基本概念：
条件变量是线程可用的另一种同步机制。条件变量给多个线程提供了一个会合的场所。条件变量与互斥量一起使用时，允许线程以无竞争的方式等待特定条件发生。
条件变量本身是互斥量保护的。线程在改变条件状态之前必须首先锁住互斥量。
在使用条件变量之前，必须先对它进行初始化。由pthread_cond_t数据类型表示的条件变量可以用两种方式初始化，可以把常量PTHREAD_COND_INITIALIZER赋给静态分配的条件变量，但是如果条件变量是动态分配的，则需要使用pthread_cond_init函数对它初始化。
一、初始化锁与销毁锁
PTHREAD_COND_DESTROY(P)POSIX Programmer's Manual PTHREAD_COND_DESTROY(P)
NAME
pthread_cond_destroy, pthread_cond_init - destroy and initialize condi‐
tion variables
SYNOPSIS
#include <pthread.h>
int pthread_cond_destroy(pthread_cond_t *cond);
int pthread_cond_init(pthread_cond_t *restrict cond,
const pthread_condattr_t *restrict attr);
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
两个函数的返回值：若成功，返回0；否则，返回错误编号
二、等待条件变真
PTHREAD_COND_TIMEDWAIT(P) POSIX Programmer's Manual PTHREAD_COND_TIMEDWAIT(P)
NAME
pthread_cond_timedwait, pthread_cond_wait - wait on a condition
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);
两个函数的返回值：若成功，返回0；否则，返回错误编号
调用者把锁住的互斥量传给函数，函数然后自动把调用线程放到等待条件的线程列表上，对互斥量解锁，pthread_cond_wait返回时，互斥量再次被锁住。pthread_cond_timedwait多了一个超时参数，需要注意的是这个时间值是一个绝对数而不是相对数。
三、唤醒等待线程
PTHREAD_COND_BROADCAST(P) POSIX Programmer's Manual PTHREAD_COND_BROADCAST(P)
NAME
pthread_cond_broadcast, pthread_cond_signal - broadcast or signal a
condition
SYNOPSIS
#include <pthread.h>
int pthread_cond_broadcast(pthread_cond_t *cond);
int pthread_cond_signal(pthread_cond_t *cond);
两个函数的返回值：若成功，返回0；否则，返回错误编号
pthread_cond_signal函数至少能唤醒一个等待该条件的线程，而pthread_cond_broadcast函数则能唤醒等待该条件的所有线程。
必须注意，一定要在改变条件状态以后再给线程发信号。
四、条件变量属性
目前条件变量支持两个属性：进程共享属性和时钟属性，这里不展开讨论。
例子1，单个唤醒 ，gcc pthread_cond_signal.c -pthread
#include <pthread.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
static int num = 0;
static int count = 100000;
static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
static pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
void Perror(const char *s)
{
perror(s);
exit(EXIT_FAILURE);
}
void* fun2(void *arg)
{
pthread_t thread_id = pthread_self();
printf("the thread2 id is %ld\n", (long)thread_id);
sleep(1);
// get num
for ( ; ; ) {
pthread_mutex_lock(&mutex);
while (num == 0) {
printf("the thread2 wait for cond\n");
pthread_cond_wait(&cond, &mutex);
}
printf("the thread2 get cond, working\n");
num -= 1;
pthread_mutex_unlock(&mutex);
}
}
void* fun3(void *arg)
{
pthread_t thread_id = pthread_self();
printf("the thread3 id is %ld\n", (long)thread_id);
sleep(1);
// get num
for ( ; ; ) {
pthread_mutex_lock(&mutex);
while (num == 0) {
printf("the thread3 wait for cond\n");
pthread_cond_wait(&cond, &mutex);
}
printf("the thread3 get cond, working\n");
num -= 1;
pthread_mutex_unlock(&mutex);
}
}
int main()
{
int err;
pthread_t thread1;
pthread_t thread2;
pthread_t thread3;
thread1 = pthread_self();
printf("the thread1 id is %ld\n", (long)thread1);
// Create thread
err = pthread_create(&thread2, NULL, fun2, NULL);
if (err != 0) {
Perror("can't create thread2\n");
}
err = pthread_create(&thread3, NULL, fun3, NULL);
if (err != 0) {
Perror("can't create thread3\n");
}
// detach thread3
err = pthread_detach(thread2);
if (err != 0) {
Perror("can't detach thread2\n");
}
err = pthread_detach(thread3);
if (err != 0) {
Perror("can't detach thread3\n");
}
// set num
for ( ; ; ) {
pthread_mutex_lock(&mutex);
num += 1;
pthread_cond_signal(&cond);
pthread_mutex_unlock(&mutex);
sleep(10);
}
return 0;
}
例子1的运行结果，每次只有一个线程被唤醒：
例子2，多个唤醒，gcc pthread_cond_broadcast.c -pthread
#include <pthread.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
static int num = 0;
static int count = 100000;
static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
static pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
void Perror(const char *s)
{
perror(s);
exit(EXIT_FAILURE);
}
void* fun2(void *arg)
{
pthread_t thread_id = pthread_self();
printf("the thread2 id is %ld\n", (long)thread_id);
sleep(1);
// get num
for ( ; ; ) {
pthread_mutex_lock(&mutex);
while (num == 0) {
printf("the thread2 wait for cond\n");
pthread_cond_wait(&cond, &mutex);
}
printf("the thread2 get cond, working\n");
num -= 1;
pthread_mutex_unlock(&mutex);
}
}
void* fun3(void *arg)
{
pthread_t thread_id = pthread_self();
printf("the thread3 id is %ld\n", (long)thread_id);
sleep(1);
// get num
for ( ; ; ) {
pthread_mutex_lock(&mutex);
while (num == 0) {
printf("the thread3 wait for cond\n");
pthread_cond_wait(&cond, &mutex);
}
printf("the thread3 get cond, working\n");
num -= 1;
pthread_mutex_unlock(&mutex);
}
}
int main()
{
int err;
pthread_t thread1;
pthread_t thread2;
pthread_t thread3;
thread1 = pthread_self();
printf("the thread1 id is %ld\n", (long)thread1);
// Create thread
err = pthread_create(&thread2, NULL, fun2, NULL);
if (err != 0) {
Perror("can't create thread2\n");
}
err = pthread_create(&thread3, NULL, fun3, NULL);
if (err != 0) {
Perror("can't create thread3\n");
}
// detach thread3
err = pthread_detach(thread2);
if (err != 0) {
Perror("can't detach thread2\n");
}
err = pthread_detach(thread3);
if (err != 0) {
Perror("can't detach thread3\n");
}
// set num
for ( ; ; ) {
pthread_mutex_lock(&mutex);
num += 1;
pthread_cond_broadcast(&cond);
pthread_mutex_unlock(&mutex);
sleep(10);
}
return 0;
}
例子2的运行结果，每次有多个线程被唤醒，但是获取到锁的时候，可能条件已经被其他唤醒的线程处理了，所以必须套在while中检查条件是否成熟：
参考：《unix环境高级编程》·第三版
本文永久更新地址：//m.ajphoenix.com/linux/23370.html