#define and functions with variable length arguments

Question

I have zillions of my_printf() function calls in a huge program. I now want to convert them all so that the function takes a new integer argument (call it x) without having to edit the zillions of calls. If my_printf only ever took exactly one string argument then I could do something like this: ``` #define my_printf(str) _my_printf(x,str) void _my_printf(int x,char *str) // changed from my_printf(char *str) { // stuff } ``` But as my_printf takes a variable number of arguments I'm not sure how to do it. Can it be done? EDIT: for those wondering why I should want to do such a thing, here's a related example: ``` #if BELT_AND_BRACES_DIAGNOSTIC_MODE #define function(x) _function(__FILE__,__LINE__,x) #else // speed critical optimised mode #define function(x) _function(x) #endif #if BELT_AND_BRACES_DIAGNOSTIC_MODE void _function(char *file,int line,int x) #else void _function(int x) #endif { // stuff #if BELT_AND_BRACES_DIAGNOSTIC_MODE if (something_went_wrong) { printf("Cock up in functi…

Accepted Answer

You may use C99 variadic macros:

```
#define my_printf(...) my_printf_(x, __VA_ARGS__)
```

As Microsoft's implementation suppresse trailing commas, the `str` argument can be added explicitly

```
#define my_printf(str, ...) my_printf_(x, str, __VA_ARGS__)
```

but this would lead to a syntax error in standard C when invoked without variadic arguments

```
my_printf("foo")
```

or an empty argument list

```
my_printf("foo",)
```

Therefore, I'd go with the first version.

#define and functions with variable length arguments

Solution

Problem