
section the exception was raised. Without this rule, there would not
be a uniform rule that exception handlers could follow regarding the
lock, and so coding would become very cumbersome.
Therefore, since some statement has to be made regarding the state of
the lock when a cancellation is delivered during a wait, a definition
has been chosen that makes application coding most convenient and
error free.
When acting on a cancellation request while a thread is blocked on a
condition variable, the implementation is required to ensure that the
thread does not consume any condition signals directed at that
condition variable if there are any other threads waiting on that
condition variable. This rule is specified in order to avoid deadlock
conditions that could occur if these two independent requests (one
acting on a thread and the other acting on the condition variable)
were not processed independently.
Performance of Mutexes and Condition Variables
Mutexes are expected to be locked only for a few instructions. This
practice is almost automatically enforced by the desire of
programmers to avoid long serial regions of execution (which would
reduce total effective parallelism).
When using mutexes and condition variables, one tries to ensure that
the usual case is to lock the mutex, access shared data, and unlock
the mutex. Waiting on a condition variable should be a relatively
rare situation. For example, when implementing a read-write lock,
code that acquires a read-lock typically needs only to increment the
count of readers (under mutual-exclusion) and return. The calling
thread would actually wait on the condition variable only when there
is already an active writer. So the efficiency of a synchronization
operation is bounded by the cost of mutex lock/unlock and not by
condition wait. Note that in the usual case there is no context
switch.
This is not to say that the efficiency of condition waiting is
unimportant. Since there needs to be at least one context switch per
Ada rendezvous, the efficiency of waiting on a condition variable is
important. The cost of waiting on a condition variable should be
little more than the minimal cost for a context switch plus the time
to unlock and lock the mutex.
Features of Mutexes and Condition Variables
It had been suggested that the mutex acquisition and release be
decoupled from condition wait. This was rejected because it is the
combined nature of the operation that, in fact, facilitates realtime
implementations. Those implementations can atomically move a high-
priority thread between the condition variable and the mutex in a
manner that is transparent to the caller. This can prevent extra
context switches and provide more deterministic acquisition of a
mutex when the waiting thread is signaled. Thus, fairness and
priority issues can be dealt with directly by the scheduling
discipline. Furthermore, the current condition wait operation
matches existing practice.
Scheduling Behavior of Mutexes and Condition Variables
Synchronization primitives that attempt to interfere with scheduling
policy by specifying an ordering rule are considered undesirable.
Threads waiting on mutexes and condition variables are selected to
proceed in an order dependent upon the scheduling policy rather than
in some fixed order (for example, FIFO or priority). Thus, the
scheduling policy determines which thread(s) are awakened and allowed
to proceed.
Timed Condition Wait
The pthread_cond_timedwait() function allows an application to give
up waiting for a particular condition after a given amount of time.
An example of its use follows:
(void) pthread_mutex_lock(&t.mn);
t.waiters++;
clock_gettime(CLOCK_REALTIME, &ts);
ts.tv_sec += 5;
rc = 0;
while (! mypredicate(&t) && rc == 0)
rc = pthread_cond_timedwait(&t.cond, &t.mn, &ts);
t.waiters--;
if (rc == 0 || mypredicate(&t))
setmystate(&t);
(void) pthread_mutex_unlock(&t.mn);
By making the timeout parameter absolute, it does not need to be
recomputed each time the program checks its blocking predicate. If
the timeout was relative, it would have to be recomputed before each
call. This would be especially difficult since such code would need
to take into account the possibility of extra wakeups that result
from extra broadcasts or signals on the condition variable that occur
before either the predicate is true or the timeout is due.
None.
pthread_cond_broadcast(3p)
The Base Definitions volume of POSIX.1‐2008, Section 4.11, Memory
Synchronization, pthread.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) In the event of any discrepancy between this version and
the original IEEE and The Open Group Standard, the original IEEE and
The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 PTHREAD_COND_TIMEDWAIT(3P)
Pages that refer to this page: pthread.h(0p), clock_nanosleep(3p), pthread_cancel(3p), pthread_condattr_getclock(3p), pthread_cond_broadcast(3p), pthread_cond_destroy(3p), pthread_mutexattr_gettype(3p)