va_list 的重用 [英] Reuse of va_list

问题描述

我需要对 va_list 执行两次(或多次)传递.我有一个一定大小的缓冲区，我想用 sprintf 将格式化的字符串写入其中.如果格式化的字符串不适合分配的空间，我想将分配的空间加倍并重复直到适合.

I need to do two (or more) passes over a va_list. I have a buffer of some size, and I want to write a formatted string with sprintf into it. If the formatted string doesn't fit in the allocated space I want to double the allocated space and repeat until it fits.

(作为旁注，我希望能够首先计算格式化字符串的长度并分配足够的空间，但我发现可以做到这一点的唯一函数是 _snprintf，它在 VS2005 中已弃用...)

(As a side-note, i would like be able to calculate the length of the formatted string first and allocate enough space, but the only function that I found that can do that is _snprintf, and it is deprecated in VS2005 ...)

现在，到目前为止没有问题:我使用 vsnprintf 并在每次调用之前调用 va_start.

Now, so far there are no problems: i use vsnprintf and call va_start before each invokation.

但我还创建了一个函数，它以 va_list 作为参数，而不是...".那我就不能再使用 va_start 了！我已经阅读了 va_copy，但它在 VS2005 中不受支持.

But I've also created a function that takes a va_list as a parameter, instead of "...". Then I cannot use va_start again! I've read about va_copy, but it is not supported in VS2005.

那么，你会怎么做?

推荐答案

我看到没有可移植的方式(我认为 va_copy 已在 C99 中引入，因为在 c89 中没有可移植的方式来实现其结果).va_list 可以是声明为

I see of no portable way (and I think that va_copy has been introduced in C99 because there was no portable way to achieve its result in c89). A va_list can be a reference type mock up declared as

typedef struct __va_list va_list[1];

(有关该技巧的另一个用户，请参阅 gmp)，这解释了围绕它们的许多语言限制.顺便说一句，如果可移植性很重要，请不要忘记 va_end.

(see gmp for another user of that trick) and that explains a lot of the language constraints around them. BTW, don't forget the va_end if portability is important.

如果可移植性不重要，我会检查 stdard.h，看看我是否可以在考虑真实声明的情况下破解某些内容.

If portability is not important, I'd check stdard.h and see if I can hack something considering the true declaration.

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