The calling function must not care where the stack pointer is currently pointing.
调用的函数一定不能顾虑栈指针当前所指的位置。
This protects against stack, buffer, and function pointer overflows, all without recompilation.
这样将保护程序代码避免堆栈、缓冲区和函数指针溢出，而且全都不需重新编译。
If a function is executing, then the stack pointer is pointing to the top of the whole stack, which is also the top of that function's activation record.
如果一个函数正在执行，那么堆栈指针就会指向整个堆栈的顶部，这也是该函数活动记录的顶部。
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