pthread_cond_wait.txt

来自「Pthread lib库完整说明文档」· 文本 代码 · 共 135 行

           
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AIX Version 4.3 Base Operating System and Extensions Technical Reference, Volume
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pthread_cond_wait or pthread_cond_timedwait Subroutine

Purpose

Blocks the calling thread on a condition.

Library

Threads Library (libpthreads.a)

Syntax

#include <pthread.h>

int pthread_cond_wait (pthread_cond_t *cond);

int pthread_cond_timedwait ( pthread_cond_t *cond, pthread_mutex_t *mutex, const
struct timespec *abstime);

Description

The pthread_cond_wait and pthread_cond_timedwait functions are used to block on a
condition variable. They are called with mutex locked by the calling thread or
undefined behaviour will result.

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_signal or
pthread_cond_broadcast in that thread behaves as if it were issued after the
about-to-block thread has blocked.

Upon successful return, the mutex has been locked and is owned by the calling
thread.

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_wait or
pthread_cond_timedwait functions may occur. Since the return from
pthread_cond_wait or pthread_cond_timedwait does not imply anything about the
value of this predicate, the predicate should be re-evaluated upon such return.

The effect of using more than one mutex for concurrent pthread_cond_wait or
pthread_cond_timedwait operations on the same condition variable is undefined;
that is, a condition variable becomes bound to a unique mutex when a thread waits
on the condition variable, and this (dynamic) binding ends when the wait returns.

A condition wait (whether timed or not) is a cancellation point. When the
cancelability enable state 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_wait or
pthread_cond_timedwait, but at that point notices the cancellation request and
instead of returning to the caller of pthread_cond_wait or
pthread_cond_timedwait, 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_wait or pthread_cond_timedwait does 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 is the same as 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 time-outs occur, pthread_cond_timedwait will
nonetheless release and reacquire the mutex referenced by mutex. The function
pthread_cond_timedwait 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 returns zero due to spurious wakeup.

Parameters

condition Specifies the condition variable to wait on.

mutex Specifies the mutex used to protect the condition variable. The mutex must
be locked when the subroutine is called.

timeout Points to the absolute time structure specifying the blocked state
timeout.

Return Values

Except in the case of ETIMEDOUT, all these error checks act as if they were
performed immediately at the beginning of processing for the function and 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 is returned. Otherwise, an error
number is returned to indicate the error.

Error Codes

The pthread_cond_timedwait function will fail if:

ETIMEDOUT The time specified by abstime to pthread_cond_timedwait has passed.

The pthread_cond_wait and pthread_cond_timedwait functions may fail if:

EINVAL The value specified by cond, mutex,orabstime is invalid.

EINVAL Different mutexes were supplied for concurrent pthread_cond_wait or
pthread_cond_timedwait operations on the same condition variable.

EINVAL The mutex was not owned by the current thread at the time of the call.

These functions will not return an error code of EINTR.

Implementation Specifics

These subroutines are part of the Base Operating System (BOS) Runtime.

Related Information

The pthread_cond_signal or pthread_cond_broadcast subroutine, the pthread.h file.

Using Condition Variables in AIX Version 4.3 General Programming Concepts:
Writing and Debugging Programs.

Threads Library Quick Reference in AIX Version 4.3 General Programming Concepts:
Writing and Debugging Programs.
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