Sitting at my desk hacking away at a project in C, I began to think back to the good ol’ days of last summer when I was knee-deep in Flex work. Now don’t get me wrong, I like lower-level C code as much as the next guy… who… well, likes lower-level C code. But there were certain niceties about ActionScript 3 that I just miss in times like these.  One of those, of course, was trace().

Sure, printf() works fine for quickly spitting out some debugging information.  But golly, it sure would be nice to add a custom prefix to those messages, maybe even have them print to stderr while I’m at it. My desire for my buddy trace() got the best of me, and after some brief searching, I found a way to create my own custom printf() wrapper functions.

The technique is rather simple.  Declare a function that accepts a format string and a varying number of further arguments.  Pass said string and arguments to fprintf.  Do anything else desired before and after.

Here’s how to do it in C/C++:

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

void trace( const char* format, ... ) {
    va_list args;
    fprintf( stderr, "[Debug] " );
    va_start( args, format );
    vfprintf( stderr, format, args );
    va_end( args );
    fprintf( stderr, "\n" );
}

And here’s the PHP equivalent:

function trace() {
    $args = func_get_args();
    $format = array_shift( $args );
    printf( '[Debug] ' );
    vprintf( $format, $args );
    printf( "\n" );
}

Not too shabby.  I ended up creating three different functions for this particular project: info(), error(), and trace().  The nice part is that I can now conditionally control whether or not these functions actually output anything via command line arguments. All that’s left is to add macros to automatically add the filename and line numbers to the output.

I whipped up a couple of tutorials over at Ozzu coving this in more detail. The C and PHP versions of the tutorial can be found below, respectively.
Writing a Custom printf() Wrapper Function in C
Writing a Custom printf() Wrapper Function in PHP

Update:
It turns out I wanted the filename and line numbers afterall, so I went ahead an added a macro (in C) to get the job done. Change the name of the function to _trace() and add two more arguments, so that your function now looks like this:

#define trace(...) _trace(__FILE__, __LINE__, __VA_ARGS__)

void _trace( char* filename, int line, const char* format, ... ) {
    va_list args;
    fprintf( stderr, "[%s:%d] " );
    va_start( args, format );
    vfprintf( stderr, format, args );
    va_end( args );
    fprintf( stderr, "\n" );
}

Every call to trace() will now print out the filename and line number of the trace call, followed by the formatted message.