pthread_mutex_lock与pthread_mutex_unlock

pthread_mutex_lock函数与pthread_mutex_unlock是一般成对使用。pthread_mutex_lock与pthread_mutex_unlock之间代码不会被被别的进程中断。类似原子类的执行。
参考一下代码，当两个线程同时访问cnt时，会发生冲突。

#include <pthread.h>
#include <stdio.h>
#include <unistd.h>#define N 10int cnt = 0;void* thread_fun_0(void* arg) {for (int i = 0; i < N; ++i) {       printf("thread id[%ld]-----%d\n",pthread_self(),cnt);++cnt;        usleep(100);}return NULL;
}void* thread_fun_1(void* arg) {for (int i = 0; i < N; ++i) {     printf("thread id[%ld]-----%d\n",pthread_self(),cnt);++cnt;     usleep(100);}return NULL;
}
int main() {pthread_t t1, t2;pthread_create(&t1, NULL, thread_fun_0, NULL);pthread_create(&t2, NULL, thread_fun_1, NULL);pthread_join(t1, NULL);pthread_join(t2, NULL);printf("cnt = %d (expect %d)\n", cnt, 2*N);return 0;
}

每个线程是的cnt 递增10次，进程结束cnt应该是20。这次运行结果有问题，按理输出的cnt应该是1~20，不会重复。但是运行结果却重复了，这是因为对cnt更新时没有锁定系统。修改后代码如下：

#include <pthread.h>
#include <stdio.h>
#include <unistd.h>#define N 10
pthread_mutex_t mux = PTHREAD_MUTEX_INITIALIZER;
int cnt = 0;void* thread_fun_0(void* arg) {for (int i = 0; i < N; ++i) {pthread_mutex_lock(&mux);printf("thread id[%ld]-----%d\n",pthread_self(),cnt);++cnt;        pthread_mutex_unlock(&mux);usleep(100);}return NULL;
}void* thread_fun_1(void* arg) {for (int i = 0; i < N; ++i) {pthread_mutex_lock(&mux);printf("thread id[%ld]-----%d\n",pthread_self(),cnt);++cnt;        pthread_mutex_unlock(&mux);usleep(100);}return NULL;
}
int main() {pthread_t t1, t2;pthread_create(&t1, NULL, thread_fun_0, NULL);pthread_create(&t2, NULL, thread_fun_1, NULL);pthread_join(t1, NULL);pthread_join(t2, NULL);printf("cnt = %d (expect %d)\n", cnt, 2*N);pthread_mutex_destroy(&mux);return 0;
}

本次的运行结果很理想，没有出现冲突。