Ç可变参数 - va_copy问题 [英] C varargs - va_copy issues
问题描述
我在C语言编写一个函数，它采用可变数量的参数。
I'm writing a function in C that takes a variable number of arguments.
size_t myprintf(char *fmt, ...);
到目前为止，一切都很好。我已经决定这是最好的做事情的正确方法™，使一个版本，采用可变参数，另一个版本，需要一个的va_list 。
size_t myprintf(char *fmt, ...);
size_t myvprintf(char *fmt, va_list args);
并不难做到。除了 my_vprintf（）需要发送的 ARGS 到两个不同的功能（第一个的snprintf （），长度为0，以确定我们的房间需要多少，然后的sprintf（）之后，我们已经分配了很大的空间）。我这样做与 va_copy 。
Not that hard to do. Except my_vprintf() needs to send its args out to two different functions (first to snprintf() with a length of 0 to determine how much room we need, then to sprintf() after we've allocated that much room). I do this with va_copy.
size_t myvprintf(char *fmt, va_list args)
{
va_list args2;
va_copy(args, args2);
// do stuff with args2
va_end(args2);
// do more stuff with args
}
这是所有罚款和花花公子，但C99是有点执行不力。我想，如果可能的话，我的code在C89以及工作，并与尽可能多的编译器和尽可能多的平台，尽可能的工作。我现在有这后的#include＆LT;＆STDDEF.H GT; 但在此之前的任何code：
This is all fine and dandy, but C99 is a bit poorly implemented. I would like, if possible, for my code to work in C89 as well, and to work with as many compilers and on as many platforms as possible. I currently have this after #include <stddef.h> but before any code:
#ifndef va_copy
# ifdef __va_copy
# define va_copy(a,b) __va_copy(a,b)
# else /* !__va_copy */
# define va_copy(a,b) ((a)=(b))
# endif /* __va_copy */
#endif /* va_copy */
我认为，导致（（A）=（B））是不可靠的，而我也许应该使用的memcpy（）或类似的东西，但是这仍然是对的：如果你不支持C99，我希望工程，而不是如果你不支持C99，从不畏惧（这是什么水平我想要）。有没有什么好的办法来解决这个限制？我见过几个解决方案 - 即吃一个参数和递归的va_list 函数，传递的va_list 两次这样两个单独的副本是由等 - 但我不知道他们会如何工作（和递归解决方案不会做的这么好，如果我只是想打电话给 vsnprintf（），现在会吗？）。
I am led to believe that ((a)=(b)) is unreliable, and that I should perhaps use memcpy() or something similar, but this is still on the level of "If you don't support C99, I hope it works" rather than "If you don't support C99, never fear" (which is what I want). Is there any good way to get around this limitation? I've seen a few solutions - va_list functions that eat one argument and recurse, passing the va_list twice so that two separate copies are made, etc. - but I don't know how well they would work (and the recursive solution won't do so well if I just want to call vsnprintf(), now, will it?).
所以我转交给你，StackOverflow的用户。有什么更多的，我可以做些什么来提供C89的兼容性，或者是没有 va_copy 用户和 __ va_copy （当然少之又少之间），只是将不得不给它吸走了吗？
So I turn to you, StackOverflow User. Is there anything more I can do to provide C89 compatibility, or are users without va_copy and __va_copy (admittedly few and far between) just going to have to suck it in and take it?
推荐答案
（A）=（B）是靠不住的。即使传递两个的va_list 是（公共函数是一个简单的包装）为va_list的可能是这样的：
(a)=(b) is unreliable. Even passing two va_list is (the public function would be a simple wrapper) as va_list can be something like:
typedef __va_list_impl va_list[1];
有关这从一个做的va_arg将修改其他（我似乎记得使用的Solaris东西这类啊，寄存器窗口...）。可悲的是，我知道没有法子，做你想做的。
for which doing a va_arg on one will be modify the other (I seem to remember that Solaris use such kind of thing, ah, the register windows...). Sadly, I know of no sure way to do what you want.
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