Making gcc "-Wformat" more verbose

[Marc Olzheim]

Hi.

When programming, I'd like to be able to make sure that what I think
what the code that I type does, is what I want it to do. Who doesn't?
Anyway, since I'm already compiling with most warnings on and I'm
linting my code, I'm trying my best to be more sure of it.

There where I find problems that could have been detected by my tools,
but weren't, I'd like to make sure that the tools get updated. This
prompted me to produce a patch for FreeBSD 5-STABLE's GCC (3.4.2).

GCC 3.4.2 takes a shortcut in checking the argument to printf()-like
functions with -Wformat. Since arguments to a varargs function smaller
than an int are promoted to an int (and floats to double), the check
doesn't care what the types originally passed to the function were
exactly, as long as after promotion it is good enough. To make things
worse, "good enough" here doesn't include checking the signdness.

I've made a simple copy-paste patch that makes sure that arguments to
those functions are checked _before_ _and_ _after_ the varargs-promotion
and that their sign meets the formatstring.

Please have a look at it and tell me whether this could be useful for
FreeBSD or whether that's a bridge too far...

The patch is at
http://www.stack.nl/~marcolz/FreeBSD/gcc-printf.patch.txt

Besides that, you'll need to include the inttypes.h at
http://www.stack.nl/~marcolz/FreeBSD/inttypes.h instead of
/usr/include/inttypes.h

If you want to compile the kernel with it, make sure to turn of
-Werror... (Or install into somewhere else then /usr/libexec and use
CFLAGS=-B<otherdir> to gcc to try it out.

I know, that other varargs functions' handling isn't modified yet; I
just thought I'd start with printf() and see whether it was useful.

Please let me know what you think.

Marc
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