进程间同步

互斥量mutex

进程间也可以使用互斥锁，来达到同步的目的。但应在pthread_mutex_init初始化之前，修改其属性为进程间共享。mutex的属性修改函数主要有以下几个。

主要应用函数：

       pthread_mutexattr_t mattr 类型：           用于定义mutex锁的【属性】

       pthread_mutexattr_init函数：               初始化一个mutex属性对象

              int pthread_mutexattr_init(pthread_mutexattr_t *attr);

       pthread_mutexattr_destroy函数：        销毁mutex属性对象 (而非销毁锁)

              int pthread_mutexattr_destroy(pthread_mutexattr_t *attr);

       pthread_mutexattr_setpshared函数：  修改mutex属性。

              int pthread_mutexattr_setpshared(pthread_mutexattr_t *attr, int pshared);

              参2：pshared取值：

                     线程锁：PTHREAD_PROCESS_PRIVATE (mutex的默认属性即为线程锁，进程间私有)

                     进程锁：PTHREAD_PROCESS_SHARED

进程间mutex示例

进程间使用mutex来实现同步：

#include <fcntl.h>
#include <pthread.h>
#include <sys/mman.h>
#include <sys/wait.h>

struct mt {
    int num;
    pthread_mutex_t mutex;
    pthread_mutexattr_t mutexattr;
};

int main(void)
{
    int fd, i;
    struct mt *mm;
    pid_t pid;

    fd = open("mt_test", O_CREAT | O_RDWR, 0777);
    ftruncate(fd, sizeof(*mm));
    mm = mmap(NULL, sizeof(*mm), PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
    close(fd);
    unlink("mt_test");
    //mm = mmap(NULL, sizeof(*mm), PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANON, -1, 0);
    memset(mm, 0, sizeof(*mm));

    pthread_mutexattr_init(&mm->mutexattr);                                  //初始化mutex属性对象
    pthread_mutexattr_setpshared(&mm->mutexattr, PTHREAD_PROCESS_SHARED);    //修改属性为进程间共享
    pthread_mutex_init(&mm->mutex, &mm->mutexattr);                          //初始化一把mutex琐

    pid = fork();
    if (pid == 0) {
        for (i = 0; i < 10; i++) {
            pthread_mutex_lock(&mm->mutex);
            (mm->num)++;
            printf("-child----num++   %d\n", mm->num);
            pthread_mutex_unlock(&mm->mutex);
            sleep(1);
        }
    } else if (pid > 0) {
        for ( i = 0; i < 10; i++) {
            sleep(1);
            pthread_mutex_lock(&mm->mutex);
            mm->num += 2;
            printf("-parent---num+=2  %d\n", mm->num);
            pthread_mutex_unlock(&mm->mutex);
        }
        wait(NULL);
    }

    pthread_mutexattr_destroy(&mm->mutexattr);          //销毁mutex属性对象
    pthread_mutex_destroy(&mm->mutex);                //销毁mutex
    munmap(mm,sizeof(*mm));                          //释放映射区
    return 0;
}        

运行结果：

ubuntu1604@ubuntu:~/wangqinghe/linux/20190821$ ./process_mutex

---------parent-------num+=2   2

-child-----------------num++   3

---------parent-------num+=2   5

-child-----------------num++   6

---------parent-------num+=2   8

-child-----------------num++   9

---------parent-------num+=2   11

-child-----------------num++   12

---------parent-------num+=2   14

-child-----------------num++   15

---------parent-------num+=2   17

-child-----------------num++   18

---------parent-------num+=2   20

-child-----------------num++   21

---------parent-------num+=2   23

-child-----------------num++   24

---------parent-------num+=2   26

-child-----------------num++   27

---------parent-------num+=2   29

-child-----------------num++   30