As many people know, I think that garbage collection is a good
solution for many memory allocation problems.
I am aware however, that nothing is "the silver bullet", not
even the GC.
A recent article in slashdot http://developers.slashdot.org/artic.../11/17/0552247
proves that.
A C# application was leaking memory, and the application would
become slower and slower because the memory was getting full and the
system was swapping like mad until it just failed.
Why?
Because a list that should be destroyed wasn't being destroyed.
This is similar to another bug that Sun discovered in their
Java implementation. The list wasn't being destroyed because
SOMEWHERE there was a reference to that list, and the GC could
not destroy it.
It is interesting to note that this bug is as difficult to trace as
a missing free or similar bugs. It required a specialized tool
to find it (yes, there are specialized tools to solve GC memory
allocation problems as there are specialized tools to solve
non-gc memory allocation problems)
The lesson to be learned is that you have to be careful (when using the
GC) to
1) Set all pointers to unused memory to NULL.
2) Do NOT store pointers to GC memory in permanent structures if you
want that data to eventually be collected.
The above bug was that the list registered itself in a global
data structure and wasn't getting destroyed.
If the GC wouldn't have been there, the programmer would have freed
the memory, what would have closed the memory leak but left
a dangling pointer in the global data structure!
Not a better alternative.
--
jacob navia
jacob at jacob point remcomp point fr
logiciels/informatique http://www.cs.virginia.edu/~lcc-win32
Nov 18 '07
84  3560 
"Tor Rustad" <to********@hot mail.comwrote in message
news:hJ******** *************@t elenor.com...
cr88192 wrote:
>"Tor Rustad" <to********@hot mail.comwrote in message
[...]
>>IMO, the main domain of C is system and embedded development. Even if
extending this domain by including communication, security development
and DB engine development, a GC seems neither important, or of much
interest.
errm, are you trying to claim that all of us good old desktop-pc
developers are all off using stuff like Java and C# or something?...
No, I say that C1X should focus on the main domain of C, and try to keep
the language small and simple. The C99 variable-length arrays was a step
too far. Adding GC to Standard C, would IMO be a major mistake.
I will agree to a point here:
those kind of arrays, IMO, do not belong within the main language.
but, GC is a runtime feature, and it is very sensible that it be left out
for embedded targets.
I was not arguing for standardized GC though, only that GC, itself, has
value.
where and for who it has value, are the people who use it.
to what extent standardization would make sense, would be in terms of a
"standardiz ed" definition of the API and provided functionality and
semantics (said, "portable" GC), but I do not expect by any means that it be
somehow "required" (as is the case with Java and C#, where GC is an assumed
and fundamental part of the language).
>must of us are in a land where we still want things like GC, but don't
feel like selling our souls to some proprietary VM framework to get it.
Nobody is stopping you from using the Boehm GC.
what do you think I was arguing through most of the thread?...
this is what I was arguing, that using Boehm is a very valid and sensible
option (however, certain other people were opposing that GC has use in C
land, ever...).
--
Tor <bw****@wvtqvm. vw | tr i-za-h a-z>
jacob navia wrote:
>
.... snip ...
>
This change doesn't look good for being able to use third party
libraries compiled with a different compiler...
You can't count on that anyhow. In fact, you expect failure.
--
Chuck F (cbfalconer at maineline dot net)
<http://cbfalconer.home .att.net>
Try the download section.
--
Posted via a free Usenet account from http://www.teranews.com
Paul Hsieh wrote:
>
.... snip ...
>
It forces call stack uniformity (the EH needs to be able to crawl up
the stack and unwind the stack in a uniform way.) So for example,
To bring things back to reality, there is no reason to have a stack
in any C system.
--
Chuck F (cbfalconer at maineline dot net)
<http://cbfalconer.home .att.net>
Try the download section.
--
Posted via a free Usenet account from http://www.teranews.com
"CBFalconer " <cb********@yah oo.comwrote in message
news:47******** *******@yahoo.c om...
jacob navia wrote:
>>
... snip ...
>>
This change doesn't look good for being able to use third party
libraries compiled with a different compiler...
You can't count on that anyhow. In fact, you expect failure.
theoretically, yes.
in practicality, one is forced to deal with this issue.
for example, between windows and linux, there are any number of compilers
you can compile code with, and they can make dlls and static libraries.
if one makes their compiler incompatible, say, by using a different calling
convention, then they are unable to use really any code (static libraries,
DLLs, ...) for their platform, and the compiler becomes essentially useless.
so, on x86, there is a singular more or less mandatory calling convention
(cdecl), and on windows, another less-common but still mandatory convention
(stdcall).
on x86-64, there are 2 calling conventions, the SystemV x86-64 convention
(followed by linux and friends), and the Win64 calling convention (windows).
and, again, to be useful on these archs, one needs to support these
conventions (even as such, I have been vaguely tempted to just use my own
convention for x86-64, but this would require, of all horrors, stubbing, to
interface external code).
what convention would I use:
I would actually just "upgrade" the existing 32-bit x86 calling convention
to work on x86-64...
that or use the Win64 convention internally, which should not be too
difficult to support (would just need to come up with some means of creating
a Win64 to SysV adaptor interface...).
or such...
--
Chuck F (cbfalconer at maineline dot net)
<http://cbfalconer.home .att.net>
Try the download section.
--
Posted via a free Usenet account from http://www.teranews.com
"CBFalconer " <cb********@yah oo.comwrote in message
news:47******** *******@yahoo.c om...
Paul Hsieh wrote:
>>
... snip ...
>>
It forces call stack uniformity (the EH needs to be able to crawl up
the stack and unwind the stack in a uniform way.) So for example,
To bring things back to reality, there is no reason to have a stack
in any C system.
errm?...
what kind of 'reality' is it you are living in?...
I guess it is theoretically possible, but it would lead to incompatibility
and what reason is there that anyone should implement such a thing?...
I guess, maybe you mean, all the call-frames are allocated on the heap, but
then one has to totally re-engineer the calling conventions, recompile all
the libraries, and likely deal with a pretty damn major performance hit.
I guess it could allow things like scheme style continuations, lazy
evaluation, and simplify using fine-grained concurrency (no more "threads",
only 'workers' and 'work queues').
the fact that pretty much every arch has a stack, and those that don't, fake
it, this is telling of its relevance...
cr88192 wrote:
"CBFalconer " <cb********@yah oo.comwrote in message
news:47******** *******@yahoo.c om...
>Paul Hsieh wrote:
... snip ...
>>It forces call stack uniformity (the EH needs to be able to crawl up
the stack and unwind the stack in a uniform way.) So for example,
To bring things back to reality, there is no reason to have a stack
in any C system.
errm?...
what kind of 'reality' is it you are living in?...
Mr Falconer has his own world, as many people here around.
A stack is not necessary, one complement or sign magnitude
representations rule the world, etc etc.
I think we have touched a trap representation
:-)
--
jacob navia
jacob at jacob point remcomp point fr
logiciels/informatique http://www.cs.virginia.edu/~lcc-win32
In article <fi**********@a ioe.org>, jacob navia <ja***@nospam.c omwrote
on Wednesday 21 Nov 2007 7:05 pm:
cr88192 wrote:
>"jacob navia" <ja***@nospam.c omwrote in message
news:fi******* ***@aioe.org...
<snip>
>but, it is maybe interesting from a theoretical perspective, aka:
a C-implementation allocating all the call frames on a heap.
so, we want a call frame?
grab it from a free-list (or, somehow, allocate more if needed).
need to store locals? grab a chunk of memory for this and attach to
call frame.
need space for args? likewise, grab a chunk of memory, evaluate and
store in the args, and attach it to the (newly created) call frame of
the function being called.
So, you are implementig the stack in software. Instead of doing a
single register subtract, you allocate memory, put that in the list
of active frames, etc etc!
That would be really bad for performance
I think that with a lot of restrictions on usage a software stack can
compete reasonably well with a hardware one. The actual memory is very
likely to be held in the CPU caches and the machine instructions to
manipulate the stack are also likely to be very similar.
Of course, using a software stack when dedicated hardware support is
available is probably unnecessary unless special flexibility is needed.
And it's not within the spirit of C, as I understand it, to chose
inefficient means to do what _can_ be done more efficiently, time and
space wise.
<snip>
"santosh" <sa*********@gm ail.comwrote in message
news:fi******** **@registered.m otzarella.org.. .
In article <fi**********@a ioe.org>, jacob navia <ja***@nospam.c omwrote
on Wednesday 21 Nov 2007 7:05 pm:
>cr88192 wrote:
>>"jacob navia" <ja***@nospam.c omwrote in message
news:fi****** ****@aioe.org.. .
<snip>
>>but, it is maybe interesting from a theoretical perspective, aka:
a C-implementation allocating all the call frames on a heap.
so, we want a call frame?
grab it from a free-list (or, somehow, allocate more if needed).
need to store locals? grab a chunk of memory for this and attach to
call frame.
need space for args? likewise, grab a chunk of memory, evaluate and
store in the args, and attach it to the (newly created) call frame of
the function being called.
So, you are implementig the stack in software. Instead of doing a
single register subtract, you allocate memory, put that in the list
of active frames, etc etc!
That would be really bad for performance
I think that with a lot of restrictions on usage a software stack can
compete reasonably well with a hardware one. The actual memory is very
likely to be held in the CPU caches and the machine instructions to
manipulate the stack are also likely to be very similar.
Of course, using a software stack when dedicated hardware support is
available is probably unnecessary unless special flexibility is needed.
And it's not within the spirit of C, as I understand it, to chose
inefficient means to do what _can_ be done more efficiently, time and
space wise.
yes, I agree...
what I was writing about was actually, something a little odd, namely
compiling C in a way very much unlike C...
in effect, it would be compiling C like it were scheme.
as for "special flexibility", at least in scheme implementations , where this
kind of thing is often done, this adds in features like easy support for
continuations and lexical scoping (neither of which are traditionally
present in C).
this is because, with a non-linear memory structure, we can determine, for
example, that something in a function will "capture" the state from that
function (such as the current continuation, part of the lexical scope, ...).
as a result, when the function returns (after this capture has occured), the
call or environment frames are not released (this is possible to determine
statically because things like 'lambda' or 'call/cc' are visible lexically,
though special considerations exist for call/cc).
this is not too terrible of a problem, since, the memory layout is
non-linear, later calls simply use different frames and different memory.
of course, probably a better approach performance-wise would be to switch
out the approach, and only really use call-frames in the case where closures
or continuations are used (closures are easier than continuations, because
although closures are always visible lexically, the use of a continuation
may not be).
luckily, the most common use of a continuation is to implement something
analogous to longjmp (said 'partial' or 'exit-only' continuations). these do
not require as much in terms of special treatment.
but, usage of full continuations are a problem, since they apply potentially
to the entire call graph.
another common approach, makes code itself fast, but call/cc very expensive.
this is an approach focusing around saving/restoring the stack (call/cc
saves the stack, and using a continuation restores it). another approach I
have heard of, is to implement each continuation as a seperate thread (how
this is done in practice I am less certain of, the basic idea is that
call/cc either forks or spawns duplicate threads, and invoking a
continuation causes executon to continue from that thread, or something...).
or such...
<snip>
cr88192 wrote:
"Tor Rustad" <to********@hot mail.comwrote in message
news:hJ******** *************@t elenor.com...
>cr88192 wrote:
>>"Tor Rustad" <to********@hot mail.comwrote in message
[...]
>>>IMO, the main domain of C is system and embedded development. Even if
extending this domain by including communication, security development
and DB engine development, a GC seems neither important, or of much
interest.
errm, are you trying to claim that all of us good old desktop-pc
developers are all off using stuff like Java and C# or something?...
No, I say that C1X should focus on the main domain of C, and try to keep
the language small and simple. The C99 variable-length arrays was a step
too far. Adding GC to Standard C, would IMO be a major mistake.
I will agree to a point here:
those kind of arrays, IMO, do not belong within the main language.
but, GC is a runtime feature, and it is very sensible that it be left out
for embedded targets.
Yes, perhaps the fragmentation of memory issue when using GC has been
solved these days, but not long ago, it wasn't AFAIK. Besides, my MISRA
C copy, prohibits non-deterministic memory allocations anyway.
I was not arguing for standardized GC though, only that GC, itself, has
value. where and for who it has value, are the people who use it.
Well, we do discuss in the context of Standard C here, so when a
non-existing features is the subject, particularly when OP is J.N. (!)
-- I assume he want it added.
>>must of us are in a land where we still want things like GC, but don't
feel like selling our souls to some proprietary VM framework to get it.
Nobody is stopping you from using the Boehm GC.
what do you think I was arguing through most of the thread?...
this is what I was arguing, that using Boehm is a very valid and sensible
option (however, certain other people were opposing that GC has use in C
land, ever...).
I have never used GC myself, but has no strong feelings against some
high-level applications using such libraries, but I wouldn't like to see
a GC during audit, in security and/or safety related software.
--
Tor <bw****@wvtqvm. vw | tr i-za-h a-z>
On Nov 20, 6:51 pm, CBFalconer <cbfalco...@yah oo.comwrote:
Paul Hsieh wrote:
... snip ...
It forces call stack uniformity (the EH needs to be able to crawl up
the stack and unwind the stack in a uniform way.) So for example,
To bring things back to reality, there is no reason to have a stack
in any C system.
Not only are you wrong, but you preface it in a way that makes you
even wronger. Amazing.
Functions calls are to push as return is to pop. It might not be
*called* a stack, but it *IS* a stack, and this has nothing to do with
system implementation details. The C *STANDARD* implicitly implements
a stack.
And as long as we are talking about reality, we should note that most
C implementations use a literal common stack for both return/link
addresses and autos.
You may have meant that you don't need a particular implementation of
a hardware stack. But that's irrelevant to the point I am making. To
run an exception the system, one way or another needs to implement
"pop (return) until catch found" at runtime without actually executing
the returns, which means that a uniform "pseudo-return" has to exist
outside of implicit execution.
Using longjmp is insufficient because you can set catch #1, then call
into something, then set catch #2, then return enough times to make
catch #2 no longer in the call stack scope, thus re-enabling catch
#1. If you throw/raise at this point, you expect catch #1 to be
triggered -- not catch #2, and not vacated entirely (and simply
leading to a runtime error.) That means these catches must exist at
each level of the call stack.
--
Paul Hsieh http://www.pobox.com/~qed/ http://bstring.sf.net/ This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
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--------
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-----------------------
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