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pthread_cond_timedwait.3p (18052B)

'\" et
.TH PTHREAD_COND_TIMEDWAIT "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH 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.
.\"
.SH NAME
pthread_cond_timedwait,
pthread_cond_wait
\(em wait on a condition
.SH SYNOPSIS
.LP
.nf
#include <pthread.h>
.P
int pthread_cond_timedwait(pthread_cond_t *restrict \fIcond\fP,
    pthread_mutex_t *restrict \fImutex\fP,
    const struct timespec *restrict \fIabstime\fP);
int pthread_cond_wait(pthread_cond_t *restrict \fIcond\fP,
    pthread_mutex_t *restrict \fImutex\fP);
.fi
.SH DESCRIPTION
The
\fIpthread_cond_timedwait\fR()
and
\fIpthread_cond_wait\fR()
functions shall block on a condition variable. The application shall
ensure that these functions are called with
.IR mutex
locked by the calling thread; otherwise, an error (for
PTHREAD_MUTEX_ERRORCHECK and robust mutexes) or undefined behavior
(for other mutexes) results.
.P
These functions atomically release
.IR mutex
and cause the calling thread to block on the condition variable
.IR 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
\fIpthread_cond_broadcast\fR()
or
\fIpthread_cond_signal\fR()
in that thread shall behave as if it were issued after the
about-to-block thread has blocked.
.P
Upon successful return, the mutex shall have been locked and shall be
owned by the calling thread. If
.IR 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.
.P
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
\fIpthread_cond_timedwait\fR()
or
\fIpthread_cond_wait\fR()
functions may occur. Since the return from
\fIpthread_cond_timedwait\fR()
or
\fIpthread_cond_wait\fR()
does not imply anything about the value of this predicate, the
predicate should be re-evaluated upon such return.
.P
When a thread waits on a condition variable, having specified a
particular mutex to either the
\fIpthread_cond_timedwait\fR()
or the
\fIpthread_cond_wait\fR()
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
\fIpthread_cond_broadcast\fR()
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.
.P
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
\fIpthread_cond_timedwait\fR()
or
\fIpthread_cond_wait\fR(),
but at that point notices the cancellation request and instead of
returning to the caller of
\fIpthread_cond_timedwait\fR()
or
\fIpthread_cond_wait\fR(),
starts the thread cancellation activities, which includes calling
cancellation cleanup handlers.
.P
A thread that has been unblocked because it has been canceled while
blocked in a call to
\fIpthread_cond_timedwait\fR()
or
\fIpthread_cond_wait\fR()
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.
.P
The
\fIpthread_cond_timedwait\fR()
function shall be equivalent to
\fIpthread_cond_wait\fR(),
except that an error is returned if the absolute time specified by
.IR abstime
passes (that is, system time equals or exceeds
.IR abstime )
before the condition
.IR cond
is signaled or broadcasted, or if the absolute time specified by
.IR abstime
has already been passed at the time of the call. When such timeouts
occur,
\fIpthread_cond_timedwait\fR()
shall nonetheless release and re-acquire the mutex referenced by
.IR mutex ,
and may consume a condition signal directed concurrently at the condition
variable.
.P
The condition variable shall have a clock attribute which specifies
the clock that shall be used to measure the time specified by the
.IR abstime
argument. The
\fIpthread_cond_timedwait\fR()
function is also a cancellation point.
.P
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.
.P
The behavior is undefined if the value specified by the
.IR cond
or
.IR mutex
argument to these functions does not refer to an initialized
condition variable or an initialized mutex object, respectively.
.SH "RETURN VALUE"
Except for
.BR [ETIMEDOUT] ,
.BR [ENOTRECOVERABLE] ,
and
.BR [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
.IR mutex
or the condition variable specified by
.IR cond .
.P
Upon successful completion, a value of zero shall be returned; otherwise,
an error number shall be returned to indicate the error.
.SH ERRORS
These functions shall fail if:
.TP
.BR ENOTRECOVERABLE
.br
The state protected by the mutex is not recoverable.
.TP
.BR EOWNERDEAD
.br
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.
.TP
.BR EPERM
The mutex type is PTHREAD_MUTEX_ERRORCHECK or the mutex
is a robust mutex, and the current thread does not own the mutex.
.P
The
\fIpthread_cond_timedwait\fR()
function shall fail if:
.TP
.BR ETIMEDOUT
The time specified by
.IR abstime
to
\fIpthread_cond_timedwait\fR()
has passed.
.TP
.BR EINVAL
The
.IR abstime
argument specified a nanosecond value less than zero or greater than
or equal to 1000 million.
.br
.P
These functions may fail if:
.TP
.BR EOWNERDEAD
.br
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.
.P
These functions shall not return an error code of
.BR [EINTR] .
.LP
.IR "The following sections are informative."
.SH EXAMPLES
None.
.SH "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
.BR [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.
.SH RATIONALE
If an implementation detects that the value specified by the
.IR cond
argument to
\fIpthread_cond_timedwait\fR()
or
\fIpthread_cond_wait\fR()
does not refer to an initialized condition variable, or detects that
the value specified by the
.IR mutex
argument to
\fIpthread_cond_timedwait\fR()
or
\fIpthread_cond_wait\fR()
does not refer to an initialized mutex object, it is recommended
that the function should fail and report an
.BR [EINVAL] 
error.
.SS "Condition Wait Semantics"
.P
It is important to note that when
\fIpthread_cond_wait\fR()
and
\fIpthread_cond_timedwait\fR()
return without error, the associated predicate may still be false.
Similarly, when
\fIpthread_cond_timedwait\fR()
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.
.P
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.
.P
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.
.P
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.
.P
It is thus recommended that a condition wait be enclosed in the
equivalent of a ``while loop'' that checks the predicate.
.SS "Timed Wait Semantics"
.P
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
\fIclock_gettime\fR()
returns the current time and
\fIcond_relative_timed_wait\fR()
uses relative timeouts:
.sp
.RS 4
.nf
clock_gettime(CLOCK_REALTIME, &now)
reltime = sleep_til_this_absolute_time -now;
cond_relative_timed_wait(c, m, &reltime);
.fi
.P
.RE
.P
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.
.P
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.
.SS "Cancellation and Condition Wait"
.P
A condition wait, whether timed or not, is a cancellation point. That
is, the functions
\fIpthread_cond_wait\fR()
or
\fIpthread_cond_timedwait\fR()
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\(emwhatever 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.
.P
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.
.P
Therefore, since
.IR 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.
.P
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.
.SS "Performance of Mutexes and Condition Variables"
.P
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).
.P
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.
.P
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.
.SS "Features of Mutexes and Condition Variables"
.P
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.
.SS "Scheduling Behavior of Mutexes and Condition Variables"
.P
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.
.SS "Timed Condition Wait"
.P
The
\fIpthread_cond_timedwait\fR()
function allows an application to give up waiting for a particular
condition after a given amount of time. An example of its use
follows:
.sp
.RS 4
.nf
(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);
.fi
.P
.RE
.P
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.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIpthread_cond_broadcast\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 4.12" ", " "Memory Synchronization",
.IR "\fB<pthread.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
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.opengroup.org/unix/online.html .
.PP
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 .