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I was wondering how does a C compiler (I tried gcc and clang) organize the argument list of functions internally. For this purpose I made a program consisting of two files, for instance:

foo.c :

double foo (double (*f) (void *, double), void * arg, double x)
{
    return f (arg, x);
}

main.c :

#include <stdio.h>

extern double foo (double (*) (double), double);

double bar (double x)
{   
    return x + x;
}

int main ()
{   
    double x = foo (&bar, 2);
    printf ("%g\n", x);
    return 0;
}

and played with the argument list and the body of the foo in the foo.c. When foo.c is not consistent with definitions in the main.c the behavior of the program should be undefined according to the C standard (if I understand correctly?).

I have tried several variants (including the one above), for which the program will print 4. The one of the more exotic is

double foo (double x, double (*f) (char, double, int), int r)
{
    char a;
    return f (a, x, r);
}

. However, if I would try something like

double foo (double x, double (*f) (char, double, double), double y)
{
    char a;
    return f (a, y, x);
}

the result would not be 4, but if I write f(a, x, y) instead, I get 4 again.

The experiment makes me think that the argument list is internally represented as a list of arrays corresponding to different types, in which the information about the order of the arguments of different types is lost. For example, the argument list (char a, double x, int i, double y, char b) would be something like (char:{a, b}, int:{i}, double:{x, y}) and the casting to the (double z, char c) would equal (char:{c = a}, double:{z = x}), where I have defined T:{} as an array of type T.

So:

Is my interpretation correct?

Is this behavior standardized somewhere?

How much can I rely on such a behavior?

This behavior allows some generic programming. Does anybody exploit it in practice?

thanks!

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1 回答 1

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如果函数的签名不匹配，则不能依赖任何行为。您的程序可以正常运行、崩溃或发射火箭导弹。这是未定义行为的定义。

如果您使用 variadict 函数或未描述参数的函数（即 f()，不要与 f(void) 混淆），则只能依赖某些东西。对于那些，有关于参数提升和转换的规则。查找 C99 标准以了解更多信息。

其余的纯属猜测。如果你知道你的处理器的汇编语言是如何工作的，你就可以观察和推断出一些行为。例如在 x86 中，调用函数和传递参数的标准方法已经定义良好......

无论如何，依赖未定义的行为并不是一个好主意。

于 2013-01-17T13:54:19.943 回答