russ@groucho.ucar.edu (Russ Rew) (03/07/89)
GNU C supports arithmetic on void pointers, but the manual implies that this is an extension to the C language. I could find no prohibition on arithmetic on void pointers in K&R second edition. What does the latest dpANS document say? Is it legal to increment a variable of type void* ? -- Russ Rew UCAR, P.O. Box 3000 INTERNET: russ@unidata.UCAR.EDU Boulder, CO 80307 UUCP: russ@scdpyr.UUCP (303) 497-8845
gwyn@smoke.BRL.MIL (Doug Gwyn ) (03/07/89)
In article <1527@ncar.ucar.edu> russ@groucho.UUCP (Russ Rew) writes: >Is it legal to increment a variable of type void* ? Of course not, because there are no void objects and therefore no way to point at the "next" one.
friedl@vsi.COM (Stephen J. Friedl) (03/07/89)
In article <1527@ncar.ucar.edu>, russ@groucho.ucar.edu (Russ Rew) writes: > What does the latest dpANS document say? Is it legal > to increment a variable of type void* ? No. When incrementing a pointer, one multiplies the increment by the size of the pointed-to object. Since a `void' has no size, you can't get to the next one. The dpANS does not directly say "You can't ++ a void*", so to prove it you need to string a couple of parts together. From the May88 dpANS: (renditions: ITALICS, /Courier/) 3.1.2.5 - Types "The /void/ type comprises an empty set of values; it is an incomplete type that cannot be completed" ### /void/ is an incomplete type 3.3.3.4 - The /sizeof/ operator "The /sizeof/ operator shall not be applied to an expression that has function type or an incomplete type [...]" ### you can't take the size of /void/ 3.3.2.4 - Postfix increment and decrement operators and 3.3.3.1 - Prefix increment and decrement operators "The operand of the prefix increment or decrement operator shall have qualified or unqualified scalar type and shall be a modifiable lvalue." ### you can't ++ or -- a /void/ I would guess that those implementations that permit ++ on void pointers do so by treating them as char pointers instead. Steve, the staggering dpANS (with apologies to Karl) -- Stephen J. Friedl / V-Systems, Inc. / Santa Ana, CA / +1 714 545 6442 3B2-kind-of-guy / friedl@vsi.com / {attmail, uunet, etc}!vsi!friedl "Cold beer, hot women, and fast compilers" - me
bill@twwells.uucp (T. William Wells) (03/07/89)
In article <1527@ncar.ucar.edu> russ@groucho.UUCP (Russ Rew) writes:
: GNU C supports arithmetic on void pointers, but the manual implies
: that this is an extension to the C language. I could find no
: prohibition on arithmetic on void pointers in K&R second edition.
: What does the latest dpANS document say? Is it legal to increment a
: variable of type void* ?
When the dpANS talks about pointer arithmetic, it speaks of pointers
to an object type. Void is an incomplete type, a pointer to void is
thus not a pointer to an object type, so a void pointer can't be used
in pointer arithmetic.
---
Bill
{ uunet | novavax } !twwells!bill
(BTW, I'm going to be looking for a new job sometime in the next
few months. If you know of a good one, do send me e-mail.)davidsen@steinmetz.ge.com (William E. Davidsen Jr) (03/08/89)
In article <1527@ncar.ucar.edu> russ@groucho.UUCP (Russ Rew) writes: | GNU C supports arithmetic on void pointers, but the manual implies | that this is an extension to the C language. I could find no | prohibition on arithmetic on void pointers in K&R second edition. | What does the latest dpANS document say? Is it legal to increment a | variable of type void* ? Let's see... a pointer increment on a char pointer adds sizeof(char) to the pointer, increment on int pointer adds sizeof(int), I guess you would conclude that increment on a void pointer would add sizeof(void). I *think* dpANS says sizeof(void) is zero (I don't have it here, so correct me if you can find the section). I conclude that if it was allowed it wouldn't do anything. I don't think it's allowed. One of my C students handed in this code fragment: void *foo, *getsym(); int m; foo = getsym(22); m = (int) foo[4]; I *think* he meant "m = ((int *) foo)[4]" which seems legal if not the clearest way to do it. -- bill davidsen (wedu@ge-crd.arpa) {uunet | philabs}!steinmetz!crdos1!davidsen "Stupidity, like virtue, is its own reward" -me
gwyn@smoke.BRL.MIL (Doug Gwyn ) (03/08/89)
In article <13323@steinmetz.ge.com> davidsen@crdos1.UUCP (bill davidsen) writes: >I *think* dpANS says sizeof(void) is zero No, it's illegal. C as it stands now has no zero-sized objects. There is an unofficial "extensions working group" to investigate ways to incorporate zero-sized objects into C, but as its point of contact I must say there has been remarkably little interest in the subject.