Passing of variable arguments in C

Question

Does anybody know how variable arguments are passed in classic C? I did some debugging today and most regular arguments are passed via stack. However it seems that this does not apply for variable arguments. Are those parameters stored somewhere else like constant strings?

Thanks in advance!

score 3 · Answer 1 · answered Mar 31 '11 at 19:41

3

It depends on the platform. /usr/include/stdarg.h is the place to start looking for details.

answered Mar 31 '11 at 19:41

geekosaur

59,309
11
123
114

On Unix, yes. On other platforms, `stdarg.h` may be elsewhere. +1, though. – Fred Foo Mar 31 '11 at 19:44
While technically true, it is also true that you would have a hard time finding many platforms where they are NOT passed on the stack. – Prof. Falken Mar 31 '11 at 19:48
@Amigable: I vaguely recall SPARC doing some interesting things with the register window for varargs, but it's been a while and I don't have access to one any more. – geekosaur Mar 31 '11 at 19:50
But Win32 is not this special, is it :) – Florian Greinacher Mar 31 '11 at 19:53
1

@Florian: see the previous comment, which suggests that some things are passed in registers on x86-64. x86 will be all stack, though: it doesn't have enough registers to even *consider* using them for argument passing. – geekosaur Mar 31 '11 at 19:58

Prof. Falken · Accepted Answer · 2011-03-31T19:54:02.257

2

They are very often passed on the stack. What you are looking for is ABI specifications for the platform you are using.

For the AMD64 platform, have a look for example here.

edited Mar 31 '11 at 19:54

answered Mar 31 '11 at 19:41

Prof. Falken

24,226
19
100
173

Thats what I expected too. But I encountered a situation where I could not locate the variable arguments in the memory window of Visual Studio... Do have any documentation on parameter passing in C? – Florian Greinacher Mar 31 '11 at 19:52
@Florian Greinacher, this is strange. I don't know what to say further, except find ABI docs for Windows. – Prof. Falken Mar 31 '11 at 19:56
Thanks, I think this will help finding an answer :) – Florian Greinacher Mar 31 '11 at 19:59

score 1 · Answer 3 · answered Mar 31 '11 at 19:40

1

You meant Variable-Length Argument Lists?

answered Mar 31 '11 at 19:40

Sadique

22,572
7
65
91

Thanks for your answer, but I know how to use them. My question is how they are passed to the callee. – Florian Greinacher Mar 31 '11 at 19:44

score 0 · Answer 4 · answered Nov 27 '11 at 04:36

0

Here's a fun trick

void func(type* values) {
    while(*values) {
        x = *values++;
        /* do whatever with x */
    }
}

func((type[]){val1,val2,val3,val4,0});

answered Nov 27 '11 at 04:36

Rose Perrone

61,572
58
208
243

Passing of variable arguments in C

4 Answers4

Linked