5 线程初始化时栈地址为什么加4

发布于 2022-09-20 08:54:47 浏览：543 订阅该版

```
static rt_err_t _rt_thread_init(struct rt_thread *thread,
                                const char       *name,
                                void (*entry)(void *parameter),
                                void             *parameter,
                                void             *stack_start,
                                rt_uint32_t       stack_size,
                                rt_uint8_t        priority,
                                rt_uint32_t       tick)
{
    /* init thread list */
    rt_list_init(&(thread->tlist));

thread->entry = (void *)entry;
    thread->parameter = parameter;

/* stack init */
    thread->stack_addr = stack_start;
    thread->stack_size = stack_size;

/* init thread stack */
    rt_memset(thread->stack_addr, '#', thread->stack_size);
#ifdef ARCH_CPU_STACK_GROWS_UPWARD
    thread->sp = (void *)rt_hw_stack_init(thread->entry, thread->parameter,
                                          (void *)((char *)thread->stack_addr),
                                          (void *)rt_thread_exit);
#else
    thread->sp = (void *)rt_hw_stack_init(thread->entry, thread->parameter,
                                          (void *)((char *)thread->stack_addr + thread->stack_size - 4),
                                          (void *)rt_thread_exit);
#endif
```
根据宏定义的不同，栈初始化函数的参数不一样。一个是stackaddr，一个是`stackaddr+stacksize-4`.
在rt_hw_stack_init函数里，
` stk  = stack_addr + sizeof(rt_uint32_t);`
说明前面减去的4，在这里被加回来了。
那看条件编译的第一个分支，栈向上生长的话，没有减4，相当于在函数里单纯的加了个4，为什么要加个4呢