ждать при условии (wait on a condition)
Пролог (Prolog)
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.
Имя (Name)
pthread_cond_timedwait, pthread_cond_wait — wait on a condition
Синопсис (Synopsis)
#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);
Описание (Description)
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.
Возвращаемое значение (Return value)
Except for [ETIMEDOUT], [ENOTRECOVERABLE], and [EOWNERDEAD], 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.
Ошибки (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.
Примеры (Examples)
None.
Использование в приложениях (Application usage)
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.
Обоснование (Rationale)
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.
Будущие направления (Future directions)
None.
Смотри также (See also)
pthread_cond_broadcast(3p)
The Base Definitions volume of POSIX.1‐2017, Section 4.12, Memory
Synchronization, pthread.h(0p)
