daveh@marob.masa.com (Dave Hammond) (10/07/89)
In article <252BFD80.194C@marob.masa.com> I wrote about problems using
time(). The mysterious clock problems turned out to be simply a matter
of a particular routine "assuming" that a standard header had properly
declared time(), when in fact that was not so. The clock "flip-flop"
effect was the result of taking the time() value as a 32-bit quantity in
some places and a 16-bit quantity in others.
I apologize to the net for wasting bandwidth with this problem, as a
simple "linting" would have pointed me in the right direction.
My thanks to Tim Olson (tim@amd.com) for replying so quickly and
pointing out what should have been obvious.
--
Dave Hammond
daveh@marob.masa.comjames@castle.ed.ac.uk (J Gillespie) (02/04/91)
Is there a simple way of getting hold of the system time from within a C program? I've read all the material I can find on time.h, but can't make head or tail of it, and have had to resort to passing in the time as a command line argument. This may be a really idiotic question, -- James Gillespie, /~~~~~~~~\ Edinurgh University. / @ @ \ "I'm not musical either - james@ed.ac.uk / < \ I play the bagpipes" ____________________/ \________/ \__________________________________________
james@castle.ed.ac.uk (J Gillespie) (02/06/91)
Thankyou to everyone who e-mailed me an explanation; I've got stacks of them now, and have got over my difficulty with all of the time functions I've tried. -- James Gillespie, /~~~~~~~~\ "Looks fast!" Edinurgh University. / @ @ \ "Yeah. That's the stripes, man." james@ed.ac.uk / < \ -- Fandango ____________________/ \________/ \__________________________________________
rana@hpqtdla.sqf.hp.com (Rana Raychoudhury) (02/07/91)
> Is there a simple way of getting hold of the system time from within a > C program? I've read all the material I can find on time.h, but can't > make head or tail of it, and have had to resort to passing in the time > as a command line argument. This may be a really idiotic question, > -- > James Gillespie, /~~~~~~~~\ > Edinurgh University. / @ @ \ "I'm not musical either - > james@ed.ac.uk / < \ I play the bagpipes" > ____________________/ \________/ \__________________________________________ (I assume you mean "time of day" and are using an ANSI compiler :- [From the 'C Programming Language, 2nd Edition pp256-7] .... char *asctime(const struct tm *tp) converts the time in the structure *tp into a string of the form Sun Jan 3 15:14:13 1988\n char *ctime(const time_t *tp) converts the calendar time *tp to local time; equivalent to :- asctime(localtime(tp)); blah blah blah. Here's a little example :- ---------------- snip snip -------------- #include <time.h> time_t now; main() { (void) time( &now ); printf("The time is : %s\n", asctime(localtime(&now))); } ---------------- snip snip -------------- which produces something like :- The time is : Wed Feb 6 17:27:15 1991 -rana-
murison@cfa.HARVARD.EDU (Marc A. Murison, RG) (02/07/91)
From article <8258@castle.ed.ac.uk>, by james@castle.ed.ac.uk (J Gillespie): > > Is there a simple way of getting hold of the system time from within a > C program? Here's how I do it: #include <time.h> #include <stdio.h> void main( void ) { time_t cur_time; /* get the current date and time */ time(&cur_time); /* ctime() converts a value of type time_t to a string */ printf( "\ncurrent time = %s", ctime(&cur_time) ); } Marc Murison Smithsonian Astrophysical Observatory murison@cfacx2.harvard.edu (the address in the header will bomb)
martin@mwtech.UUCP (Martin Weitzel) (05/21/91)
In article <588@sherpa.UUCP> rac@sherpa.UUCP (Roger Cornelius) writes: [...] > long seed = time(0L); ^^--------- wrong time((long *)0); ^^^^^^^^^--- right Note that the argument to the time system call is a `long *', aka pointer to long. Repeat after me: POINTER to long. But I've seen this too often, even in examples printed in some manuals. Though I had not much confidence into those particular manuals where I found it (for reasons that probably aren't of any interrest to the readers of this group), I'm curious if anybody has ideas from where this misconception originated. Or, if you have two minutes and you have some source code from which you suppose that it contains a time system call, just grep through it and look if it's done right. I'm always wondering why people can't grasp the difference between a plain integer, a long and pointers to any of it. -- Martin Weitzel, email: martin@mwtech.UUCP, voice: 49-(0)6151-6 56 83
lothar@tmcsys.UUCP (L. Hirschbiegel) (05/22/91)
In article <1141@mwtech.UUCP> martin@mwtech.UUCP (Martin Weitzel) writes: >In article <588@sherpa.UUCP> rac@sherpa.UUCP (Roger Cornelius) writes: >[...] >> long seed = time(0L); > ^^--------- wrong > time((long *)0); > ^^^^^^^^^--- right > You're a cousin of "lint", aren't you :-) ? >Note that the argument to the time system call is a `long *', aka >pointer to long. Ever tried to RTFM for yourself? NAME time - get time DESCRIPTION The time system call returns the value of time in seconds since 00:00:00 Greenwich Mean Time (GMT), January 1, 1970. If tloc is non-zero, the return value is also stored in the ^^^^^^^^^^^ location to which tloc points. If you still don't understand: I'll help you! For this little test program: main() { time(0L); time((long *)0); } the assembler output from my C compiler is: .file "main.c" gcc_compiled.: .text .align 4 .globl main main: pushl %ebp movl %esp,%ebp pushl $0 call time pushl $0 call time .L1: leave ret >Repeat after me: POINTER to long. Repeat after me: IT DOESN'T MATTER :-) >I'm always wondering why people can't grasp >the difference between a plain integer, a long and pointers to any of it. So do I... >-- >Martin Weitzel, email: martin@mwtech.UUCP, voice: 49-(0)6151-6 56 83 Lothar Hirschbiegel -- ----------------------------------------------- L. Hirschbiegel, AEG - A84, Frankfurt (Germany) email: unido!aega84!lh tel: -49-69-66414316 -----------------------------------------------
gwyn@smoke.brl.mil (Doug Gwyn) (05/22/91)
In article <1141@mwtech.UUCP> martin@mwtech.UUCP (Martin Weitzel) writes: >> long seed = time(0L); >I'm curious if anybody has ideas from where this misconception originated. The roots of many such errors are probably unknown even to the original culprits. In this particular case, my best guess is that the common "time(0)" usage was "repaired" during port to a system where it mattered, and the person doing the repair was thinking merely of the required representation for the argument to work right, not more abstractly in terms of its type. You can see this also in the common #define NULL 0L
gwyn@smoke.brl.mil (Doug Gwyn) (05/22/91)
In article <381@tmcsys.UUCP> lh@aega84.UUCP (L. Hirschbiegel) writes: >Ever tried to RTFM for yourself? > If tloc is non-zero, the return value is also stored in the > ^^^^^^^^^^^ That's what you get for looking in a buggy FM! >If you still don't understand: I'll help you! Martin understands just fine. >Repeat after me: IT DOESN'T MATTER :-) Oh, yes it does -- if your application is any good, some day it might be ported to a platform other than the specific environment you used in your testing.
ckp@grebyn.com (Checkpoint Technologies) (05/22/91)
In article <381@tmcsys.UUCP> lh@aega84.UUCP (L. Hirschbiegel) writes: >In article <1141@mwtech.UUCP> martin@mwtech.UUCP (Martin Weitzel) writes: >>In article <588@sherpa.UUCP> rac@sherpa.UUCP (Roger Cornelius) writes: >>[...] >>> long seed = time(0L); >> ^^--------- wrong >> time((long *)0); >> ^^^^^^^^^--- right >> >>Note that the argument to the time system call is a `long *', aka >>pointer to long. > >Ever tried to RTFM for yourself? > > NAME > time - get time > > DESCRIPTION > The time system call returns the value of time in seconds > since 00:00:00 Greenwich Mean Time (GMT), January 1, 1970. > > If tloc is non-zero, the return value is also stored in the > ^^^^^^^^^^^ > location to which tloc points. You omitted the SYNOPSIS for time: "time_t time(tloc) time_t *tloc;", which clearly shows that the argument to time() is a pointer to a time_t (which BTW is the *real* correct answer). This man page uses an archaic style when using the term "non-zero". Read the FAQ for the whole poop on NULL and 0; there is a place in C where 0 is treated as NULL, but arguments to functions sans prototype isn't one of them. >For this little test program: > >main() >{ >time(0L); >time((long *)0); >} > >the assembler output from my C compiler is: > [ omitted; irrelevant ] BIG mistake. The assembly language output from the C compiler shows all the implementation details, including the ones which are not guanteed to be portable. You will not learn portable C programming this way, quite the opposite. There's no favor from anyone in comp.lang.c for arguing how widely applicable implementation details are; all the world's NOT a VAX or i386... >>Repeat after me: POINTER to long. >Repeat after me: IT DOESN'T MATTER :-) Too many programmers repeat after you as it is. It does matter. -- Richard Krehbiel, private citizen ckp@grebyn.com (Who needs a fancy .signature?)
jeff@uf.msc.umn.edu (Jeff Turner) (05/22/91)
In article <1141@mwtech.UUCP> martin@mwtech.UUCP (Martin Weitzel) writes: >In article <588@sherpa.UUCP> rac@sherpa.UUCP (Roger Cornelius) writes: >[...] >> long seed = time(0L); > ^^--------- wrong > time((long *)0); > ^^^^^^^^^--- right > >Note that the argument to the time system call is a `long *', aka >pointer to long. Repeat after me: POINTER to long. You are right -- to ensure portability, you should always pass what is expected (i.e. a pointer and not a long). This way, if someday a machine is created on which a pointer to a long is a different size then a long, the program will still work. However, I don't know of any machines on which the two differ. Perhaps you are confusing time(0) with time(0L). Calling time(0) on a 16-bit machine (e.g. PCs not running 386 UNIX) will push only a 16 bit value onto the stack instead of the 32 bit address that time() is expecting (and is going to use). Since half of the 32-bit address will have a random (and probably non-zero) value, time() will use it as an address to store the current time into, and bash your memory or cause range-error core-dump. I encountered this problem frequently back when I was running uPort's SysV/AT. I found that passing NULL to time() (or to any other similar function) worked in all cases as the value of NULL changes with the memory model (i.e. small memory model defined it simply as 0, whereas large memory model defined it as 0L). Of course by definition, passing (long*)0 is the only truely correct way to pass a long pointer that points to address 0, but that is toooooooo much to type :-) -Jeff --- Jeff Turner EMAIL: jeff@msc.edu Minnesota Supercomputer Center, Inc. VOICE: (612) 626-0544 Minneapolis, Minnesota 55415 FAX: (612) 624-6550
tim@dell.co.uk (Tim Wright) (05/22/91)
In <381@tmcsys.UUCP> lothar@tmcsys.UUCP (L. Hirschbiegel) writes: >In article <1141@mwtech.UUCP> martin@mwtech.UUCP (Martin Weitzel) writes: >>In article <588@sherpa.UUCP> rac@sherpa.UUCP (Roger Cornelius) writes: >>[...] >>> long seed = time(0L); >> ^^--------- wrong >> time((long *)0); >> ^^^^^^^^^--- right >> >You're a cousin of "lint", aren't you :-) ? >>Note that the argument to the time system call is a `long *', aka >>pointer to long. >Ever tried to RTFM for yourself? > NAME > time - get time > DESCRIPTION > The time system call returns the value of time in seconds > since 00:00:00 Greenwich Mean Time (GMT), January 1, 1970. > If tloc is non-zero, the return value is also stored in the > ^^^^^^^^^^^ > location to which tloc points. >If you still don't understand: I'll help you! >For this little test program: Yes but it's a pointer i.e. they are talking about a non-zero or non-null pointer. If the compiler allows prototypes, it can cast a 0 correctly. Without prototypes it cannot. >main() >{ >time(0L); >time((long *)0); >} >the assembler output from my C compiler is: ... irrelevant :-) >>Repeat after me: POINTER to long. >Repeat after me: IT DOESN'T MATTER :-) *WRONG* *WRONG* *WRONG*. Just because it doesn't happen to matter on *YOUR* machine with *YOUR* C compiler does not mean it does not matter. On a non-ansi compliant compiler on a '286 system it is possible to choose a memory model where sizeof(long) != sizeof(long *). In this case the code generated will be somewhat different :-) Tim -- Tim Wright, Dell Computer Corp., Bracknell | Domain: tim@dell.co.uk Berkshire, UK, RG12 1RW. Tel: +44-344-860456 | Uucp: ...!ukc!delluk!tim Smoke me a Kipper, I'll be back for breakfast - Red Dwarf
david@sco.COM (David Fiander) (05/22/91)
In article <381@tmcsys.UUCP> lh@aega84.UUCP (L. Hirschbiegel) writes: > >You're a cousin of "lint", aren't you :-) ? > No, he's somebody that knows the difference between a null pointer and the integer zero. >Ever tried to RTFM for yourself? > > NAME > time - get time > > DESCRIPTION > The time system call returns the value of time in seconds > since 00:00:00 Greenwich Mean Time (GMT), January 1, 1970. You missed a line which appears in _my_ FM: long *tloc; >If you still don't understand: I'll help you! >For this little test program: > >main() >{ >time(0L); >time((long *)0); >} > >the assembler output from my C compiler is: > > [assembler code deleted, I try to avoid it] > So you're using GCC on an intel platform (i386 obviously, it wouldn't fit on anything else). Lucky you, you've never had to program an Cyber 180. If you tried that trick on any CDC machine , you would get a "Ring-0" hardware exception and, if you hadn't installed a handler, your program could get blown off (I think. I never used NOS/VE, and what I used just caused a disgusting penalty hit, since we trapped to the OS, it said "shithead programmer," and returned). There exist architectures on which the NULL pointer is different from "all bits 0". In fact, if you can get the compiler people to agree to it, the best value for the NULL pointer is something that is an invalid virtual memory address, because then, when somebody tries to dereference it, you get a core dump (note that 0 might be a valid address which would never be referred to by the user, for example: the address of the C startup code). The large part of a project I was involved was spent inserting necessary casts into function calls, in order to _fix_ code written by people that believed > >Repeat after me: IT DOESN'T MATTER :-) -- David J. Fiander SCO MMDF Development Team SCO Canada, Inc.
clewis@ferret.ocunix.on.ca (Chris Lewis) (05/22/91)
In article <381@tmcsys.UUCP> lh@aega84.UUCP (L. Hirschbiegel) writes: >In article <1141@mwtech.UUCP> martin@mwtech.UUCP (Martin Weitzel) writes: >>In article <588@sherpa.UUCP> rac@sherpa.UUCP (Roger Cornelius) writes: >>[...] >>> long seed = time(0L); >> ^^--------- wrong >> time((long *)0); >> ^^^^^^^^^--- right >Ever tried to RTFM for yourself? > DESCRIPTION > The time system call returns the value of time in seconds > since 00:00:00 Greenwich Mean Time (GMT), January 1, 1970. > If tloc is non-zero, the return value is also stored in the > ^^^^^^^^^^^ > location to which tloc points. You omitted the part where it defined time as: long time (long *tloc) (Or, for that matter, "time_t time (time_t *tloc);") Within context, the "non-zero" really means "non-null pointer to long(time_t)" [I've even seen manual pages where they are out-and-out wrong. I've seen time(0) in some manual pages, and that won't work either] >If you still don't understand: I'll help you! >For this little test program: Lothar goes on to "prove" his point by test compiling a bit of C and showing that "time(0L)" and "time((long *) 0)" compiles the same way. (on what looks like a 386 according to the assembler) You're lucky. Not all machines have pointers that are the same length as longs. Try this code on a PDP-11 or on many small mode 8086 compilers: int func(a, b, c) long *b; int a,c; { printf("%d:%d\n", a, c); } func(4, 0L, 4); It won't print "4:4". Some compilers will print "4:0", and some'll print "0:4", and some will print garbage. func(4, (long*)0, 4) will print 4:4. Lint and Martin are correct. Pointers aren't longs. They ain't ints either. Didn't your math teacher tell you that you can't prove a theory on a single example? On the other hand, any theory can be disproved by a single counter-example. -- Chris Lewis, Phone: (613) 832-0541, Domain: clewis@ferret.ocunix.on.ca UUCP: ...!cunews!latour!ecicrl!clewis; Ferret Mailing List: ferret-request@eci386; Psroff (not Adobe Transcript) enquiries: psroff-request@eci386 or Canada 416-832-0541. Psroff 3.0 in c.s.u soon!
ed@mtxinu.COM (Ed Gould) (05/22/91)
>You are right -- to ensure portability, you should always pass what is expected >(i.e. a pointer and not a long). This way, if someday a machine is created >on which a pointer to a long is a different size then a long, the program will >still work. However, I don't know of any machines on which the two differ. The machine on which this stuff was first developed - the PDP-11 - is one. It has 16-bit pointers. So does any machine with a 64K address space. In fact, the early (Sixth Edition and before) form of the time() call was time(t) int t[2]; In those days, "long" ints were represented explicitly as arrays of two ints. The time() function took (a pointer to) one of those. It had no meaningful return value. In Seventh Edition UNIX, by which time the C language had been extended to include longs and typedefs, time() became time_t time(t) time_t *t; Not too long ago, I saw a piece of code (in a large, now commercial, software system that will remain nameless) that looked like this: time_t now; #if some_machine_type || some_other_machine_type now = time(0); #else now = time(); #endif Evidently, time() failed on those machines when passed no arguments, so somebody "fixed" the bug. I wish people would take the 15 or 20 seconds it takes to get something like this right. -- Ed Gould No longer formally affiliated with, ed@mtxinu.COM and certainly not speaking for, mt Xinu. "I'll fight them as a woman, not a lady. I'll fight them as an engineer."
torek@elf.ee.lbl.gov (Chris Torek) (05/23/91)
In article <1141@mwtech.UUCP> martin@mwtech.UUCP (Martin Weitzel) writes: >... I'm curious if anybody has ideas from where this misconception >[about the type of the argument to time()] originated. Perhaps it dates back to Version 6 Unix, when C did not have a `long' type. The time() system call took a pointer to the first of two `ints'. In V7 this became a pointer to a long, but of course in V6 pointers and integers were freely interchangeable, and people wrote such gems as struct xxreg { int csr; /* control/status */ int bar; /* unibus address */ }; ... 0177440->bar = bp->b_addr; 0177440->csr = XXREAD | XXGO; One can speculate that `long' went in before `pointers are not ints', and people changed their time(vec); calls to t = time(0); calls, but for backwards compatibility the time() function still accepted pointers. It still does; it should be `time_t time(void)', but compatibility with 20-year-old software is oh so terribly important :-) . Perhaps if dmr is indulging his news habit and sees this, he might comment.... -- In-Real-Life: Chris Torek, Lawrence Berkeley Lab CSE/EE (+1 415 486 5427) Berkeley, CA Domain: torek@ee.lbl.gov
jim@sco.COM (Jim Sullivan) (05/23/91)
lothar@tmcsys.UUCP (L. Hirschbiegel) writes: >In article <1141@mwtech.UUCP> martin@mwtech.UUCP (Martin Weitzel) writes: >>In article <588@sherpa.UUCP> rac@sherpa.UUCP (Roger Cornelius) writes: >>[...] >>> long seed = time(0L); >> ^^--------- wrong >> time((long *)0); >> ^^^^^^^^^--- right > >If you still don't understand: I'll help you! >For this little test program: > >main() >{ >time(0L); >time((long *)0); >} > >the assembler output from my C compiler is: < assembly code illustrating that 0L and (long *)0 both map to 0 > This if from YOUR C compiler. And on many systems, similar code would be produced. But on many systems 0L and (long *)0 are not equal, so writers of portable code should do the right thing and not assume that the entire work is the same as their machine. > >Repeat after me: IT DOESN'T MATTER :-) But it does. >>I'm always wondering why people can't grasp >>the difference between a plain integer, a long and pointers to any of it. Here! Here! >>Martin Weitzel, email: martin@mwtech.UUCP, voice: 49-(0)6151-6 56 83 >Lothar Hirschbiegel -- Jim Sullivan "Revenue Canada: SCO Canada, Inc. You Make It Toronto, Ontario, Canada We Take It" jim@scocan.com, ...!uunet!scocan!jim - Backbench comic in the Globe
sef@kithrup.COM (Sean Eric Fagan) (05/23/91)
In article <1141@mwtech.UUCP> martin@mwtech.UUCP (Martin Weitzel) writes: >In article <588@sherpa.UUCP> rac@sherpa.UUCP (Roger Cornelius) writes: >> long seed = time(0L); > ^^--------- wrong > time((long *)0); > ^^^^^^^^^--- right >Note that the argument to the time system call is a `long *', aka >pointer to long. Repeat after me: POINTER to long. On the other hand, if you have an ansi-conformant compiler, and a prototype is in scope, then either solution will work properly. -- Sean Eric Fagan | "I made the universe, but please don't blame me for it; sef@kithrup.COM | I had a bellyache at the time." -----------------+ -- The Turtle (Stephen King, _It_) Any opinions expressed are my own, and generally unpopular with others.
cpcahil@virtech.uucp (Conor P. Cahill) (05/23/91)
lothar@tmcsys.UUCP (L. Hirschbiegel) writes: >Ever tried to RTFM for yourself? >If you still don't understand: I'll help you! >For this little test program: >main() >{ >time(0L); >time((long *)0); >} >the assembler output from my C compiler is [the same for both.] >Repeat after me: IT DOESN'T MATTER :-) Boy, you just showed the world that you are a junior programmer who hasn't yet realized that just because something works on your system doesn't mean it is right. The code you show won't work reliably on systems where the size of a pointer is different from the size of an integer (for ex: large model programs on an 8086 or 80286). Before you jump on someone elses case, be sure you know what you are talking about. -- Conor P. Cahill (703)430-9247 Virtual Technologies, Inc. uunet!virtech!cpcahil 46030 Manekin Plaza, Suite 160 Sterling, VA 22170
toy@pegasus.crd.ge.com (Raymond L. Toy) (05/23/91)
In article <4138@uc.msc.umn.edu> you write: |> In article <1141@mwtech.UUCP> martin@mwtech.UUCP (Martin Weitzel) writes: |> >In article <588@sherpa.UUCP> rac@sherpa.UUCP (Roger Cornelius) writes: |>[...] |> |> You are right -- to ensure portability, you should always pass what is expected |> (i.e. a pointer and not a long). This way, if someday a machine is created |> on which a pointer to a long is a different size then a long, the program will |> still work. However, I don't know of any machines on which the two differ. Such machines do exist and are very common. In particular, a PC has this property when using the small memory model: a pointer is 16 bits long, but a long is 32 bits. Also, at school, we had a Harris H800 mini, a 24-bit machine. A pointer was 24 bits long (really only 22!!!), and a long was really 47 bits long!!!! This was a rather bizarre machine, to say the least. It's dead and gone now, fortunately. Ray ray ----> toy@soho.crd.ge.com |> |> -Jeff |> --- |> Jeff Turner EMAIL: jeff@msc.edu |> Minnesota Supercomputer Center, Inc. VOICE: (612) 626-0544 |> Minneapolis, Minnesota 55415 FAX: (612) 624-6550
scotte@applix.com (Scott Evernden) (05/23/91)
Someone, please, anyone- Name for me any modern computer in any kind of common use which will barf at: time(0L); -scott
barmar@think.com (Barry Margolin) (05/23/91)
In article <1200@applix.com> scotte@applix.UUCP (Scott Evernden) writes: >Name for me any modern computer in any kind of common use which will >barf at: > time(0L); I frequently respond to this frequent question with one or two machines I'm familiar with, but I'm not going to do that this time. Suffice it to say that there *are* systems where the null pointer is not implemented as the same bit pattern as 0L. No, these systems are not "in common use". 90% of all C programs are probably running on 80*86, 680x0, VAX, and SPARC processors, but does that mean that all other systems should be ignored? Prototypes are the most appropriate solution to this problem. With a prototype in scope, the above call works right on any ANSI-conforming implementation. But if you need to be portable to pre-ANSI implementations, then is "(char *)0" so hard to write? Not only does it inform the compiler of the correct code to generate (in implementations that care), but it also makes the intent clearer to *human* readers of the code. What's the point of looking for a way around something that's so clearly right? -- Barry Margolin, Thinking Machines Corp. barmar@think.com {uunet,harvard}!think!barmar
boyd@prl.dec.com (Boyd Roberts) (05/23/91)
In article <13455@dog.ee.lbl.gov>, torek@elf.ee.lbl.gov (Chris Torek) writes: > > Perhaps it dates back to Version 6 Unix, ... I think Chris is right. If you look at any routine that requires a long (in the `standard' C library) it'll be passed by a pointer. One Computing Centre once changed ctime(3) to take a long instead of a long *. Boy, were they sorry. Broken code from here to backups... Boyd Roberts boyd@prl.dec.com ``When the going gets wierd, the weird turn pro...''
conor@lion.inmos.co.uk (Conor O'Neill) (05/23/91)
In article <4138@uc.msc.umn.edu> jeff@uf.UUCP (Jeff Turner) writes: >You are right -- to ensure portability, you should always pass what is expected >(i.e. a pointer and not a long). This way, if someday a machine is created >on which a pointer to a long is a different size then a long, the program will >still work. However, I don't know of any machines on which the two differ. Think of a 16-bit machine, where pointers are 16-bits, but longs are 32-bits. Not a particularly obscure possibility. (Oh - by the way - we sell one - the IMS T222 transputer). --- Conor O'Neill, Software Group, INMOS Ltd., UK. UK: conor@inmos.co.uk US: conor@inmos.com "It's state-of-the-art" "But it doesn't work!" "That is the state-of-the-art".
imc@prg.ox.ac.uk (Ian Collier) (05/23/91)
In article <381@tmcsys.UUCP>, lh@aega84.UUCP (L. Hirschbiegel) wrote: [about the argument of time()] > NAME > time - get time > > DESCRIPTION > The time system call returns the value of time in seconds > since 00:00:00 Greenwich Mean Time (GMT), January 1, 1970. > > If tloc is non-zero, the return value is also stored in the > ^^^^^^^^^^^ > location to which tloc points. Obviously the writers of your manual don't know the difference between a long and a pointer, then. Ours seems to have it about right... SYNOPSIS ... time_t time(tloc) time_t *tloc; DESCRIPTION time() returns the time since 00:00:00 GMT, Jan. 1, 1970, measured in seconds. If tloc is non-NULL, the return value is also stored in the ^^^^ location to which tloc points. >For this little test program: ... >the assembler output from my C compiler is: ... All that means is that your computer represents a NULL pointer by zero (also evident from your manual page). So for time(0L) it pushes a long zero, and for time((long *)0) it pushes a null pointer, which is just a long zero. That will happen on most computers, but not all. (As someone has pointed out, however, if a prototype for time() is in scope then a zero will be converted into a null pointer, probably without a warning message). Ian Collier Ian.Collier@prg.ox.ac.uk | imc@ecs.ox.ac.uk
pt@geovision.gvc.com (Paul Tomblin) (05/23/91)
scotte@applix.com (Scott Evernden) writes: >Someone, please, anyone- >Name for me any modern computer in any kind of common use which will >barf at: > time(0L); Well, when I worked on a MVS/XA system, it probably would have barfed on that line, but only because we didn't have a C compiler on it..... :-) //GO.SYSIN DD *, DO-DAH, DO-DAH -- Paul Tomblin, Department of Redundancy Department. ! My employer does Two roads diverged in a wood, and I/ I took the one less ! not stand by my travelled by/ And that is why I'm lost, dammit... ! opinions.... pt@geovision.gvc.com or {cognos,uunet}!geovision!pt ! Me neither.
6sigma2@polari.UUCP (Brian Matthews) (05/23/91)
In article <381@tmcsys.UUCP> lh@aega84.UUCP (L. Hirschbiegel) writes: |In article <1141@mwtech.UUCP> martin@mwtech.UUCP (Martin Weitzel) writes: |>In article <588@sherpa.UUCP> rac@sherpa.UUCP (Roger Cornelius) writes: |>[...] |>> long seed = time(0L); |> ^^--------- wrong |> time((long *)0); |> ^^^^^^^^^--- right |Ever tried to RTFM for yourself? | | NAME | time - get time | If tloc is non-zero, the return value is also stored in the | ^^^^^^^^^^^ | location to which tloc points. You'll get yourself in trouble by taking parts of TFM out of context. The part you left out reads something like: long time ((long *) 0) long time (tloc) long *tloc; Tloc still has to be of the correct type, whether zero or non-zero. This can happen by having a prototype in scope or with an appropriate cast. |If you still don't understand: I'll help you! |For this little test program: |[program calling time with (incorrect) 0L and (correct) (long *)0] This just proves that on your particular OS at some particular release level and version with some particular version of gcc, it happens to work. In general, portable code must assume time(0L) won't work. |Repeat after me: IT DOESN'T MATTER :-) Please remind me never to use any code you've written. It will undoubtedly fail in many strange and mysterious (to you at least) ways.
jim@segue.segue.com (Jim Balter) (05/24/91)
In article <4138@uc.msc.umn.edu> jeff@uf.UUCP (Jeff Turner) writes: >Of course by definition, passing (long*)0 is the only truely correct >way to pass a long pointer that points to address 0, but that is toooooooo >much to type :-) By the ANSI standard, if there is a prototype in scope, a zero value of any type is "correct". Without a prototype, only (time_t *)0 is correct.
gwyn@smoke.brl.mil (Doug Gwyn) (05/24/91)
In article <1200@applix.com> scotte@applix.UUCP (Scott Evernden) writes: >Name for me any modern computer in any kind of common use which will >barf at: > time(0L); Look, you're wasting our time. Examples have been given many times in this newsgroup previously, but anyway since it is NOT guaranteed to work why don't just just accept that and do it right?
lothar@tmcsys.UUCP (L. Hirschbiegel) (05/24/91)
In article <1991May22.190908.5311@virtech.uucp> cpcahil@virtech.uucp (Conor P. Cahill) writes: > >The code you show won't work reliably on systems where the size of >a pointer is different from the size of an integer (for ex: large >model programs on an 8086 or 80286). This was posted to newsgroup "comp.UNIX.SYSV386", right?? This was NOT posted to newsgroup "comp.unix.any_intel_product", right?? The original poster had problems with SCO-UNIX password generator, not with a password generator running under MS-C6.0 in tiny-mode, right?? Martin was referring to a "time" syscall argument mismatch under SCO-UNIX. I pointed out, that it really makes no difference for the argument value itself. I did NOT say this is a general rule-of-thumb for all kinds of cpus and all kinds of compilers?!?! >Before you jump on someone elses case, be sure you know what >you are talking about. Before you answer in a newsgroup make sure you realise what that newsgroup is about. >Conor P. Cahill (703)430-9247 Virtual Technologies, Inc. -- ----------------------------------------------- L. Hirschbiegel, AEG - A84, Frankfurt (Germany) email: unido!aega84!lh tel: -49-69-66414316 -----------------------------------------------
jim@segue.segue.com (Jim Balter) (05/24/91)
In article <1991May22.130713.25852@sco.COM> david@sco.COM (David Fiander) writes: >There exist architectures on which the NULL pointer is different from >"all bits 0". This isn't relevant if using ANSI prototypes, since any form of zero is implicitly converted to the appropriate null pointer, no matter how it is represented. If you aren't using ANSI prototypes, then you should manually cast everything into the proper type, and run lint religiously. Hopefully, some day, we will only concerned with the first case. >In fact, if you can get the compiler people to agree to >it, the best value for the NULL pointer is something that is an invalid >virtual memory address, because then, when somebody tries to dereference >it, you get a core dump (note that 0 might be a valid address which would >never be referred to by the user, for example: the address of the C >startup code). This doesn't require a non-zero representation of NULL, since page 0 of the virtual address space can usually be made inaccessible. Most UNIX systems don't do it because there are too many bad UNIX programs written by too many bad UNIX programmers, and any desire to do the "right" thing is overridden by the market demand to allow programs that dereference NULL to continue to "work".
cpcahil@virtech.uucp (Conor P. Cahill) (05/24/91)
lothar@tmcsys.UUCP (L. Hirschbiegel) writes: >This was posted to newsgroup "comp.UNIX.SYSV386", right?? NO. It was cross-posted to comp.unix.sysv386 AND comp.lang.c >Martin was referring to a "time" syscall argument mismatch under SCO-UNIX. Martin was saying that it was incorrect useage to call time(0L) period. >I pointed out, that it really makes no difference for the argument value itself. >I did NOT say this is a general rule-of-thumb for all kinds of cpus >and all kinds of compilers?!?! You did not say "it makes on difference under SCO UNIX". You said it makes no difference. Either way (on SCO UNIX or not) it is incorrect to code time(0L) as opposed to time((long*)0). >Before you answer in a newsgroup make sure you realise what that newsgroup >is about. Learn how to read headers. -- Conor P. Cahill (703)430-9247 Virtual Technologies, Inc. uunet!virtech!cpcahil 46030 Manekin Plaza, Suite 160 Sterling, VA 22170
jja@wsl.ie (John Allen on wsl) (05/24/91)
In article <1200@applix.com> scotte@applix.UUCP (Scott Evernden) writes: >Someone, please, anyone- >Name for me any modern computer in any kind of common use which will >barf at: > > time(0L); > >-scott I suppose this is a good a reason as you can come up with to do it inorrectly. You belong to the old school of 'we used to do it wrong, we still do it wrong'
lbr@holos0.uucp (Len Reed) (05/24/91)
In article <1991May22.130713.25852@sco.COM> david@sco.COM (David Fiander) writes: >There exist architectures on which the NULL pointer is different from >"all bits 0". In fact, if you can get the compiler people to agree to >it, the best value for the NULL pointer is something that is an invalid >virtual memory address, because then, when somebody tries to dereference >it, you get a core dump (note that 0 might be a valid address which would >never be referred to by the user, for example: the address of the C >startup code). Under what architecture is a NULL pointer not "all bits 0"? The problems, as many have said, result when a pointer has fewer or more bits than an integer (or a long). NULL pointers *are* all zeros, though, since casting zero to a pointer must yield a null pointer. (Maybe there's a loophole here in ANSI that says that (void *)0 is a null pointer but the compiler could implement that casted zero as somthing other than a pattern of zeros. Not likely in the real world.) You're certainly right that a NULL that produced a virtual memory fault upon any use would be better than a virtual zero address. But it's not the "compiler people" who would have to agree to this. Far too much existing code would be broken by a move to this, I'm afraid. -- Len Reed Holos Software, Inc. Voice: (404) 496-1358 UUCP: ...!gatech!holos0!lbr
bright@nazgul.UUCP (Walter Bright) (05/25/91)
In article <4138@uc.msc.umn.edu> jeff@uf.UUCP (Jeff Turner) writes:
/Perhaps you are confusing time(0) with time(0L). Calling time(0) on a 16-bit
/machine (e.g. PCs not running 386 UNIX) will push only a 16 bit value onto the
/stack instead of the 32 bit address that time() is expecting (and is going to
/use). Since half of the 32-bit address will have a random (and probably
/non-zero) value, time() will use it as an address to store the current time
/into, and bash your memory or cause range-error core-dump.
Actually, in all the 16 bit compilers I'm currently familiar with,
time() is prototyped such that the argument is correctly cast, and the
problem doesn't arise. For instance, time(long *) being the prototype,
time(0) becomes time((long*)0) which will always work correctly.
The time you unavoidably get into trouble is if it is a varargs function
like printf().lothar@tmcsys.UUCP (L. Hirschbiegel) (05/25/91)
>This just proves that on your particular OS at some particular release >level and version with some particular version of gcc, it happens to >work. In general, portable code must assume time(0L) won't work. To come to an end with this really annoying 'discussion' please accept the following statement: the whole thread of time(0L) vs. time((long *)0) was about the **Intel i386 processor** running under **ISC/SCO-UNIX** (Martin Weitzel's posting as a reply and my own followup). I didn't intend to crosspost this to com.lang.c but it happened somehow. This admittedly gave it a different kind of taste... I did not even realize this was also in comp.lang.c (thanks, steve, for the warning!). Since I had included assembler output for the i386 and also a part of the SysV/386 man page for 'time' I thought it would be clear what I was talking about. It was also obvious from the subject line about SCO-UNIX. It was most interesting to see, how some people declared my included assembler listing as "irrelevant", thus pushing the topic to a level of abstraction it was never ment to be. It was EXACTLY for i386 assembler on a box running UNIX, nothing else. If you can't find a reason to flame - just create one... Now everybody is telling me this will not run on an 8086, i286 or some kind of superprocessor with 124 bit pointers and 12 bit longs. Thanks for the help! One guy (forgot the name, but he must have been recently out of school, 'cause he still was arguing with me about his math teacher) even proved it doesn't work on a PDP11. Quite irritating, since nobody claimed it would run on a PDP11. Nice proof, anyway. My own proof was (only!) for the i386 under ISC-UNIX - my fault that I didn't state it loud n'clear enough in my original posting. One can of course argue about what people think is "good programming practice", but that's different from saying "you don't know what you are talking about" or "you fool think your unix box is the center of the world". At least in my opinion. So, *please stop* sending me any more hate mail. I'm already enlighted as a "junior programmer" (this was one of the more modest things I received by mail; some nice guys hanging around here). -- ----------------------------------------------- L. Hirschbiegel, AEG - A84, Frankfurt (Germany) email: unido!aega84!lh tel: -49-69-66414316 -----------------------------------------------
gwyn@smoke.brl.mil (Doug Gwyn) (05/25/91)
In article <7649@segue.segue.com> jim@segue.segue.com (Jim Balter) writes: >In article <1991May22.130713.25852@sco.COM> david@sco.COM (David Fiander) writes: >>There exist architectures on which the NULL pointer is different from >>"all bits 0". >This isn't relevant if using ANSI prototypes, since any form of zero is >implicitly converted to the appropriate null pointer, no matter how it is >represented. Well, no, that's not quite right. First of all, an appropriate prototype must be in scope for automatic argument coercions to be performed; second, only certain ways of writing "zero" will be converted to a null pointer constant. The bit pattern is definitely not relevant.
gwyn@smoke.brl.mil (Doug Gwyn) (05/25/91)
In article <384@tmcsys.UUCP> lothar@tmcsys.UUCP (L. Hirschbiegel) writes: >At least in my opinion. But the original question clearly indicated an understanding that this was a portability issue, and it was YOU who decided to turn it into a 386-specific issue and claim that the difference between correct portable practice and 386-dependent coding "doesn't matter". It did matter to the original purpose of the question. It should also matter even if you are at the moment programming with the expectation that the program will run only on a 386, because there is no guarantee that the non-portable practice will continue to "work" even using future releases of the "same" C compiler on the "same" hardware platform. Thus, it would be much wiser to code this properly in the first place, especially given that it is easy to do so. In MY opinion, you deserved to be flamed, on several counts.
sef@kithrup.COM (Sean Eric Fagan) (05/25/91)
In article <1991May24.151350.22705@holos0.uucp> lbr@holos0.uucp (Len Reed) writes: >Under what architecture is a NULL pointer not "all bits 0"? Pr1me, Cyber 180, and another weird machine whose name I forget but might have been a Burroughs (excuse me, Unisys) machine. >NULL pointers *are* all zeros, though, since >casting zero to a pointer must yield a null pointer. That is a non-sequitor. Casting a constant 0 to a pointer must yeild a null pointer. Casting an integer, whose value is 0, to a pointer does not necessarily yield a null pointer. >(Maybe there's >a loophole here in ANSI that says that (void *)0 is a null pointer but >the compiler could implement that casted zero as somthing other than >a pattern of zeros. Not likely in the real world.) ANSI says exactly that. You should *never* assume that that things are otherwise. Talk to Henry, Doug, or Chris about such things; I'll just get into an argument and end up calling you a fool or worse. (HGoC might *think* so, but I don't think they'll *say* so. Well, actually, I'm not so sure about that, now that I think about it.) >You're certainly right that a NULL that produced a virtual memory fault >upon any use would be better than a virtual zero address. But it's not >the "compiler people" who would have to agree to this. Far too much >existing code would be broken by a move to this, I'm afraid. Many, many people have made address 0 result in an address fault (who said *anything* about virtual memory? lots of machines don't have virtual memory, and some of them still manage to compile C). Some people have made it work, simply to placate stupid programmer, such as you appear to be. Frankly, I would *much* rther have my OS complain, so I can track down the error (for that's what it is) during development. (SCO, for example, has address 0 readable and writable, because too many applications people bitched about it. I prefer having a core dump, myself.) -- Sean Eric Fagan | "I made the universe, but please don't blame me for it; sef@kithrup.COM | I had a bellyache at the time." -----------------+ -- The Turtle (Stephen King, _It_) Any opinions expressed are my own, and generally unpopular with others.
scs@adam.mit.edu (Steve Summit) (05/25/91)
In article <1991May24.151350.22705@holos0.uucp> lbr@holos0.uucp (Len Reed) writes: >Under what architecture is a NULL pointer not "all bits 0"? Would it be at all possible for anyone who feels the urge to contribute to this thread to read the comp.lang.c frequently- asked questions list first, before posting anything more? (If you just joined us, this is a *very* old topic, at least for comp.lang.c, and nothing new is being said. If comp.unix.sysv386 finds this fascinating, please remove comp.lang.c from the Newsgroups: line.) Steve Summit scs@adam.mit.edu
gwyn@smoke.brl.mil (Doug Gwyn) (05/25/91)
In article <1991May24.151350.22705@holos0.uucp> lbr@holos0.uucp (Len Reed) writes: >Under what architecture is a NULL pointer not "all bits 0"? The problems, >as many have said, result when a pointer has fewer or more bits than >an integer (or a long). NULL pointers *are* all zeros, though, since >casting zero to a pointer must yield a null pointer. (Maybe there's >a loophole here in ANSI that says that (void *)0 is a null pointer but >the compiler could implement that casted zero as somthing other than >a pattern of zeros. Not likely in the real world.) Please don't post assertions like these when you're not sure of your facts; it adds confusion, not enlightenment. Innumerable times in the past in this newsgroup (and in comp.std.c), correct discussions of this issue have been posted, and I'm pretty sure it's in the FAQ list. In fact, in both theory and the real world, null pointers need not be represented as all zero bit patterns, nor need pointers have the same size as some integral type, nor need all pointers have the same size. There are several C implementations where one or more of those assumptions would be in error. Furthermore, these characteristics pose no problem for a programmer who avoids making such assumptions.
gwyn@smoke.brl.mil (Doug Gwyn) (05/25/91)
In article <1991May25.002706.27552@kithrup.COM> sef@kithrup.COM (Sean Eric Fagan) writes: >Talk to Henry, Doug, or Chris about such things; I'll just get >into an argument and end up calling you a fool or worse. (HGoC might >*think* so, but I don't think they'll *say* so. Well, actually, I'm not so >sure about that, now that I think about it.) "Sean, you ignorant slut." I think it's entirely a function of how annoyed we are by hearing the umpteen-zillionth repetition of the same error, when sources of correct information are relatively easy to come by. :-)
mouse@thunder.mcrcim.mcgill.edu (der Mouse) (05/25/91)
In article <383@tmcsys.UUCP>, lothar@tmcsys.UUCP (L. Hirschbiegel) writes: > In article <1991May22.190908.5311@virtech.uucp> cpcahil@virtech.uucp (Conor P. Cahill) writes: >> The code you show won't work reliably on systems where the size of a >> pointer is different from the size of an integer (for ex: large >> model programs on an 8086 or 80286). > This was posted to newsgroup "comp.UNIX.SYSV386", right?? It was *also* posted to comp.lang.c. > Martin was referring to a "time" syscall argument mismatch under > SCO-UNIX. I pointed out, that it really makes no difference for the > argument value itself. I did NOT say this is a general rule-of-thumb > for all kinds of cpus and all kinds of compilers?!?! Then you should have removed comp.lang.c from the Newsgroups: line before posting. der Mouse old: mcgill-vision!mouse new: mouse@larry.mcrcim.mcgill.edu
allbery@NCoast.ORG (Brandon S. Allbery KF8NH) (05/26/91)
As quoted from <1991May25.002706.27552@kithrup.COM> by sef@kithrup.COM (Sean Eric Fagan): +--------------- | Many, many people have made address 0 result in an address fault (who said | *anything* about virtual memory? lots of machines don't have virtual | memory, and some of them still manage to compile C). Some people have made | it work, simply to placate stupid programmer, such as you appear to be. | Frankly, I would *much* rther have my OS complain, so I can track down the | error (for that's what it is) during development. (SCO, for example, has | address 0 readable and writable, because too many applications people | bitched about it. I prefer having a core dump, myself.) +--------------- Whatever happened to (System V) ld using a default ld script in /lib? Plexus System V did this, and it was trivial to change it from the default to not mapping page 0 (they even provided a script for this, and a special ld option that used that script instead; but changing the default script worked fine). ++Brandon -- Me: Brandon S. Allbery KF8NH: DC to LIGHT! [44.70.4.88] Internet: allbery@NCoast.ORG Delphi: ALLBERY uunet!usenet.ins.cwru.edu!ncoast!allbery
rac@sherpa.UUCP (Roger Cornelius) (05/26/91)
From article <1991May24.044751.5933@virtech.uucp>, by cpcahil@virtech.uucp (Conor P. Cahill): > lothar@tmcsys.UUCP (L. Hirschbiegel) writes: > >>This was posted to newsgroup "comp.UNIX.SYSV386", right?? > > NO. It was cross-posted to comp.unix.sysv386 AND comp.lang.c No, the original article, as well as my followup (I posted the code everyone has been arguing about) were posted only to comp.unix.sysv386. Some later follow-upper must have done the cross-posting. Here are the headers from my post: Subject: Re: SCO password generator Newsgroups: comp.unix.sysv386 References: <1991May14.040042.15199@jpradley.jpr.com> Regardless, of where it was posted, I think everybody now knows that time(0L) != time((long*)0). I'm sure I do. For those interested, I've included a patch below :-). Hopefully this will end this thread. Roger --- cut here --- *** a1.c Sat May 25 16:29:31 1991 --- a2.c Sat May 25 16:29:58 1991 *************** *** 11,17 **** { char word[MAX], hyword[2*MAX]; ! long seed = time(0L); set_auth_parameters(ac,av); --- 11,17 ---- { char word[MAX], hyword[2*MAX]; ! long seed = time((long*)0); set_auth_parameters(ac,av); --- cut here --- -- Roger Cornelius rac@sherpa.UUCP uunet!sherpa!rac
henry@zoo.toronto.edu (Henry Spencer) (05/26/91)
In article <1991May25.002706.27552@kithrup.COM> sef@kithrup.COM (Sean Eric Fagan) writes: >...Talk to Henry, Doug, or Chris about such things; I'll just get >into an argument and end up calling you a fool or worse. (HGoC might >*think* so, but I don't think they'll *say* so. Well, actually, I'm not so >sure about that, now that I think about it.) Chris is an extremely nice and amazingly patient guy and doesn't say such things. Doug and I have our limits. :-) Actually, the people Len really wants to talk to are the IBM AS/400 people, whose machine is an exception to almost every naive misconception about C you have ever seen. Rumor hath it that if you think "ANSI C *surely* could have promised XYZ instead of the watered-down version they actually gave us", there is a high probability that the AS/400 was a major counterexample. -- "We're thinking about upgrading from | Henry Spencer @ U of Toronto Zoology SunOS 4.1.1 to SunOS 3.5." | henry@zoo.toronto.edu utzoo!henry
david@sco.COM (David Fiander) (05/27/91)
In article <1991May24.151350.22705@holos0.uucp> lbr@holos0.uucp (Len Reed) writes: > >You're certainly right that a NULL that produced a virtual memory fault >upon any use would be better than a virtual zero address. But it's not >the "compiler people" who would have to agree to this. Far too much >existing code would be broken by a move to this, I'm afraid. No, all I have to do is convince the compiler people. They don't particularly care if somebody else's broken code breaks; just ask anybody that has worked on an optimizer. Most of the bugs reported against HCR-PCO (the Portable Code Optimizer) turned out to be bugs in the application source which worked until an agressive optimizer hacked the code into something else.
lbr@holos0.uucp (Len Reed) (05/29/91)
In article <1991May25.221530.16119@zoo.toronto.edu> henry@zoo.toronto.edu (Henry Spencer) writes: >Actually, the people Len really wants to talk to are the IBM AS/400 people, >whose machine is an exception to almost every naive misconception about C >you have ever seen. Note: I've dropped the 386 group out of the distribution. Well, several folks have set my straight on NULL not not necessarily being all bits zero on some machines. It is true that (type *)0 must be the NULL pointer of type, even if done as part of an implicit cast. E.g., extern void some_routine(int *); .... some_routine(0); /* implicit cast here */ Obviously this is not well-coded, even if legal. I myself always use NULL instead of zero. Okay, what about static and global pointers not explicitly initialized? Older Unices (not sure about the newest) depended upon the kernel's zeroing out of the BSS to hit global and static data with zeros. (This is in some sense a side-effect of zeroing out data for security reasons: you can peek at what the last process did with the memory you get.) Are these zero or NULL? The compiler could catch these and make them NULL--does it? Example: static char *s_ptr; /* Is s_ptr NULL or all-zeros? */ What about pointers inside structs? E.g., struct my_struct { int *pointer; int abc; } instance; Is instance.pointer NULL or all zeros? Finally, what if I malloc a my_struct and memset it to zeros? No way this can work: if (a_ptr = malloc(sizeof(struct my_struct))) != NULL) { memset(a_ptr, 0, sizeof(struct my_struct)); } There's no way a_ptr->pointer is going to be NULL after this runs since the compiler can't get this one. I know that this'll break some of my code. I use this mechanism for various kinds of linked structures. If the static init works (instance.pointer above), I could define a constant template for initialization and use structure assignment instead of memset: struct my_struct initer = {NULL, 0}; or just struct my_struct initer; /* trust compiler to init this correctly */ .... *a_ptr = initer; /* instead of memset */ But I don't do it now, and I know lots of folks who don't. -- Len Reed Holos Software, Inc. Voice: (404) 496-1358 UUCP: ...!gatech!holos0!lbr
sef@kithrup.COM (Sean Eric Fagan) (05/29/91)
In article <1991May28.175246.2160@holos0.uucp> lbr@holos0.uucp (Len Reed) writes: >Okay, what about static and global pointers not explicitly initialized? They are guaranteed to hold the same value as if they had been explicitly initialized to 0. >static char *s_ptr; /* Is s_ptr NULL or all-zeros? */ This is equivalent to static char *s_ptr = 0; >What about pointers inside structs? E.g., > >struct my_struct { > int *pointer; > int abc; >} instance; I believe that this is equivalent to struct my_struct { int *pointer; int abc; } instance = { 0 }; That is, the first element is set to 0. Which will get treated as NULL for whatever machine it's compiled for. (For the vast majority of cases, no problem, for those that are, they may have several "bss" segments, or some initialization code that set things up appropriately.) >Finally, what if I malloc a my_struct and memset it to zeros? No way this >can work: You're right, there is no way it can work. Which means you can't use calloc() for some things that used to be done. What you have to do is, if you want to be portable, go through and initialize everything yourself, instead of using a memory-setting routine to do it for you. >But I don't do it now, and I know lots of folks who don't. And I know lots of folks who make lots of assumptions about word sizes, which causes even more problems, or about a writable (or, worse, readable) location 0. Doesn't mean it's good practice. -- Sean Eric Fagan | "I made the universe, but please don't blame me for it; sef@kithrup.COM | I had a bellyache at the time." -----------------+ -- The Turtle (Stephen King, _It_) Any opinions expressed are my own, and generally unpopular with others.
gwyn@smoke.brl.mil (Doug Gwyn) (05/29/91)
In article <1991May28.175246.2160@holos0.uucp> lbr@holos0.uucp (Len Reed) writes: >Okay, what about static and global pointers not explicitly initialized? Before any reference to them, they must somehow be initialized by the implementation to null pointers of the appropriate types. >Older Unices (not sure about the newest) depended upon the kernel's >zeroing out of the BSS to hit global and static data with zeros. On the PDP-11 and VAX, that was a valid implementation technique. Note that it also default-initialized floating-point variables to zero values of the correct types. Not every environment has such pleasant properties; in other situations the compiler may have to emit explicit initialization code (e.g. .ADCON 0xFFFF directives). >Finally, what if I malloc a my_struct and memset it to zeros? That properly initializes the integral (sub)members but not necessarily other basic types. >But I don't do it now, and I know lots of folks who don't. Why not? You should certainly properly set the value of a variable before trying to use it. I even write explicit initializers for static objects when the initial value matters, as an indication that (a) I have thought about the initial contents and (b) the particular initial value is necessary for correct operation of the code. By the way, you may find that your code adapts better, for example to parallelization, if you initialize using assignment instead of counting on the static initializer.
gwyn@smoke.brl.mil (Doug Gwyn) (05/29/91)
In article <1991May29.002529.22232@kithrup.COM> sef@kithrup.COM (Sean Eric Fagan) writes: >That is, the first element is set to 0. *All* the members of the struct will receive zero or null initializers.
sef@kithrup.COM (Sean Eric Fagan) (05/30/91)
In article <16295@smoke.brl.mil> gwyn@smoke.brl.mil (Doug Gwyn) writes: >In article <1991May29.002529.22232@kithrup.COM> sef@kithrup.COM (Sean Eric Fagan) writes: >>That is, the first element is set to 0. >*All* the members of the struct will receive zero or null initializers. Oops. I was thinking about unions at the time. -- Sean Eric Fagan | "I made the universe, but please don't blame me for it; sef@kithrup.COM | I had a bellyache at the time." -----------------+ -- The Turtle (Stephen King, _It_) Any opinions expressed are my own, and generally unpopular with others.
chap@art-sy.detroit.mi.us (j chapman flack) (06/01/91)
In article <1991May23.040054.23555@Think.COM> barmar@think.com writes: >No, these systems are not "in common use". 90% of all C programs are >probably running on 80*86, 680x0, VAX, and SPARC processors, but does that Mr. Margolin implies that time(0L) is not a problem on the 80*86. For x > 2, the 80x86 supports two flavors of pointer (near and far) and two sizes of offset (16 and 32 bit). The combinations are: near, 16: sizeof(*long) != sizeof(long) far, 16: sizeof(*long) == sizeof(long) BUT null value not all-zero in the implementations I know of far, 32: sizeof(*long) != sizeof(long) near, 32: sizeof(*long) == sizeof(long) and null value COULD be all-zero if the compiler implementor felt like it. So time(0L) *is* incorrect in (at least) 3 out of 4 cases on an 80x86, x > 2. It's incorrect in *all* cases on earlier 80x86's (though I could imagine an implementation that used 0x0000:0x0000 as the null pointer; the ones I know of all use segment:0x0000.) Of course all my sizeof() comparisons above are based on a 32-bit long, which is yet another assumption.... I replied to Mr. Evernden by mail before I saw this article. Sorry for the duplication. -- Chap Flack Their tanks will rust. Our songs will last. chap@art-sy.detroit.mi.us -MIKHS 0EODWPAKHS Nothing I say represents Appropriate Roles for Technology unless I say it does.
andre@targon.UUCP (Andre van Dalen) (06/02/91)
In article <381@tmcsys.UUCP> lh@aega84.UUCP (L. Hirschbiegel) writes: >In article <1141@mwtech.UUCP> martin@mwtech.UUCP (Martin Weitzel) writes: >>In article <588@sherpa.UUCP> rac@sherpa.UUCP (Roger Cornelius) writes: >>[...] >>> long seed = time(0L); >> ^^--------- wrong >> time((long *)0); >> ^^^^^^^^^--- right [ removed assembler listing (gives us a clue :-) ] >>Repeat after me: POINTER to long. > >Repeat after me: IT DOESN'T MATTER :-) Never worked under MSDOeSn't in different memory models or on 64 bit machines have you? -- The mail| AAA DDDD It's not the kill, but the thrill of the chase. demon...| AA AAvv vvDD DD Ketchup is a vegetable. hits!.@&| AAAAAAAvv vvDD DD {nixbur|nixtor}!adalen.via --more--| AAA AAAvvvDDDDDD Andre van Dalen, uunet!hp4nl!targon!andre
sef@kithrup.COM (Sean Eric Fagan) (06/02/91)
In article <9106011048.aa06266@art-sy.detroit.mi.us> chap@art-sy.detroit.mi.us (j chapman flack) writes: >For x > 2, the 80x86 supports two flavors of pointer (near and far) and two >sizes of offset (16 and 32 bit). The combinations are: Uhm, better go back and reread your manual. For 808[86] and 80?8[86], there are two flavours of pointers: 16-bit ("near"), and 32-bit (actually 20 bit on the '286 and previous), aka "far" in some compilers. >So time(0L) *is* incorrect in (at least) 3 out of 4 cases on an 80x86, x > 2. But it won't cause things to fail, except in some very limited cases (basicly, where one more 16-bit word will cause a stack fault). >It's incorrect in *all* cases on earlier 80x86's (though I could imagine an >implementation that used 0x0000:0x0000 as the null pointer; the ones I know >of all use segment:0x0000.) Actually, most use segment 0, offset 0, since most people like being able to use address 0 of an array (yes, the compiler could fudge things, most don't, though). >Of course all my sizeof() comparisons above are based on a 32-bit long, which >is yet another assumption.... long's are guaranteed to be at least 32 bits, as per ANSI C. Since most (all?) popular compilers for the iAPX86 architecture (and descendents, of course) at least claim some measure of ANSI conformance, and 32 bits is really the only feasible size of a long (other than 16, which defeats the purpose of a long), you can pretty much rest assured that a long is 32 bits on an Intel *86 system. -- Sean Eric Fagan | "I made the universe, but please don't blame me for it; sef@kithrup.COM | I had a bellyache at the time." -----------------+ -- The Turtle (Stephen King, _It_) Any opinions expressed are my own, and generally unpopular with others.