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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]