互斥锁线程

Question

我是多线程/流程编程的新手.所以这就是我需要澄清的内容.

处理代码

pthread_mutex_lock()
    pthread_create(fooAPI(sharedResource)) //fooAPI creates another thread with shared resource that shares across processes.
pthread_mutex_unlock()

使用上面的伪代码,sharedResource如果未解锁互斥锁,进程B是否能够访问？

如何正确访问进程B中的sharedResource？

有没有明确的可视图解释互斥体,线程和进程之间的关系？

Answer 1

你需要做的是调用pthread_mutex_lock来保护互斥锁,如下所示:

pthread_mutex_lock(&mutex);

执行此操作后,pthread_mutex_lock(mutex)在您调用pthread_mutex_unlock此线程之前,任何其他调用都不会返回.因此,如果您尝试调用pthread_create,您将能够创建一个新线程,并且该线程将能够(错误地)使用共享资源.您应该pthread_mutex_lock在fooAPI函数内部调用,这将导致函数等待共享资源可用.

所以你会有这样的事情:

#include <pthread.h>
#include <stdio.h>

int sharedResource = 0;
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

void* fooAPI(void* param)
{
    pthread_mutex_lock(&mutex);
    printf("Changing the shared resource now.\n");
    sharedResource = 42;
    pthread_mutex_unlock(&mutex);
    return 0;
}

int main()
{
    pthread_t thread;

    // Really not locking for any reason other than to make the point.
    pthread_mutex_lock(&mutex);
    pthread_create(&thread, NULL, fooAPI, NULL);
    sleep(1);
    pthread_mutex_unlock(&mutex);

    // Now we need to lock to use the shared resource.
    pthread_mutex_lock(&mutex);
    printf("%d\n", sharedResource);
    pthread_mutex_unlock(&mutex);
}

编辑:跨进程使用资源遵循相同的基本方法,但您需要将内存映射到其他进程.这是一个使用shmem的例子:

#include <stdio.h>
#include <unistd.h>
#include <sys/file.h>
#include <sys/mman.h>
#include <sys/wait.h>

struct shared {
    pthread_mutex_t mutex;
    int sharedResource;
};

int main()
{
    int fd = shm_open("/foo", O_CREAT | O_TRUNC | O_RDWR, 0600);
    ftruncate(fd, sizeof(struct shared));

    struct shared *p = (struct shared*)mmap(0, sizeof(struct shared),
        PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

    p->sharedResource = 0;

    // Make sure it can be shared across processes
    pthread_mutexattr_t shared;
    pthread_mutexattr_init(&shared);
    pthread_mutexattr_setpshared(&shared, PTHREAD_PROCESS_SHARED);

    pthread_mutex_init(&(p->mutex), &shared);

    int i;
    for (i = 0; i < 100; i++) {
        pthread_mutex_lock(&(p->mutex));
        printf("%d\n", p->sharedResource);
        pthread_mutex_unlock(&(p->mutex));
        sleep(1);
    }

    munmap(p, sizeof(struct shared*));
    shm_unlink("/foo");
}

编写程序以对p-> sharedResource进行更改留给读者练习.:-)

顺便说一下,忘记注意互斥锁必须设置PTHREAD_PROCESS_SHARED属性,以便pthreads可以跨进程工作.