2014_5001@uwovax.uwo.ca (08/16/89)
Every so often someone mentions a trigraph. What is a trigraph??? I have programmed in C for 3 years, and the only time I met the word was when I was reading a graph-theory book (and I cannot remember what that definition was). Please enlighten me. Post or reply directly. -- Alexander Pruss, at one of: Department of Applied Mathematics, Astronomy, Mathematics, or Physics University of Western Ontario pruss@uwovax.uwo.ca pruss@uwovax.BITNET A5001@nve.uwo.ca
lhf@aries5 (Luiz H. deFigueiredo) (08/16/89)
A trigraph is a 'escape sequence' to allow C sources to be written in
machines that do not have special characters like #{}.
The format of a trigraph is:
??= meaning #
??( meaning {
??) meaning }
There are others but I don't remember them now. I'd have to check.
Anyway, it is not the kind of beast you'd be likely to meet...
(sorry to post but problems with mailer.)
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Luiz Henrique de Figueiredo internet: lhf@aries5.waterloo.edu
Computer Systems Group bitnet: lhf@watcsg
University of Waterloo
(possible domains are waterloo.edu and uwaterloo.ca)
-------------------------------------------------------------------------------swh@hpcupt1.HP.COM (Steve Harrold) (08/17/89)
Re: Trigraphs Poster suggests that trigraphs are not likely to be met in "everyday" C code, but I'd like to caution you that they will show up in some very unexpected ways. For example, say you have a string literal in your code that reads something like "...Are you sure you want to do this??.....". The inncocent pairing of the two question marks will cause the Ansi-conformant compiler to hunt for a trigraph. If the character immediately following the "??" completes a legal trigraph, your string is no longer what you thought it was. The compilation will succeed without comment, but your run-time results may vary.
gwyn@smoke.BRL.MIL (Doug Gwyn) (08/17/89)
In article <3566@uwovax.uwo.ca> 2014_5001@uwovax.uwo.ca writes: >Every so often someone mentions a trigraph. What is a trigraph??? There's one (the "???"). Standard C requires that, very early in the phases of translation, certain sequences of three source characters starting with "??" be mapped into equivalent single characters. This is intended to allow C programs to be expressed entirely using the ISO-646 basic character set, which does not include {, |, and several other essential C source characters. It is not expected that real programming environments would ever require programmers to type these trigraph sequences; they're intended primarily for program interchange among different sites. Unless you're importing a lot of code from European sites or are implementing a C compiler, you shouldn't have to be concerned about trigraphs, since you'll never use nor see them.
bengsig@oracle.nl (Bjorn Engsig) (08/18/89)
Article <10762@smoke.BRL.MIL> by gwyn@brl.arpa (Doug Gwyn) says: |Unless you're importing a lot of code from |European sites or are implementing a C compiler, you shouldn't have to |be concerned about trigraphs, ... Well, I don't think that any Europeans use trigraphs. We use one of the alter- natives: - Get a terminal like yours (i.e. US-style) and give a damm about our own funny characters - we never program in our own language anyway. - Use a terminal (and software such as vi, sh, etc.) with 8-bit support - Use nationalized terminals, where e.g. } looks like an 'a' with a small ring above it (try typing ComposeCharacter a * on a vt220). You can actually get used to such ugly looking C programs :-( -- Bjorn Engsig, ORACLE Europe \ / "Hofstadter's Law: It always takes Path: mcvax!orcenl!bengsig X longer than you expect, even if you Domain: bengsig@oracle.nl / \ take into account Hofstadter's Law"
mcdonald@uxe.cso.uiuc.edu (08/18/89)
>Unless you're importing a lot of code from >European sites or are implementing a C compiler, you shouldn't have to >be concerned about trigraphs, since you'll never use nor see them. puts("Should I have to worry about trigraphs in ANSI C??!"); What does the above print? According to K&R I it should print Should I have to worry about trigraphs in ANSI C??! Does it? Alfred E. Neumann (mcdonald@uxe.cso.uiuc.edu)
walter@hpclwjm.HP.COM (Walter Murray) (08/18/89)
Doug Gwyn writes: >In article <3566@uwovax.uwo.ca> 2014_5001@uwovax.uwo.ca writes: >>Every so often someone mentions a trigraph. What is a trigraph??? >There's one (the "???"). Nope. Doug must not have had his dpANS handy. The "???" smells a little like a trigraph, but it isn't one. Of all three-character sequences beginning with "??", only nine are trigraphs, and "???" isn't one of them! Walter Murray ----------
karl@haddock.ima.isc.com (Karl Heuer) (08/19/89)
In article <10762@smoke.BRL.MIL> gwyn@brl.arpa (Doug Gwyn) writes: >In article <3566@uwovax.uwo.ca> 2014_5001@uwovax.uwo.ca writes: >>What is a trigraph??? > >There's one (the "???"). Lest anyone take that too literally, I'll point out what I'm sure Doug already knows: although all trigraphs begin with "??", not everything beginning with "??" is a trigraph. In particular, "???" is not one of the nine trigraphs, and so it would not be interpreted specially. (Unless the next character is one of ( ) ! = < > / - ' in which case the first `?' is literal and the other two are the first two-thirds of a trigraph.) In article <5940008@hpcupt1.HP.COM> swh@hpcupt1.HP.COM (Steve Harrold) writes: >[It's possible to accidentally create a trigraph in a string literal.] >The compilation will succeed without comment... Hopefully, most complete% implementations of ANSI C will warn about "possibly unintended trigraph", especially if a trigraphable character (i.e., one of the nine characters [ ] | # { } \ ~ ^ ) is encountered in the same source. Karl W. Z. Heuer (ima!haddock!karl or karl@haddock.isc.com), The Walking Lint % There will undoubtedly be many "incomplete" implementations that don't support trigraphs. I reserve judgement on whether this is good or bad.
gwyn@smoke.BRL.MIL (Doug Gwyn) (08/19/89)
In article <479.nlhp3@oracle.nl> bengsig@oracle.nl (Bjorn Engsig) writes: >Article <10762@smoke.BRL.MIL> by gwyn@brl.arpa (Doug Gwyn) says: >|Unless you're importing a lot of code from >|European sites or are implementing a C compiler, you shouldn't have to >|be concerned about trigraphs, ... >Well, I don't think that any Europeans use trigraphs. I didn't say they did. In the future, once there is a sufficient desity of standard-conforming C implementations, it is possible that C source files being transported between sites that operate in radically different environments will often be mapped to/from trigraphs to avoid national character-set problems during the port.
ado@elsie.UUCP (Arthur David Olson) (08/19/89)
In article <5940008@hpcupt1.HP.COM>, swh@hpcupt1.HP.COM (Steve Harrold) writes: > . . .say you have a string literal in your code. . .like > "...Are you sure you want to do this??.....". The. . .two question marks > will cause the Ansi-conformant compiler to hunt for a trigraph. > If the character immediately following the "??" completes > a legal trigraph, your string is no longer what you thought it was. > > The compilation will succeed without comment, but your run-time results > may vary. Script started on Fri Aug 18 20:26:28 1989 elsie$ cat try.c #include <stdio.h> int main(argc, argv) int argc; char * argv[]; { (void) printf("Hello, world??!\n"); return 0; } elsie$ /usr/local/bin/gcc try.c elsie$ a.out Hello, world??! elsie$ /usr/local/bin/gcc -trigraphs try.c elsie$ a.out Hello, world| elsie$ /usr/local/bin/gcc -trigraphs -Wtrigraphs try.c >>>> try.c:1: warning: file contains 1 trigraph(s) elsie$ a.out Hello, world| elsie$ exit script done on Fri Aug 18 20:27:01 1989 The compilation will not *necessarily* succeed without comment. -- Arthur David Olson ado@alw.nih.gov ADO is a trademark of Ampex.
paul@csnz.co.nz (Paul Gillingwater) (08/20/89)
In article <3566@uwovax.uwo.ca> 2014_5001@uwovax.uwo.ca writes: >Every so often someone mentions a trigraph. What is a trigraph??? May I refer you to page 229 of the Kernighan and Ritchie Bible, "The C Programming Language" (2nd. ed.). Also, page 259 (Appendix C, "Summary of Changes") states: 'Trigraph sequences introduced by ?? allow representation of characters lacking in some character sets.' This substitution is usually performed by a preprocessor. For instance, the following are valid trigraphs: ??= # ??( [ ??< { ??/ \ ??) ] ??> } ??' ^ ??! | ??- ~ >I have programmed in C for 3 years, and the only time I met the word was when I >was reading a graph-theory book (and I cannot remember what that definition >was). You've programmed in C for 3 years and haven't read the Bible according to K & R (revised ed.) ?? >Please enlighten me. The pointer to NULL is not the whole of the dhamma. Mu! >Alexander Pruss, at one of: Department of Applied Mathematics, Astronomy, -- Paul Gillingwater, Computer Sciences of New Zealand Limited Bang: ..!uunet!dsiramd!csnz!paul Domain: paul@csnz.co.nz Call Magic Tower BBS V21/23/22/22bis 24 hrs +0064 4 767 326