c - 何时唤醒读写器问题中的作者?在释放互斥锁之前还是之后?
问题描述
我想知道在我的情况下最好的做法是什么。在下面的代码中,我在解锁互斥锁之前向作者发出信号
void* Reader(void *context)
{
while(TRUE)
{
LOCK(&g_mutex);
++g_activeReaders;
UNLOCK(&g_mutex);
printf("reader: %ld counter val: %d ", (long)context, g_counter);
LOCK(&g_mutex);
--g_activeReaders;
printf("g_activeReaders: %d \n", g_activeReaders);
if(0 == g_activeReaders)
{
SIGNAL(&g_cv);
}
UNLOCK(&g_mutex);
}
return NULL;
}
我想知道这是否出于某种原因更好,以防止死锁
void* Reader(void *context)
{
int signalFlag;
while(TRUE)
{
signalFlag = 0;
LOCK(&g_mutex);
++g_activeReaders;
UNLOCK(&g_mutex);
printf("reader: %ld counter val: %d ", (long)context, g_counter);
LOCK(&g_mutex);
--g_activeReaders;
printf("g_activeReaders: %d \n", g_activeReaders);
if(0 == g_activeReaders)
{
signalFlag = 1;
}
UNLOCK(&g_mutex);
if(signalFlag)
SIGNAL(&g_cv);
}
return NULL;
}
我的完整程序是:
#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>
#include <unistd.h>
pthread_mutex_t g_mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t g_cv = PTHREAD_COND_INITIALIZER;
int g_activeReaders = 0;
int g_counter;
#define LOCK(X) pthread_mutex_lock(X)
#define UNLOCK(X) pthread_mutex_unlock(X)
#define WAIT pthread_cond_wait
#define WAKE_ALL pthread_cond_broadcast
#define SIGNAL pthread_cond_signal
#define JOIN pthread_join
#define TRUE 1
#define N 3
void* Writer(void * context)
{
while(TRUE)
{
LOCK(&g_mutex);
while(g_activeReaders)
{
WAIT(&g_cv, &g_mutex);
}
++g_counter;
UNLOCK(&g_mutex);
}
return NULL;
}
void* Reader(void *context)
{
while(TRUE)
{
LOCK(&g_mutex);
++g_activeReaders;
UNLOCK(&g_mutex);
printf("reader: %ld counter val: %d ", (long)context, g_counter);
LOCK(&g_mutex);
--g_activeReaders;
printf("g_activeReaders: %d \n", g_activeReaders);
if(0 == g_activeReaders)
{
SIGNAL(&g_cv);
}
UNLOCK(&g_mutex);
}
return NULL;
}
void InitWriters(pthread_t* writers, int count)
{
int status;
int i;
for(i = 0; i < count; ++i)
{
status = pthread_create(&writers[i], NULL, Writer, NULL);
if(status)
{
fprintf(stderr, "Writer create fail\n");
exit(-1);
}
}
}
void InitReaders(pthread_t* readers, int count)
{
int status;
int i;
for(i = 0; i < count; ++i)
{
long e = i;
status = pthread_create(&readers[i], NULL, Reader, (void*) e);
if(status)
{
fprintf(stderr, "readers create fail\n");
exit(-1);
}
}
}
void JoinThreads(pthread_t* threads, int count)
{
int status;
int i;
for(i = 0; i < count; ++i)
{
status = pthread_join(threads[i], NULL);
if(status)
{
fprintf(stderr, "readers create fail\n");
exit(-1);
}
}
}
int main(void)
{
pthread_t writer;
pthread_t readers[N];
InitWriters(&writer, 1);
InitReaders(readers, N);
JoinThreads(&writer, 1);
JoinThreads(readers, N);
pthread_cond_destroy(&g_cv);
pthread_mutex_destroy(&g_mutex);
return 0;
}
我的实现基于找到的伪代码(此处)[ C 中的读取器/写入器实现
解决方案
正确性无关紧要-两者都可以,并且没有死锁。
在您使用不同线程优先级的情况下,在解锁互斥锁之前发出条件变量信号的实现可以确保等待条件变量的高优先级线程将优先于低优先级线程获得互斥锁那只是在互斥体上等待。
推荐阅读
- java - PDFBox 处理隐藏/不可见文本?
- python - 如何使用 Kivy 的 remove_widget 删除所有子元素
- javascript - 用于跟踪 Open Weather Map API 调用的 Javascript 计数器
- python - 无法重现自然图像的光谱功率随频率下降 1/f**2 的说法
- flutter - 从 Flutter 应用程序中的 Firebase 实时数据库事件获取父节点
- c# - 导致问题的服务分离问题
- postgresql - 在分区表上追加成本非常高
- python - 想知道如何从使用列表中获取奇数
- vba - 女士 Access VBA 太快无法刷新 excel 列表对象
- r - r 秩和不同于单个向量和由这些向量组成的矩阵