[Bug c/55892] New: Bogus compiler warning

["sworddragon2 at aol dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55892

             Bug #: 55892
           Summary: Bogus compiler warning
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: sworddragon2@aol.com


Created attachment 29092
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=29092
Example code

In the attachments is the example file for this bug report. Compiling it with
"gcc -O3 -Wall -Wextra -o /dev/null -pedantic test.c" will result in a warning:

In file included from /usr/include/stdio.h:937:0,
                 from test.c:2:
In function 'fgets',
    inlined from 'main' at test.c:17:11:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:261:2: warning: call to
'__fgets_chk_warn' declared with attribute warning: fgets called with bigger
size than length of destination buffer [enabled by default]


There are 2 potential errors on this behavior:

- If the warning triggers on line 33 it should also be triggered on all 4
lines.
- In this code the limit of fgets can never exceed a length of 8. The compiler
thinks because read_limit can be 2147483647 (INT32_MAX) limit in fgets can it
be too (which as already said can never happen in this code). Removing the 3
lines which assign INT32_MAX removes the compiler warning.


I have a suggestion for a solution:

- Like conditional return values the compiler could check if the code has a
fixed construct which makes it possible to calculate if the limit will be
exceeded (this would match on the example code and trigger no compiler
warnings). If the code has a dynamic construct which makes it not possible to
calculate if the limit would be exceeded a comiler warning is thrown.