Zoltán Kócsi wrote: > On Tue, 27 Jan 2009 07:08:51 -0500 > Robert Dewar <de...@adacore.com> wrote: > >> James Dennett wrote: >> >>> I don't know how much work it would be to disable this optimization >>> in gcc. >> To me, it is always troublesome to talk of "disable this optimization" >> in a context like this. The program in question is not C, and its >> semantics cannot be determined from the C standard. If the appropriate >> gcc switch to "disable" the optimization is set, then the status of >> the program does not change, it is still undefined nonsense. Yes, it >> may happen to "work" for some definition of "work" that is in your >> mind, but is not defined or guaranteed by any coherent language >> definition. It seems worrisome to be operating under such >> circumstances, so surely the best advice in a situation like >> this is to fix the program. > > I don't mean to complain, but I happen to work with embedded > systems. I program them in C, or at least in a language that uses > the syntactic elements of C. While it might not be a C program and > is utter nonsense from a linguistic view, in the embedded world > dereferencing a NULL pointer is often legal and actually > unavoidable.
No, that's not true: accessing memory at address zero may be necessary, but that is not the same as dereferencing a NULL pointer. This may sound like a minor and pedantic point, but I don't think it is: it goes to the very core of this misunderstanding. > Many embedded systems run on CPUs without MMU and I'd dare to state > that there are many more of these lesser beasts out there than big > multicore, superscalar CPUs with paged MMUs and vector processing > units and FPUs. Now on many of these at location 0 you find a vector > table or memory mapped registers or simple memory or even the > dreaded zero-page where you can do things that you can't do anywhere > else. On every one of those chips it is legal to dereference a NULL > pointer as long as you have the notion of a pointer being an address > of something. If it's an embedded system, and your program is tied to particular hardware with system data at address zero, you might as well access that data with an asm; that's not going to reduce portability any. > I openly admit that the test case was sloppy (and admitted it in my > OP). I do accept that due to the elevation of the C language from > the low level system programming language it used to be to the > linguisticly pure high-level object oriented metalanguage that C-99 > apparently is, dereferencing a NULL pointer became meaningless > nonsense these days. No, this is since C90; nothing has changed in this area. NULL doesn't mean "address 0", it means "nothing". The C statement if (ptr) doesn't mean "if ptr does not point to address zero", it means "if ptr points to something". > However, I'd like to point out one thing: the compiler is there to > help the programmer. Derefencing a NULL pointer might be a bad thing > on one system and perfectly normal on others. > From a C language standard purist point of view such code might be > nonsense, but chances are that you own and use some gadget every day > that was programmed by such low-life engineers like me who live in > those by-gone days where a pointer held a memory address and work > with chips where at address 0 there are some useful things. I think you perhaps need to be a little less patronizing. Many of us, myself included, have done a great deal of embedded programming and we know what the issues are. You have written an incorrect program, and you now know what was incorrect about it. > So, pretty please, when the compiler detects that a language > resembling to, but not really C is used and removes assumedly > (albeit unprovenly) superfluos chunks of code to purify the > misguided programmer's intent, could it please at least send a > warning? In practice that's a lot harder than you might think. If we were to issue a warning for every transformation we made based on the semantics of the C language I'm sure people would complain. "You can't dereference a NULL pointer" is a fundamental part of the language. Andrew.
