paul@vixie.UUCP (Paul Vixie Esq) (12/15/86)
I tried to compile and use multivol today. It compiled without incident,
but it and testdata dump core during 'make wfile'. My debugger is hosed,
and refuses to print a stack trace, so I started looking through the code.
In testdata.c, I found:
...
if (no_eoln = (*argv[1] == '-' && *++argv[1] == 'n')) {
...
Now, since C doesn't guarantee order of evaluation, this seems blatently
wrong. I don't know neccessarily that this is the cause of my problem,
but if the rest of the code has errors like this, I don't want to bother
trying to find the cause of the core dumps. Mission: Impossible :-), and
I refuse to accept it.
Is there an alternative to (or more recent version of) multivol?
--
Paul A. Vixie {ptsfa, qantel, crash, winfree}!vixie!paul
329 Noe Street dual!ptsfa!vixie!paul@ucbvax.Berkeley.EDU
San Francisco nike!ptsfa!vixie!paul@seismo.CSS.GOV
CA 94116 paul@vixie.UUCP (415) 864-7013tsf@theory.cs.cmu.edu (Timothy Freeman) (12/16/86)
"The left operand of '&&' is fully evaluated first." Page 181, "C a reference manual", by Harbison & Steele, 1984. What was it that you were saying was evaluated in an unspecified order? Tim Freeman Arpanet: tsf@theory.cs.cmu.edu Uucp: ...!seismo!theory.cs.cmu.edu!tsf
campbell@maynard.BSW.COM (Larry Campbell) (12/16/86)
In article <403@vixie.UUCP> paul@vixie.UUCP (Paul Vixie Esq) writes: ... >In testdata.c, I found: > > if (no_eoln = (*argv[1] == '-' && *++argv[1] == 'n')) { > >Now, since C doesn't guarantee order of evaluation, this seems blatently >wrong. I don't know neccessarily that this is the cause of my problem, >but if the rest of the code has errors like this, I don't want to bother >trying to find the cause of the core dumps. Mission: Impossible :-), and >I refuse to accept it. You are suffering from an all too common misconception. There are several clearly documented operators for which the order of evaluation IS guaranteed, and && is one of them. I quote from section 7.11 (pg. 190) of K&R: Unlike &, && guarantees left-to-right evaluation; moreover the second operand is not evaluated if the first operand is 0. There may well be bugs in multivol, but the example quoted above isn't one of them. -- Larry Campbell The Boston Software Works, Inc. Internet: campbell@maynard.bsw.com 120 Fulton Street, Boston MA 02109 uucp: {alliant,wjh12}!maynard!campbell +1 617 367 6846 ARPA: campbell%maynard.uucp@harvisr.harvard.edu MCI: LCAMPBELL
ted@blia.BLI.COM (Ted Marshall) (12/16/86)
In article <403@vixie.UUCP>, paul@vixie.UUCP (Paul Vixie Esq) writes: > In testdata.c, I found: > if (no_eoln = (*argv[1] == '-' && *++argv[1] == 'n')) { > Now, since C doesn't guarantee order of evaluation, this seems blatently > wrong... Sorry, but this not wrong. Look at K&R page 19: "Expressions connected by && or || are evaluated left to right and it is guaranteed that evaluation will stop as soon as the truth or falsehood is known." Of course, this doesn't help you find the bug (I'm a VMS programmer so I'm not familiar with multivol. Sorry.) Ted Marshall @ Britton Lee, Inc UUCP: ...!ucbvax!mtxinu!blia!ted
psfales@ihlpl.UUCP (Peter Fales) (12/16/86)
> In testdata.c, I found: > ... > if (no_eoln = (*argv[1] == '-' && *++argv[1] == 'n')) { > ... > Now, since C doesn't guarantee order of evaluation, this seems blatently > wrong. Kerhighan & Ritchie, p. 190: "Unlike &, && guarantees left-to-right evaluation; moreover the second operand is not evaluated if if the first operand is 0." Peter Fales ihnp4!ihlpl!psfales
john@basser.oz (John Mackin) (12/17/86)
In article <403@vixie.UUCP> paul@vixie.UUCP (Paul Vixie Esq) writes: > if (no_eoln = (*argv[1] == '-' && *++argv[1] == 'n')) { > > Now, since C doesn't guarantee order of evaluation, this seems blatently > wrong. I don't know neccessarily that this is the cause of my problem, > but if the rest of the code has errors like this, I don't want to bother > trying to find the cause of the core dumps. Oh dear. Restraint, tolerance, good karma. Remember those good loops you flew over the dam this morning. Deep breaths. NOW... Look here! I am not going to say that multivol is well coded. I can't comment on that, as I have never looked at its internals. But I am MOST CERTAINLY going to say that your specific criticism is totally incorrect, unjustified, and unwarranted, and in fact you do not know what you are talking about. A little knowledge, it would seem, is a dangerous thing. You state that ``C does not guarantee order of evaluation''. This is patently false. If you look in the index of K&R, you will see "order of evaluation" indexed in quite a few places. The VERY FIRST PLACE IT IS MENTIONED is on page 19, section 1.5, under "Word Counting", where it states: Expressions connected by && or || are evaluated left to right ... Sure sounds like a guarantee to me! Certainly, order of evalution is undefined for many of C's operators; you will see this if you look at some of the other places mentioned in the index entry. It IS defined for && and ||. Please find out what you are talking about before you post to the net. Thousands of people worldwide will read your articles. If you post misinformation, you will confuse some of them. They may write code which is wrong or form ideas which are wrong, due to your posted misinformation. Please, get it right. John Mackin, Basser Department of Computer Science, University of Sydney, Sydney, Australia john@basser.oz.AU (john%basser.oz@SEISMO.CSS.GOV) {seismo,hplabs,mcvax,ukc,nttlab}!munnari!basser.oz!john