Ellipsis and va_args trick needed

Question

TraceMessage is an WinAPI function with variable number of arguments. It is a tracing function, with a notation similar to printf, which generates a trace message in Windows tracing. The weird part here is that it receive a format string as part of the ellipsis, not as a dedicated argument. It is possible to 'override' this function with a function of my own, which then needs to call TraceMessageVa (which is the same as TraceMessage, just with va_args rather than ellipsis).

So far so good; but now I want to access the traced message using a sprintf-like function, which has the format string out of the ellipsis. Thus I need to
- get the format string argument out of the ellipsis ;
- create a new va_list without the first argument.

Any idea about to how do it? Solutions specific to Visual Studio compiler are also acceptable. Thanks!

The docs for TraceMessage do not say that it does what you say it does. — , Apr 23 '11 at 09:35
That is correct, but you should believe me or just treat this as a question about ellipsis and va_args, regardless of the tracing context... — Uri Cohen, Apr 23 '11 at 10:29

score 4 · Accepted Answer · answered Apr 23 '11 at 09:38

With a va_list you can pass it to a function which takes a va_list after having used va_arg on it already to have extracted one or more arguments. The va_list will then act like it "contains" only the rest of the arguments.

I have no experience with TraceMessage itself, but I've given an example using standard vprintf and a test function. You should be able to adapt as appropriate.

E.g.

#include <stdio.h>
#include <stdarg.h>

void test(int a, ...)
{
    va_list va;
    const char* x;

    va_start(va, a);
    x = va_arg(va, const char*);

    vprintf(x, va);

    va_end(va);
}

int main(void)
{
    test(5, "%d\n", 6);
    return 0;
}

Ellipsis and va_args trick needed

1 Answers1