调用具有未知数量的参数的可变函数 [英] Calling a variadic function with an unknown number of parameters

问题描述

说我有一个函数，它接受可变数量的参数：我想从其他地方调用这个函数，构建参数列表，但不知道提前需要多少参数。

Say I have a function that takes a variable number of parameters: I want to call this function from somewhere else, building the list of parameters, but without knowing in advance how many parameters I'll need.

抱歉，解释不太清楚，希望这个程式码能让我的问题更清楚：

Sorry that's not well explained, hopefully this code makes my question clearer:

void foo(int n, ...) {
    va_list vl;
    va_start(vl,n);

    for (int i = 0; i<n; i++) {
      // Do something to each passed variable
    }
}

该函数正在从这个函数调用：

That function's being called from this one:

void bar(int howManyParams) {

  // Here I want to call foo() with howManyParams parameters
  // (values are irrelevant for the question)
  // 
  // I.e. for howManyParams = 1, we should call foo(0)
  //      for howManyParams = 2, we should call foo(0,0)
  //      for howManyParams = 3, we should call foo(0,0,0)
  // etc.
  //

}

推荐答案

实际上在运行时构建一个可变长度的参数列表 - 这是我确定你想要做的 - 是很棘手。

Actually building a variable-length argument list at run-time -- which is what I'm pretty sure you're trying to do -- is pretty tricky. There's no way to do it at all in Standard C, but there are various tricks you can try.

也许最好的是外部函数接口库在 http://sourceware.org/libffi/ 。

Perhaps the best is the "Foreign Function Interface Library" at http://sourceware.org/libffi/ .

另请参阅C常见问题列表中的问题15.13： http://c-faq.com/varargs/invvarargs .html

See also question 15.13 in the C FAQ list: http://c-faq.com/varargs/invvarargs.html

另请参阅以前的Stackoverflow问题：

使用未知参数的后期绑定

如何通过在C中传递多个参数的指针调用函数？

See also these previous Stackoverflow questions:
C late binding with unknown arguments
How to call functions by their pointers passing multiple arguments in C?

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