到底是va_end用来做什么？是否总是需要调用它呢？ [英] What exactly is va_end for? Is it always necessary to call it?

问题描述

va_end用来 - 宏重置 arg_ptr

访问变量参数列表之后， arg_ptr 指针通常是与重置va_end用来（）。我明白，如果你想再次重申列表中它是必需的，但是否真的需要，如果你不打算？难道仅仅是很好的做法，像治始终有一个默认值：在开关

After accessing a variable argument list, the arg_ptr pointer is usually reset with va_end(). I understand that it is required if you want to re-iterate the list, but is it really needed if you aren't going to? Is it just good practice, like the rule "always have a default: in your switch"?

推荐答案

va_end用来是用来做清理工作。你不想破坏堆栈，你呢？

va_end is used to do cleanup. You don't want to smash the stack, do you?

从男人的va_start ：

va_end用来（）

va_end()

的va_start的每次调用（）必须由va_end用来（）中相同功能的相应调用相匹配。呼叫va_end用来（AP）之后的变量Ap是不确定的。列表的多个遍历，各由的va_start（）va_end用来（）括号内是可能的。 va_end用来（）可以是宏或功能

Each invocation of va_start() must be matched by a corresponding invocation of va_end() in the same function. After the call va_end(ap) the variable ap is undefined. Multiple traversals of the list, each bracketed by va_start() and va_end() are possible. va_end() may be a macro or a function.

请注意这个词的的presence必须

该堆栈就被破坏了，因为的你不知道什么是的va_start（）做的。在 VA _ * 宏是为了被视为黑盒子。每个平台上每一个编译器可以为所欲为那里。它可能什么也不做，也可以做很多事情。

The stack could become corrupted because you don't know what va_start() is doing. The va_* macros are meant to be treated as black boxes. Every compiler on every platform can do whatever it wants there. It may do nothing, or it may do a lot.

一些ABI在寄存器中传递的前几ARGS，并在堆栈中剩余部分。 A 在va_arg（）有可能更为复杂。您可以查找一个给定的实施如何做可变参数，这可能是有趣的，但以书面便携code，你应该把它们看作是不透明的操作。

Some ABIs pass the first few args in registers, and the remainder on the stack. A va_arg() there may be more complicated. You can look up how a given implementation does varargs, which may be interesting, but in writing portable code you should treat them as opaque operations.

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