Passing variable arguments to another function

[greeneka]

Hi All,

I'm writing a debug logging function and was wondering if there's any way to pass variable arguments straight to another function without touching them.

For example I want to do something like this:

#include <stdio.h>

const int maxlevel = 3;
void debugLog(int debuglevel, char *msg, ...);

void main(){
	debugLog(2, &quot;should be printed%d&quot;, 5);
	debugLog(5, &quot;should not be printed%d&quot;, 8);
}

void debugLog(int debuglevel, char *msg, ...){
	if(debuglevel < maxlevel){
		printf(msg, ...);
	}
}

This doesn't compile, but you can see the idea - passing the var args straight through the debugLog function to the printf function without having to touch them.

Any help or pointers would be much appreciated.

Thanks in advance.

 

[palbano]

>> any way to pass variable arguments straight to another
>> function without touching them.

No.


-pete

 

[Salem]

Seems easy enough...

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

const int maxlevel = 3;
void debugLog(int debuglevel, char *msg, ...);

int main ( ) { //!! main returns int
    debugLog(2, &quot;should be printed %d\n&quot;, 5);
    debugLog(5, &quot;should not be printed %d\n&quot;, 8);
    return 0;
}

void debugLog(int debuglevel, char *msg, ...){
    va_list ap;
    va_start(ap,msg);
    if(debuglevel < maxlevel){
        vprintf(msg, ap);
    }
    va_end(ap);
}

--

http://www.catb.org/~esr/faqs/smart-questions.html

 

[palbano]

Salem,

Oh right! You didn't touch them, you used your keyboard

-pete

 

[greeneka]

Thanks for the replies.

Salem - The example I gave was contrived, it's not printf I'm calling but a different function that takes variable arguments that does more than just print to the console (I just thought printf would be the easiest example). So I was hoping to be able to modify my debug code and still be able to call the same function.
I *could* split the function I want to call in two (one taking variable arguments, and the other taking a va_list) but I'd rather not!

Palbano - After some digging around last night I think your answer is (unfortunately!) the one I needed. For those interested, I came accross this while trying to figure it out:

http://www.eskimo.com/~scs/C-faq/varargs/invvarargs.19920714.html

Apparently this problem has already been named the "inverse varargs problem"!

Anyway, thanks for the help.

 

[PerFnurt]

Note that in a world of overloaded operators and functions specified by their parameters, the ... solution is, well, obsolete.


/Per
[sub]
if (typos) cout << &quot;My fingers are faster than my brain. Sorry for the typos.&quot;;
[/sub]

 

[greeneka]

P.S. Apologies - I meant to post the solution I'm using:

I vsprintf'ed the data into a buffer and used that:

void debugLog(int debuglevel, char *msg, ...){
    if(debuglevel < maxlevel){
 		char buffer[500];
		va_list argptr;
		va_start(argptr,msg);
		vsprintf( buffer, msg, argptr ); 
		printf(buffer); // well not printf but you get the idea!:)
	}
}

 

[greeneka]

PerFnurt - true, but I have to call a previously defined function that uses the ellipsis, so I have to work with it!

 

[Salem]

> it's not printf I'm calling but a different function that takes variable arguments
I would say that the &quot;proper&quot; answer would be to make it all more printf-like.

Eg. if you have

void foo ( char *msg, ... );

then you should also have

void vfoo ( char *msg, va_list ap );

Where foo is actually implemented as a simple wrapper for calling vfoo(), and vfoo() does all the work foo() was doing anyway.

void foo ( char *msg, ... ) {
  va_list ap;
  va_start(ap,msg);
  vfoo(msg,ap);
  va_end(ap);
}

Then from the application, you can call foo() for convenience, and vfoo() if necessary.

If you don't own foo(), then go and hassle the author to provide this interface

--

http://www.catb.org/~esr/faqs/smart-questions.html