warning message "decimal constant is unsigned only in ISO C90"

[Richard Earnshaw]

> I'm getting this warning when trying to initialize a signed int with
> the most natural form of a constant for the maximum negative value:
> 
>   $ cat x.c
>   signed int maxneg1 = -2147483648;       /* This gives a warning */
>   signed int maxneg2 = -2147483647 - 1;   /* This is OK */
>   signed int maxneg3 = 0x80000000;        /* This is also OK */
> 
>   $ /usr/local/sourceware/bin/gcc -c x.c
>   x.c:1: warning: this decimal constant is unsigned only in ISO C90
> 
>   $ /usr/local/sourceware/bin/gcc --v
>   Reading specs from /links1/usr/local/sourceware/bin/../lib/gcc-lib/i686-pc-linux-gnu/3.4/specs
>   Configured with: /src/sourceware/gcc/gcc/configure -v --prefix=/usr/local/sourceware --cache-file=config.cache --enable-languages=c,c++
>   Thread model: posix
>   gcc version 3.4 20030401 (experimental)
> 
> Is this some sort of bug or just a quirk of how integer constants are
> converted from external to internal format on a 32 bit two's
> complement machine?

It's because c90 says that the constant is 2147483648 (which is not 
representable as a signed 32-bit value).  To that constant you've then 
applied the negation operation.

c90 doesn't have negative constants, only positive ones that have been 
negated...

R.