Question

pthread问题：

似乎只有在另一个线程调用pthread_cond_notify之前调用pthread_cond_wait时，条件变量才有效。如果在等待之前以某种方式通知，则等待将被卡住。

我的问题是： 什么时候应该使用条件变量？

调度程序可以抢占线程，并且可以在等待之前发生通知。

等待信号量没有这个问题 - 这些都有一个反击。

条件变量何时优于信号量？

这是一个测试：

**** file condvar.c

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>

// ******
// test of conditional variables;
// if cond-var is notified before wait starts, then wait never wakes up !!!
// better to use semaphores than this crap.
// ******

pthread_mutex_t cond_var_lock =  PTHREAD_MUTEX_INITIALIZER; 
pthread_cond_t cond_var = PTHREAD_COND_INITIALIZER;

int wait_first = 1;

void *tfunc(void *arg)
{
(void) arg;

if (!wait_first)
    sleep(1);

fprintf(stderr,"on enter cond_var_lock %lx\n", pthread_self());
pthread_mutex_lock( &cond_var_lock);
fprintf(stderr,"before pthread_cond_wait %lx\n", pthread_self());
pthread_cond_wait( &cond_var, &cond_var_lock);
fprintf(stderr,"after pthread_cond_wait %lx\n", pthread_self());
pthread_mutex_unlock( &cond_var_lock);
fprintf(stderr,"after exit cond_var_lock %lx\n", pthread_self());

return 0;
}

int main(int argc, char *argv[])
{
pthread_t th;

if (argc > 0) 
    wait_first = atoi( argv[1] );

if (wait_first)
{
    fprintf(stderr,"********* Wait first ***********\n");
} else {
    fprintf(stderr,"********* Notify first *********\n");
}


pthread_create( &th, 0, tfunc, 0 );

if (wait_first)
{
    sleep(1);
} 

fprintf(stderr, "! on enter cond_var_lock %lx\n", pthread_self());
pthread_mutex_lock( &cond_var_lock);
fprintf(stderr, "! before pthread_cond_signal %lx\n", pthread_self());
pthread_cond_signal( &cond_var );
fprintf(stderr, "! after pthread_cond_signal %lx\n", pthread_self());
pthread_mutex_unlock( &cond_var_lock);
fprintf(stderr, "! after exit cond_var_lock %lx\n", pthread_self());

sleep(5);
return 0;    
}

**** file test.sh

#!/bin/sh

set -e
set -x

gcc condvar.c -o condvar -lpthread

./condvar 1

./condvar 0

**** test output

Output:

+ gcc condvar.c -o condvar -lpthread
+ ./condvar 1
********* Wait first ***********
on enter cond_var_lock b7779b70
before pthread_cond_wait b7779b70
! on enter cond_var_lock b777a6c0
! before pthread_cond_signal b777a6c0
! after pthread_cond_signal b777a6c0
! after exit cond_var_lock b777a6c0
after pthread_cond_wait b7779b70
after exit cond_var_lock b7779b70
+ ./condvar 0
********* Notify first *********
! on enter cond_var_lock b785c6c0
! before pthread_cond_signal b785c6c0
! after pthread_cond_signal b785c6c0
! after exit cond_var_lock b785c6c0
on enter cond_var_lock b785bb70
before pthread_cond_wait b785bb70

Answer 1

条件变量应该用作等待和通知的地方。它们不是条件本身，它们不是事件。条件包含在周围的编程逻辑中。条件变量的典型使用模式是

// safely examine the condition, prevent other threads from
// altering it
pthread_mutex_lock (&lock);
while ( SOME-CONDITION is false)
    pthread_cond_wait (&cond, &lock);

// Do whatever you need to do when condition becomes true
do_stuff();
pthread_mutex_unlock (&lock);

另一方面，一个线程，发出条件变量的信号，通常看起来像

// ensure we have exclusive access to whathever comprises the condition
pthread_mutex_lock (&lock);

ALTER-CONDITION

// Wakeup at least one of the threads that are waiting on the condition (if any)
pthread_cond_signal (&cond);

// allow others to proceed
pthread_mutex_unlock (&lock)

何时使用pthread条件变量？

1 个答案: