C函数中的变量参数 [英] Variable arguments in C functions

问题描述

我已经阅读了有关变量参数函数"int func(int, ...)"的信息. 这些函数的参数在哪里分配(堆栈或堆)?

I've read about variable arguments functions "int func(int, ...)". Where do the arguments of these functions get allocated (stack or heap)?

因为我读到va_end()宏释放了分配给va_list的空间，所以"frees"一词引起了我的注意.

Because I read that the va_end() macro frees space assigned to va_list, so that word "frees" caught my eyes.

注意:我知道常规函数会进入堆栈，但是这种类型的函数很有趣，因为参数的数量未知.

Note: I know that regular functions go to stack, but this type of function is interesting as the number of arguments is not known.

我只想确定这不像没有预定义空间的数组； 我们在最后使用malloc()和free().

I just want to know for sure that it's not like arrays with no pre-defined space; we use malloc() and free() at the end.

链接: https://www.tutorialspoint.com/cprogramming/c_variable_arguments.htm

推荐答案

c 本身未指定堆"或堆栈"之类的内容，因此对标准和可移植的

c itself doesn't specify things like "heap" or "stack", so programming standard and portable c, you should better think in categories of the c standard: static, automatic and dynamic storage.

尽管如此，在典型的实现中，自动存储"转换为用于堆栈的堆栈".函数参数就是这种情况，可变参数函数也不例外.

Nevertheless, in a typical implementation, "automatic storage" translates to "the stack is used for it". This is the case for function arguments and variadic functions are no exception here.

va_end()可能释放一些动态存储(通常是:分配在堆上)的原因是va_arg()宏通常需要一些上下文信息来找到 next 参数. va_start()将为该信息分配内存(而不是为参数本身分配内存)，并以某种方式对其进行初始化，以使第一个va_arg()调用返回第一个可变参数.

The reason va_end() might free some dynamic storage (typically: allocated on the heap) is that the va_arg() macro typically needs some context information to find the next argument. va_start() will allocate the memory for that information (not for the arguments themselves) and initialize it in a way so that the first va_arg() call returns the first variadic argument.

请注意，va_start()的实现没有必须分配内存. va_list的定义方式可以为所需的上下文信息提供空间.但是作为程序员，您应该完全没有兴趣，所有va_*内容对您来说都是黑匣子，并且如果文档中注明致电完成清理工作后，您就可以做到；)

Note that an implementation of va_start() doesn't have to allocate memory. va_list might be defined in a way that it provides the space for the required context information. But that should be of no interest at all to you as the programmer, all the va_* stuff is a black box for you and if the documentation states "call va_end() when done for cleanup", you just do it ;)

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