[Bug c/20951] bogus error passing va_list to va_list*
--- Additional Comments From joseph at codesourcery dot com 2005-04-13 20:42 --- Subject: Re: bogus error passing va_list to va_list* On Mon, 11 Apr 2005, sebor at roguewave dot com wrote: Yes, I read that comment but I still don't see anything in the standard the footnote is in conflict with and I don't see it on the WG14 DR list(*). If the footnote is bogus and va_list can't be universally implemented to allow programs to initialize a va_list* with the address of a va_list function argument,it ought to be taken out of the standard. If you are not aware of an issue against the C standard I'll be happy to send it to the committee myself. I see nothing wrong with the footnote. You can create a pointer to a va_list, as the footnote notes. To do so, you need an object of type va_list. Declaring a function parameter as va_list va doesn't declare such a object if va_list is an array type; it declares one of type va_list_member * where va_list is an array va_list_member[number_of_elements]. Just because it says you can create a pointer doesn't mean you can do so with va if va doesn't have va_list type, any more that it means you can do so with random expression I'd like to have va_list * type but which has nothing to do with va_list; they are in exactly the same position. If your expression has type va_list * according to the semantics of the language and is the address of a va_list object, you can use it. (comment#1 in this bug is bogus. va isn't va[0], it's the address of the pointer argument rather than the address of an element. Both of these are of course different from the address of the va_list object itself which has type va_list * (i.e. va_list_member (*)[number_of_elements]) instead of va_list_member ** (which va has) or va_list_member * (which va[0] has). comment#8 in bug 14557 is just confused as well; ignore it and read comments 10-14 instead.) -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20951
[Bug c/20951] bogus error passing va_list to va_list*
--- Additional Comments From sebor at roguewave dot com 2005-04-13 22:06 --- (In reply to comment #5) Thanks for the pointer. Let me try again to explain why I object to the footnote: The footnote assumes that the reader will make the extrapolation that 1) since va_list is an object type, and 2) since array types are (also) object types, and 3) since array types in function arguments are converted to pointers, the actual type of a va_list object when declared as a function argument need not in fact be va_list but va_list*, despite the fact the declaration clearly appears to say otherwise. I am not a casual reader of the standard -- I read and interpret standards for a living. It seems that if I have trouble making that leap above it may not be so obvious to others either (as demonstrated by the lively discussion in the referenced bug). Perhaps removing the footnote is an unnecessarily drastic change. But clarifying it by mentioning the caveat above would, IMO, go a long way toward helping readers avoid the trap that I (and others) fell into. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20951
[Bug c/20951] bogus error passing va_list to va_list*
--- Additional Comments From sebor at roguewave dot com 2005-04-11 17:33 --- (In reply to comment #1) Right. I understand why it doesn't compile and how to work around it (with gcc at least). What I'm still not convinced of is that it's not a strict conformance bug. The same point was raised in the discussion you pointed me to but I couldn't find where it was addressed or if it was communicated to WG14. I.e., even if the requirement in C99 (that makes the test case well-formed) is non-normative and bogus, strictly speaking this would still be a conformance bug in the gcc implementation of va_list (after all, it is a gcc builtin, so it seems that the compiler could do some magic whereby taking the address of a va_list declared as an argument to a function would produce va_list* and not va_list**). Do you happen to know whether there's a C99 issue to fix the footnote? -- What|Removed |Added Component|target |c http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20951
[Bug c/20951] bogus error passing va_list to va_list*
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-04-11 17:35 --- (In reply to comment #2) Do you happen to know whether there's a C99 issue to fix the footnote? If you read that bug's comment, you will see in comment #8: My recollection from last time we were discussing this is that text above is an footnote that is not normative and in conflict with rest of the standard, so it needs to be ignored. While I agree that this behaviour is very unfortuante side effect of va-arg implementation, there are some other ABIS around (PPC SysV) doing the same so the only choice now is probably to avoid this construct. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20951
[Bug c/20951] bogus error passing va_list to va_list*
--- Additional Comments From sebor at roguewave dot com 2005-04-11 17:51 --- Yes, I read that comment but I still don't see anything in the standard the footnote is in conflict with and I don't see it on the WG14 DR list(*). If the footnote is bogus and va_list can't be universally implemented to allow programs to initialize a va_list* with the address of a va_list function argument,it ought to be taken out of the standard. If you are not aware of an issue against the C standard I'll be happy to send it to the committee myself. (*) http://www.open-std.org/jtc1/sc22/wg14/www/docs/summary.htm -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20951
