When I first started writing C code I riddled my code with a bunch of printf statements to log information for debugging purposes. When I did this I found myself writing a lot of boilerplate code over and over again. For example, the format string of my printf always ended with a '\n' newline character, and I wished there as a way I could skip having to type those two characters over and over again. Luckily I was able to make my wish come true. As I learned more about C I discovered that it gives you the ability to write custom printf style functions and it is a real easy thing to do.

To get started, you should include the <stdio.h> and <stdarg.h> libraries:

#include <stdio.h>

#include <stdarg.h>

The <stdarg.h> library gives you the structures needed to accept a variable number of arguments to the end of your function, as printf does. The <stdio.h> library gives you the 'vprintf' and related function that accept the variable argument structures for printing.

A custom printf function should have in its signature a const char pointer to the format you want printed and a set of ellipses as the last parameter:

void log(const char *format, ...);

The ellipses indicate the function accepts a variable number of arguments. The basic definition will look something like this:

void log(const char *format, ...)


    va_list vargs;

    va_start(vargs, format);

    vprintf(format, vargs);




va_list is a pointer type that the variable arguments list is assigned to and cleaned up by the va_start and va_end functions. After calling va_start on the 'vargs' variable it will point to the variable parameters passed to it. vprintf is a special function that accepts the format and va_list and performs the standard printf operation.

Note that stdio.h also contains a vfprintf functions that will allow you to print to file or a standard stream such as stderr.