On 12/10/2010 03:54 AM, Andrew W. Nosenko wrote: >> > Doesn't this suffice, without dragging Autoconf into it? >> > >> > #if -1 >> 1 == -1 > No. It won't work. Your preprocessor directive checks behavior of > the preprocessor (/bin/cpp for example) that may have nothing with the > behavior of the compiler and behavior of the code generated by > compiler. If that is the issue, then even Autoconf won't work. Nothing in the C standard says that -1 >> 1 must evaluate to anything in particular; the result is implementation-defined, which means that it has to be *something* (there can't be a core-dump), and it has to be documented, but it can depend on the word size or even (in theory) the phase of the moon. For example: return (-1 >> 1) == (-1L >> 1); might return 0. The usual assumption, of course, is that the compiler picks one interpretation of >> and sticks with it. If that is true, then "#if -1 >> 1 == -1" works, and any preprocessor that behaves differently from the compiler is simply buggy. If it is false, then code should not depend on any consistent result from shifting negative values right. _______________________________________________ Autoconf mailing list Autoconf@xxxxxxx http://lists.gnu.org/mailman/listinfo/autoconf
