CODEWITHSUNDEEP
c++winapivisual-c++traceetw
Uri Cohen
Uri Cohen

Reputation: 3608

Ellipsis and va_args trick needed

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!

Upvotes: 3

Views: 787

Answers (1)

CB Bailey
CB Bailey

Reputation: 792069

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;
}

Upvotes: 4

Related Questions