CODEWITHSUNDEEP
c++parsingvariadic-functions
Tox1k
Tox1k

Reputation: 105

Getting ellipses function parameters without an initial argument

So I've been making a custom parser for a scripting language, and I wanted to be able to pass only ellipses arguments. I don't need or want an initial variable, however Microsoft and C seem to want something else. FYI, see bottom for info.

I've looked at the va_* definitions

#define _crt_va_start(ap,v)  ( ap = (va_list)_ADDRESSOF(v) + _INTSIZEOF(v) )
#define _crt_va_arg(ap,t)    ( *(t *)((ap += _INTSIZEOF(t)) - _INTSIZEOF(t)) )
#define _crt_va_end(ap)      ( ap = (va_list)0 )

and the part I don't want is the v in va_start. As a little background I'm competent in goasm and I know how the stack works so I know what's happening here. I was wondering if there is a way to get the function stack base without having to use inline assembly.

Ideas I've had:

#define im_va_start(ap) (__asm { mov [ap], ebp })

and etc... but really I feel like that's messy and I'm doing it wrong.

struct function_table {
    const char* fname;
    (void)(*fptr)(...);
    unsigned char maxArgs;
};
function_table mytable[] = {
{ "MessageBox", &tMessageBoxA, 4 } };

... some function that sorts through a const char* passed to it to find the matching function in mytable and calls tMessageBoxA with the params. Also, the maxArgs argument is just so I can check that a valid number of parameters is being sent. I have personal reasons for not wanting to send it in the function, but in the meantime we can just say it's because I'm curious.

This is just an example; custom libraries are what I would be implementing so it wouldn't just be calling WinAPI stuff.

void tMessageBoxA(...) {
// stuff to load args passed
MessageBoxA(arg1, arg2, arg3, arg4);
}

I'm using the __cdecl calling convention and I've looked up ways to reliably get a pointer to the base of the stack (not the top) but I can't seem to find any. Also, I'm not worried about function security or typechecking.

edit: Thanks for input, it appears it wasn't possible.

My fix has ended up being

    #define im_va_start(ap) {\
        __asm push eax\
        __asm mov eax, ebp\
        __asm add eax, 8h\
        __asm mov ap, eax\
        __asm pop eax\
    }

And then I can continue as normal.

As for why I require it, I'm doing some (unique) read: unsafe tricks and using a struct array with a pointer to the function as defined above. Since each function is unique, and most are from my custom library, they have... different behaviors. I don't really know how to explain it, but I'll release the source when I finish the POC.

I'm not really worried about portability so that'll have to work. Also, for counting the args I did: 

#define im_va_count(ap, num, t) {\
for(num = 0; *(t*)((ap += _INTSIZEOF(t)) - _INTSIZEOF(t)) > 0; ++num){ }\
--num;\
im_va_start(argptr);\
}

which works for me. If anyone's interested...

Upvotes: 2

Views: 1327

Answers (1)

Seth Carnegie
Seth Carnegie

Reputation: 75150

Unfortunately this is not possible, the C standard says:

A function may be called with a variable number of arguments of varying types. As described in 6.9.1, its parameter list contains one or more parameters.

And the ... doesn't count as the "one or more parameters". Further,

The va_start macro shall be invoked before any access to the unnamed arguments.

That it should be called like

va_start(va_list, parmN)

And that

The parameter parmN is the identifier of the rightmost parameter in the variable parameter list in the function definition (the one just before the , ...).

So as you can see, you cannot have a variadic function without at least one parameter before the ellipsis in standard C++. The non-portable assembly trick is the closest you can come.

Upvotes: 2

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