may be undefined operation warning

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may be undefined operation warning
\ Wesley Smith (25 Feb 2008)
. \ Diego Novillo (25 Feb 2008)
. . \ Wesley Smith (25 Feb 2008)
. . . \ Wesley Smith (25 Feb 2008)
. . . \ Diego Novillo (25 Feb 2008)
. . . . \ Wesley Smith (25 Feb 2008)
. . . . . \ Diego Novillo (25 Feb 2008)
. . . . . . \ Wesley Smith (25 Feb 2008)
. . . . . . . \ Diego Novillo (25 Feb 2008)
. \ John Love-Jensen (25 Feb 2008)
. . \ Diego Novillo (25 Feb 2008)

2 msg	non virtual dtor warning
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5 msg	gcc 3.2

Subject:	may be undefined operation warning
Group:	Gcc-help
From:	Wesley Smith
Date:	25 Feb 2008

Hi,
I'm wondering why GCC is giving a warning "operation on 'c' may be
undefined"? Here's the code:

#define MAX_CHAR(data, accum) ((255 - (data)) > (accum)) ? ((accum) +
(data)) : 255

uchar *c;
uchar accum

c = out_pix + (voffset - *cp++)*out_rowstride;
*c++ = MAX_CHAR(*c, accum); ///<------------- warning on this line

This is in a C file with

gcc -v
Using built-in specs.
Target: i686-apple-darwin8
Configured with: /private/var/tmp/gcc/gcc-5367.obj~1/src/configure
--disable-checking -enable-werror --prefix=/usr --mandir=/share/man
--enable-languages=c,objc,c++,obj-c++
--program-transform-name=/^[cg][^.-]*$/s/$/-4.0/
--with-gxx-include-dir=/include/c++/4.0.0 --with-slibdir=/usr/lib
--build=powerpc-apple-darwin8 --with-arch=nocona --with-tune=generic
--program-prefix= --host=i686-apple-darwin8
--target=i686-apple-darwin8
Thread model: posix
gcc version 4.0.1 (Apple Computer, Inc. build 5367)

Any ideas? Thanks,
wes