-Wformat-length and floats

[Gerald Pfeifer]

This took me a bit to get to the bottom of, but I know believe 
that we need to work both on the documentation and implementation 
of -Wformat-length, in particular when it comes to floats.

  #include <stdio.h>

  typedef struct M {
      float a, b, c;
  } M;

  char *foo(M *m) {
      static char buf[64];
      sprintf(buf, "%.2f %.2f %.2f", m->a, m->b, m->c);
      return buf;
  }

First of all, it turns out that floats are not covered in the
documentation at all.  I've had a look at the code, and think
I'll be able to propose a doc change latest this coming weekend.

Now to what actually happens in the example above:

  # gcc -c -o x.o -Wall x.c
  x.c: In function Β‘fooΒ’:
  x.c:9:24: warning: Β‘%.2fΒ’ directive writing between 4 and 317 bytes into a region of size 0 [-Wformat-length=]
     sprintf(buf, "%.2f %.2f %.2f", m->a, m->b, m->c);
                        ^~~~
  x.c:9:5: note: format output between 15 and 954 bytes into a destination of size 64
     sprintf(buf, "%.2f %.2f %.2f", m->a, m->b, m->c);
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

A couple of issues I find really confusing:

 - Where is the number of 317 bytes as the upper limit for a sole
   %.2f coming from?
 - Where is the number of 954 bytes coming from for the full call?
   
 - What is a region of size 0?  Why 0?

 - And what is the difference between a region and a destination?

I'll see what I can do about documentation; any input on the above
related to that will be helpful.

And something tells me that there may be in issue with the diagnostics
code?

Gerald