IsUnsigned() function?

[Will Crowder]

In article <8118 at ncar.ucar.edu> steve at groucho.ucar.edu (Steve Emmerson) writes:
>In <1990Jul27.161339.14712 at kfw.COM> will at kfw.COM (Will Crowder) writes:
>
>>What's wrong with:
>
>>#define IsUnsigned(type)	((type)0 - 1 > 0)
>
>Unless I'm mistaken, in ANSI C "(char)0" will, on most machines,
>promote to "signed int" due to the value-preserving rule; consequently,
>the above expression will always be false on those machines, regardless
>of the signedness of "char".
>
>Steve Emmerson        steve at unidata.ucar.edu        ...!ncar!unidata!steve

You're absolutely right, which Karl Heuer also pointed out to me via
e-mail.

I hate the fact that signedness is not preserved, so I probably blocked
it out of my memory due to severe emotional trauma.  :) :) :)  (In other
words, I blew it.)

I've tried ((type)(-1) > 0) on Turbo C and Gnu CC v1.36, and it seems to
work on both.  It does not, however, work on Sun cc.  A compile-time
switch on Turbo C lets you choose whether you want signed or unsigned
chars, which is handy.

Side note: can anyone out there give a good explanation as to why
chars were *ever* signed in the first place, and why ANSI decided not
to require either signed or unsigned chars (leaving it implementation
dependent)?

Will