互斥量mutex

程序間也可以使用互斥鎖，來達到同步的目的。但應在pthread_mutex_init初始化之前，修改其屬性為程序間共享。mutex的屬性修改函式主要有以下幾個。

互斥鎖API

pthread_mutexattr_t mattr 型別 //用於定義mutex鎖的【屬性】

int pthread_mutexattr_init(pthread_mutexattr_tattr); //初始化一個mutex屬性物件 int pthread_mutexattr_destroy(pthread_mutexattr_tattr); //銷燬mutex屬性物件 (而非銷燬鎖)

intpthread_mutexattr_setpshared(pthread_mutexattr_t *attr, int pshared); //修改mutex屬性

• 參2：pshared取值
  • 執行緒鎖：PTHREAD_PROCESS_PRIVATE(mutex的預設屬性即為執行緒鎖，程序間私有)
  • 程序鎖：PTHREAD_PROCESS_SHARED

示例程式碼

#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屬性物件
    pthread_mutexattr_setpshared(&mm->mutexattr, PTHREAD_PROCESS_SHARED);    //修改屬性為程序間共享
    pthread_mutex_init(&mm->mutex, &mm->mutexattr);                          //初始化一把mutex瑣
 
    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);          //銷燬mutex屬性物件
    pthread_mutex_destroy(&mm->mutex);                //銷燬mutex
    munmap(mm,sizeof(*mm));                          //釋放對映區
    return 0;
}