# Re: [PATCH] Trivial warning fix for imap-send.c 5 messages from 2006-03-13 to 2006-12-05. Participants: Horst von Brand, Linus Torvalds, Jeff King, Horst H. von Brand. Thread: https://gitlist.dev/t/3641 ## Horst von Brand, 2006-03-13 04:12 Subject: Re: [PATCH] Use explicit pointers for execl...() sentinels. Message-ID: <200603130412.k2D4CW1b011631@laptop11.inf.utfsm.cl> URL: https://gitlist.dev/e/200603130412.k2D4CW1b011631%40laptop11.inf.utfsm.cl In-Reply-To: ``` Jeff King wrote: > On Sun, Mar 12, 2006 at 08:08:12PM +0200, Timo Hirvonen wrote: > > NULL pointer does not point to any data, it just says it's 'empty'. So > > it doesn't need to be same type pointer as specified in the function > > prototype. Pointers are just addresses, it doesn't matter from to code > > generation point of view whether it is (char *)0 or (void *)0. > Sorry, but I think you're wrong according to the C standard. Pointers of > different types do NOT have to share the same representation (e.g., > there have been some platforms where char* and int* were different > sizes). A void pointer must be capable of representing any type of > pointer (for example, holding the largest possible type). However, if > sizeof(void *) == 8 and sizeof(char *) == 4, Very improbable, they'll be the same normally ("void *" is a way of getting rid of the overloading of the meaning of "char *" for this before ANSI C). Sure, sizeof(int *) might be 4, but I think that is pretty far off. > you have a problem with > variadic functions which are expecting to pull 4 byte off the stack. There are special rules for variadic functions, probably pointers would be cast to/from void * in such a case by the compiler. > In a non-variadic function, the compiler would do the right implicit > casting. In a variadic function, it can't. It sure can. The rules where defined so that it works. > The real question is, does git want to care about portability to such > platforms. Broken platform, on which the compiler fails miserably in doing its job? No, it doesn't. > If you remain unconvinced, I can try to find chapter and verse of the > standard. Please do. > > sizeof(unsigned long) is sizeof(void *) in real world. > Are you saying that because it encompasses all of the platforms you've > worked on, or do you have some evidence that it is largely the case? It > certainly isn't guaranteed by the C standard. More because a machine with pointers that are much larger than the largest "normal" integer would be pretty weird (sure, on intel 8086 they where 32 ("far" pointer, segment + offset) and 16 bits, but... -- Dr. Horst H. von Brand User #22616 counter.li.org Departamento de Informatica Fono: +56 32 654431 Universidad Tecnica Federico Santa Maria +56 32 654239 Casilla 110-V, Valparaiso, Chile Fax: +56 32 797513 ``` ## Horst von Brand, 2006-03-13 04:14 Subject: Re: [PATCH] Trivial warning fix for imap-send.c Message-ID: <200603130414.k2D4EXcX011651@laptop11.inf.utfsm.cl> URL: https://gitlist.dev/e/200603130414.k2D4EXcX011651%40laptop11.inf.utfsm.cl In-Reply-To: ``` Jeff King wrote: [...] > A void pointer is guaranteed to be able to hold any type of pointer > (either char * or struct foo * or whatever). The declaration of malloc > indicates a return of void *. On a platform where it matters, the > compiler generates code so that > struct foo *bar = malloc(100); > converts the void * pointer into the correct size (in the same way that > assigning between differently sized integers works). Right. > This breaks down with variadic functions, which have no typing > information. So doing this: > execl("foo", "bar", my_struct_foo); > doesn't give the compiler a chance to do the implicit cast and you get > subtle breakage (in the same way that you would if you passed a long to > a variadic function expecting a short). It just passes 3 "void *"'s, and casts back. What is so strange? -- Dr. Horst H. von Brand User #22616 counter.li.org Departamento de Informatica Fono: +56 32 654431 Universidad Tecnica Federico Santa Maria +56 32 654239 Casilla 110-V, Valparaiso, Chile Fax: +56 32 797513 ``` ## Linus Torvalds, 2006-03-13 16:26 Subject: Re: [PATCH] Trivial warning fix for imap-send.c Message-ID: URL: https://gitlist.dev/e/Pine.LNX.4.64.0603130759320.3618%40g5.osdl.org In-Reply-To: <200603130414.k2D4EXcX011651@laptop11.inf.utfsm.cl> ``` On Mon, 13 Mar 2006, Horst von Brand wrote: >> > > This breaks down with variadic functions, which have no typing > > information. So doing this: > > execl("foo", "bar", my_struct_foo); > > doesn't give the compiler a chance to do the implicit cast and you get > > subtle breakage (in the same way that you would if you passed a long to > > a variadic function expecting a short). > > It just passes 3 "void *"'s, and casts back. What is so strange? It doesn't actually pass 3 "void *". Variadic functions pass their arguments as-is (apart from the normal integer promotion for small integer types: chars are passed as integers if "char" is smaller than "int"). So no actual casting takes place for the arguments, and execl("foo", "bar", my_struct_foo, NULL); passes in four pointers: two of type "char *", one of whatever my_struct_foo is ("struct foo *") and finally hopefully one of "void *" (modulo broken compilers). They might - in theory - have different sizes and representations. Now, the interesting effect of this is that simple things like printf("Pointer value %p\n", myfunction); is actually NOT STRICTLY PORTABLE CODE! Why? Because "%p" wants a void pointer, but "myfunction" is of a different pointer type, which may actually have a different size and a different representation entirely, so you can get total garbage printed out. So you have two choices: - be sane, and ignore insane architectures. In this case, the above is perfectly fine C code. - be insane, and care about it. In this case, you really do have to add the casts to be safe in theory. The "%p" for printf() is actually a wonderful example of why you really really really should ignore language lawyers. According to language lawyers, you should add that "(void *)" cast. But look around for how many such casts you can find in real code, and realize that the language lawyers just don't matter. A C compiler environment that requires it is simply broken, and sane people will refuse to use it for anything than small embedded work, because it's simply not usable. So while it's not true in theory, in _practice_ you should expect all pointers to be of the same size and use the same representation. Anything else is just too painful to be ever worth bothering with. (Remember near and far pointers from 16-bit DOS/Windows? Those environments at least had _explicit_ pointer sizes, making the problem less horrible, but that was clearly a disaster nonetheless. Pointers that implicitly have different sizes because they point to different types are even _worse_). Linus PS. Final words: while knowing that all pointer representations must be the same, and that NULL actually must be the bitwise binary "all zero" value (otherwise "memset(structptr, 0, structsize)" would break), it's equally important to know that the standard doesn't _guarantee_ it. Why? Because that way, when somebody tries to tell you that your code isn't standards conformant and might be unportable, you can say "yeah, I know, but only a total loser would ever care". Instead of incorrectly trying to argue that your code is "correct". ``` ## Jeff King, 2006-03-14 00:42 Subject: [OT] Re: [PATCH] Use explicit pointers for execl...() sentinels. Message-ID: <20060314004253.GA24668@coredump.intra.peff.net> URL: https://gitlist.dev/e/20060314004253.GA24668%40coredump.intra.peff.net In-Reply-To: <200603130412.k2D4CW1b011631@laptop11.inf.utfsm.cl> ``` On Mon, Mar 13, 2006 at 12:12:31AM -0400, Horst von Brand wrote: > Very improbable, they'll be the same normally ("void *" is a way of getting > rid of the overloading of the meaning of "char *" for this before ANSI C). > Sure, sizeof(int *) might be 4, but I think that is pretty far off. Let me clarify my position. The STANDARD doesn't guarantee such things. In PRACTICE, for modern machines you can assume that all pointers are the same size (and things like all-bits-zero is a null pointer) if it makes your code cleaner. In other words, I agree with Linus: git should follow what works in practice, but you should at least recognize that you're violating the standard. That being said, you appear to be making the argument that passing a 'foo *' to a variadic function expecting a 'bar *' doesn't violate the standard. I believe it invokes undefined behavior. > There are special rules for variadic functions, probably pointers would be > cast to/from void * in such a case by the compiler. The rules indicate that arguments matching the '...' follow "default argument promotion". See section 6.5.2.2, paragraph 7. This default promotion is the same as what would happen if there were no prototype for the function, and is defined in paragraph 6: ...the integer promotions are performed on each argument, and arguments that have type float are promoted to double. I don't see anything about promoting pointers to void. Furthermore, when accessing the arguments using va_arg, the types must match or the behavior is undefined, UNLESS (7.5.1.1, para 2): - one type is signed and the other is the matching unsigned type - one type is a pointer to void and the other is a pointer of character type IOW, the standard does promise that void* and char* pointers are represented the same, but nothing else. > > If you remain unconvinced, I can try to find chapter and verse of the > > standard. > Please do. See above. -Peff ``` ## Horst H. von Brand, 2006-12-05 17:34 Subject: Re: [PATCH] git-explain Message-ID: <200612051734.kB5HYBUs016133@laptop13.inf.utfsm.cl> URL: https://gitlist.dev/e/200612051734.kB5HYBUs016133%40laptop13.inf.utfsm.cl In-Reply-To: ``` Jeff King wrote: [...] > As long as the safety valves don't come up _routinely_ in certain > workflows, it seems OK to bypass them with a '-f' force switch. I > suspect the best way to figure out if such workflows are in use is to > put in the safety valves and see who complains; otherwise we're stuck > with brainstorming workflows and deciding whether they make sense. Problem is that nobody reads the manuals, next to nobody complains, and when it doesn't work out via road A you try plan B. A might have been exactly right, but it is blocked, and you'll never know if it is because of fundamental reasons or by decree. -- Dr. Horst H. von Brand User #22616 counter.li.org Departamento de Informatica Fono: +56 32 2654431 Universidad Tecnica Federico Santa Maria +56 32 2654239 ```