如何将本地结构作为参数传递给pthread_create？

Question

The following function doesn't work. pin_thread_function sometimes receive garbage instead of the struct data. What is wrong? I know that is some basic scope related problem, but I can't explain.

typedef void (*state_callback_t)(int state);

struct pin_thread_params
{
    char pin[3 + 1];
    state_callback_t callback_onchange;
};

extern int pin_monitor_create(const char * pin,
        state_callback_t callback_onchange)
{
    int ret;
    unsigned long int thread;
    struct pin_thread_params params;

    //Setup struct
    strcpy(params.pin, "123");
    params.callback_onchange = callback_onchange;

    //Create thread with struc as parameter
    ret = pthread_create(&thread, NULL, pin_thread_function, &params);
    if (!ret)
    {
        ret = pthread_detach(thread);
    }

    return ret;
}

static void * pin_thread_function(void * context)
{
    struct pin_thread_params params;
    memcpy(&params, context, sizeof(params));

    //Sometimes the correct string, most time garbage 
    printf("Started monitoring %s", params.pin);
}

When params is malloc'ed before pthread_create, everything works fine, like this:

...
    struct pin_thread_params * params;

    //Setup struct with malloc
    params = malloc(sizeof(struct pin_thread_params));
    strcpy(params->pin, "123");
    params->callback_onchange = callback_onchange;
...

Answer 1

struct pin_thread_params params is statically allocated and the address of it is not safe to use once the scope of it is over (ie after pin_monitor_create has returned). It may happen that sometimes the thread execution starts before the pin_monitor_create has exited and you see the valid data in params . However, the dynamically allocated memory is from heap and will be usable until you free it, so you always get valid data in params within pin_thread_function .

Answer 2

我对pthreads并不是特别了解（还不能发表评论），但是您正在向该线程传递一个指向堆栈的已分配内存的指针，该指针最终将被进行函数调用所破坏。