C：如何安全，正确地传递几个参数到pthread的？ [英] C: How to safely and properly pass several arguments to a pthread?

问题描述

考虑一个简单的code：

Consider this simple code:

void* threadFunction(void* arg) {

    int argument=(int)arg;

    printf("%d recieved\n", argument);

    return NULL;
}


int main(int argv, char* argc[]) {

    int error;
    int i=0;
    pthread_t thread;

    int argument_to_thread=0;

    if ((error=pthread_create(&thread, NULL, threadFunction, (void*)argument_to_thread))!=0) {
        printf("Can't create thread: [%s]\n", strerror(error));
        return 1;
    }

    pthread_join(thread, NULL);


    return 0;
}

这工作，但两件事情在这里打扰我。结果
首先，我要多于一个参数发送到threadFunction（）。结果
当然，我可以传递一个指向数组的指针，但如果我想通过不同类型的两个参数是什么？ （比方说一个 INT 和的char * ）如何能不能做到？

This works, but two things bother me here.
First, I want to send more than one argument to threadFunction().
Of course, I can pass a pointer to an array, but what if I want to pass two arguments of different types? (say an int and char*) How can it be done?

在这里困扰我的第二件事，就是编制以上时，我得到了警告...

The second thing that bothers me here, is the warnings I get when compiling the above...

test2.c: In function ‘threadFunction’:
test2.c:8:15: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
  int argument=(int)arg;
               ^
test2.c: In function ‘main’:
test2.c:24:59: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
  if ((error=pthread_create(&thread, NULL, threadFunction, (void*)argument_to_thread))!=0) {
                                                           ^

现在，我可以让这个去如这样做：

Now, I can make this go away by doing this:

if ((error=pthread_create(&thread, NULL, threadFunction, (void*)&argument_to_thread))!=0) {
    printf("Can't create thread: [%s]\n", strerror(error));
    return 1;
}

但是，让我们说，我不希望通过引用传递吧...有没有办法作为参数传递，比如......一个int，由值，而编译器警告我？

But let's say I don't want to pass it by reference... Is there way to pass, say... an int, by value as an argument without the compiler warning me?

推荐答案

有两种方法可以做到这一点：

There are two ways to do this:

1. 使用的malloc 来获得的参数结构存储，并有新的线程负责调用免费一旦它的参数完成。 （注意：如果在pthread_create 失败，则需要免费此存储中的失败路径。）

1. Use malloc to obtain storage for the arguments structure, and have the new thread be responsible for calling free once it's done with the arguments. (Note: if pthread_create fails, you need to free this storage in the failure path.)

使用信号量（或者其他同步，但信号是迄今为止最轻）。

Use a semaphore (or other synchronization, but a semaphore is by far the lightest).

下面是后者的一个例子：

Here's an example of the latter:

struct args {
    int a;
    char *b;
    sem_t sem;
};

void *thread_func(void *p)
{
    struct args *args = p;
    int a = args->a;
    char *b = args->b;
    sem_post(args->sem);
    ...
}

    /* in caller */
    struct args args;
    args.a = ...;
    args.b = ...;
    sem_init(&args.sem, 0, 0);
    pthread_create(&tid, 0, thread_func, &args);
    sem_wait(&args.sem);
    ...

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