Newsgroups: comp.lang.c Path: utzoo!henry From: henry@utzoo.uucp (Henry Spencer) Subject: Re: What's a C expert? Message-ID: <1989Aug13.015137.29760@utzoo.uucp> Organization: U of Toronto Zoology References: <12214@well.UUCP> <6057@microsoft.UUCP> <4722@alvin.mcnc.org> <25999@amdcad.AMD.COM> <4724@alvin.mcnc.org> Date: Sun, 13 Aug 89 01:51:37 GMT In article <4724@alvin.mcnc.org> spl@mcnc.org.UUCP (Steve Lamont) writes: >>Having the sign of chars be undefined allows the implementation to be as >>efficient as possible with respect to converting between chars and ints. > >Huh? Are you telling us that the standard *allows* such a horrible >thing? Aaaaaaarrrrrgh! :-+ (<-- smiley sucking on a persimmon) I >thought the standard was supposed to clarify things, not confuse the >issue... I see it's time to repost my commentary on this from some time ago: ----------- All potential participants in this debate please attend to the following. - There exist machines (e.g. pdp11) on which unsigned chars are a lot less efficient than signed chars. - There exist machines (e.g. ibm370) on which signed chars are a lot less efficient than unsigned chars. - Many applications do not care whether the chars are signed or unsigned, so long as they can be twiddled efficiently. - For this reason, char is intended to be the more efficient of the two. - Many old programs assume that char is signed; this does not make it so. Those programs are wrong, and have been all along. Alas, this is not a comfort if you have to run them. - The Father, the Son, and the Holy Ghost (K&R1, H&S, and X3J11 resp.) all agree that characters in the "source character set" (roughly, those one uses to write C) must look positive. Actually, the Father and the Son gave considerably broader guarantees, but the Holy Ghost had to water them down a bit. - The "unsigned char" type exists (in most newer compilers) because there are a number of situations where sign extension is very awkward. For example, getchar() wants to do a non-sign-extended conversion from char to int. - X3J11, in its semi-infinite wisdom, has decided that it would be nice to have a signed counterpart to "unsigned char", to wit "signed char". Therefore it is reasonable to expect that most new compilers, and old ones brought into conformance with the yet-to-be-issued standard, will give you the full choice: signed char if you need signs, unsigned char if you need everything positive, and char if you don't care but want it to run fast. - Given that many compilers have not yet been upgraded to match even the current X3J11 drafts, much less the final endproduct (which doesn't exist yet), any application which cares about signedness should use typedefs or macros for its char types, so that the definitions can be revised later. - The only things you can safely put into a char variable, and depend on having them come out unchanged, are characters from the native character set and small *positive* integers. - Dennis Ritchie is on record, as I recall, as saying that if he had to do it all over again, he would consider changing his mind about making chars signed on the pdp11 (which is how this mess got started). The pdp11 hardware strongly encouraged this, but it *has* caused a lot of trouble since. It is, however, much too late to make such a change to C. ----------- -- V7 /bin/mail source: 554 lines.| Henry Spencer at U of Toronto Zoology 1989 X.400 specs: 2200+ pages. | uunet!attcan!utzoo!henry henry@zoo.toronto.edu