1、互斥锁
用于解决竞争问题的一种机制。
什么是竞争，竞争就是多个实体同时获取一个资源，例如多个线程写一个全局变量。
2、Linux如何使用互斥锁
以pthread为例，锁的创建和使用如下：
/* 创建锁 */
pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
/* 加锁*/
pthread_mutex_lock(&lock);
/* 解锁 */
pthread_mutex_unlock(&lock);
3、多写者问题
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <dirent.h>
#include <stdlib.h>
#include <pthread.h>
/* global variable */
int gValue = 0;
pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
void *threadFuncReader(void *arg)
{
while(1)
{
gValue = 1;
if(1 != gValue)
{
printf("\nerror");
}
// printf("\nthis is [1], read value is [%d]", gValue);
usleep(1);
}
}
void *threadFuncWriter(void *arg)
{
while(1)
{
gValue = 2;
if(2 !=gValue )
{
printf("\nerror");
}
else
{
// printf("\nthis is [2], read value is [%d]", gValue);
}
usleep(1);
}
}
void *threadFuncWriter1(void *arg)
{
while(1)
{
gValue = 3;
if(3 != gValue )
{
printf("\nerror");
}
else
{
//printf("\nthis is [3], read value is [%d]", gValue);
}
usleep(1);
}
}
void *threadFuncWriter2(void *arg)
{
while(1)
{
gValue = 4;
if(4 != gValue )
{
printf("\nerror");
}
else
{
printf("\nthis is [4], read value is [%d]", gValue);
}
sleep(1);
}
}
int main(int argc, int **argv)
{
/* shared resources */
pthread_t tid_reader;
pthread_t tid_writer;
pthread_t tid_writer1;
pthread_t tid_writer2;
pthread_create(&tid_reader, NULL, threadFuncReader, NULL);
pthread_create(&tid_writer, NULL, threadFuncWriter, NULL);
pthread_create(&tid_writer1, NULL, threadFuncWriter1, NULL);
pthread_create(&tid_writer2, NULL, threadFuncWriter2, NULL);
pthread_join(tid_reader, NULL);
pthread_join(tid_writer, NULL);
pthread_join(tid_writer1, NULL);
pthread_join(tid_writer2, NULL);
printf("\n hello world in Linux.");
return 0;
}上述代码由于没有锁，运行结果如下：
this is [4], read value is [4]
this is [4], read value is [4]
this is [4], read value is [4]
error
this is [4], read value is [4]
加锁后的代码：
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <dirent.h>
#include <stdlib.h>
#include <pthread.h>
/* global variable */
int gValue = 0;
pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
void *threadFuncReader(void *arg)
{
while(1)
{
pthread_mutex_lock(&lock);
gValue = 1;
if(1 != gValue)
{
printf("\nerror");
}
// printf("\nthis is [1], read value is [%d]", gValue);
pthread_mutex_unlock(&lock);
usleep(1);
}
}
void *threadFuncWriter(void *arg)
{
while(1)
{
pthread_mutex_lock(&lock);
gValue = 2;
if(2 !=gValue )
{
printf("\nerror");
}
else
{
// printf("\nthis is [2], read value is [%d]", gValue);
}
pthread_mutex_unlock(&lock);
usleep(1);
}
}
void *threadFuncWriter1(void *arg)
{
while(1)
{
pthread_mutex_lock(&lock);
gValue = 3;
if(3 != gValue )
{
printf("\nerror");
}
else
{
//printf("\nthis is [3], read value is [%d]", gValue);
}
pthread_mutex_unlock(&lock);
usleep(1);
}
}
void *threadFuncWriter2(void *arg)
{
while(1)
{
pthread_mutex_lock(&lock);
gValue = 4;
if(4 != gValue )
{
printf("\nerror");
}
else
{
printf("\nthis is [4], read value is [%d]", gValue);
}
pthread_mutex_unlock(&lock);
sleep(1);
}
}
int main(int argc, int **argv)
{
/* shared resources */
pthread_t tid_reader;
pthread_t tid_writer;
pthread_t tid_writer1;
pthread_t tid_writer2;
pthread_create(&tid_reader, NULL, threadFuncReader, NULL);
pthread_create(&tid_writer, NULL, threadFuncWriter, NULL);
pthread_create(&tid_writer1, NULL, threadFuncWriter1, NULL);
pthread_create(&tid_writer2, NULL, threadFuncWriter2, NULL);
pthread_join(tid_reader, NULL);
pthread_join(tid_writer, NULL);
pthread_join(tid_writer1, NULL);
pthread_join(tid_writer2, NULL);
printf("\n hello world in Linux.");
return 0;
}运行结果：
this is [4], read value is [4]
this is [4], read value is [4]
this is [4], read value is [4]
this is [4], read value is [4]
this is [4], read value is [4]
this is [4], read value is [4]
this is [4], read value is [4]
this is [4], read value is [4]
this is [4], read value is [4]
this is [4], read value is [4]
this is [4], read value is [4]
this is [4], read value is [4]
this is [4], read value is [4]
this is [4], read value is [4]
this is [4], read value is [4]
