Custom support for __attribute__((format))

Question

Both GCC and Clang have a support to make compile-time checks on variable argument functions like printf. These compilers accept syntax like:

extern void dprintf(int dlevel, const char *format, ...)
  __attribute__((format(printf, 2, 3)));  /* 2=format 3=params */

On OSX, the Cocoa framework also use an extension of this for NSString:

#define NS_FORMAT_FUNCTION(F,A) __attribute__((format(__NSString__, F, A)))

In our company, we have a custom C++ framework with a bunch of classes like BaseString all deriving from BaseObject. In BaseString there are a few variable argument methods similar to sprintf, but with some extensions. For example, "%S" expects an argument of type BaseString*, and "%@" expects a BaseObject* argument.

I would like to perform a compile-time check of the arguments in our projects, but because of the extensions, __attribute__((format(printf))) give lots of false positive warnings.

Is there a way to customize the support of __attribute__((format)) for one of the two compilers ? If this requires a patch to the compiler source, is it doable in a reasonable amount of time ? Alternatively, are there other lint like tools that could perform the check ?

Answer 1

With recent version of GCC (I recommend 4.7 or newer, but you could try with a GCC 4.6) you can add your own variables and functions attributes thru a GCC plugin (with the PLUGIN_ATTRIBUTES hook), or a MELT extension. MELT is a domain specific language to extend GCC (implemented as a [meta-]plugin).

If using a plugin (e.g. MELT) you won't need to recompile the source code of GCC. But you need a plugin-enabled GCC (check with gcc -v).

In 2020, MELT is not updated any more (because of lack of funding); however you could write your own GCC plugin for GCC 10 in C++, doing such checks.

^{Some Linux distributions don't enable plugins in their gcc - please complain to your distribution vendor; others provide a package for GCC plugin development, e.g. gcc-4.7-plugin-dev for Debian or Ubuntu.}