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Groups > comp.lang.c > #148245
| From | David Brown <david.brown@hesbynett.no> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: OT: improve this simple algorithm? |
| Date | 2019-11-13 22:49 +0100 |
| Organization | A noiseless patient Spider |
| Message-ID | <qqhtp6$m0m$1@dont-email.me> (permalink) |
| References | (4 earlier) <E8SyF.324056$NI41.13621@fx12.am4> <qqguti$n3d$1@dont-email.me> <WITyF.3176$XV5.2873@fx15.am4> <qqh9dd$lo3$1@dont-email.me> <ebWyF.46153$7z4.34438@fx09.am4> |
On 13/11/2019 17:28, Bart wrote:
> On 13/11/2019 16:01, David Brown wrote:
>> On 13/11/2019 14:40, Bart wrote:
>
>> Why do you think that is relevant? What is it about the OP's posts that
>> suggests he wants to handle such large lists of numbers? What makes you
>> think it is better to be able to work with 500 K lists, while still
>> failing with 500 M lists dues to lack of heap?
>>
>> The point is, VLA's are perfectly good here. They are a tool in the
>> toolbox - use them when they make sense. As with /every/ tool, they
>> have limitations, but you don't need to invent problems.
>
>>> Now call checkedmalloc(100000000), it will most likely work, but if not
>>> it will fail gracefully. Call uncheckedvla(25000000) (for same number of
>>> bytes), it will crash. Unless you have an unusually large stack
>>> allocated.
>>>
>>
>> Call checkedmalloc(100 000 000 000) and there is a fair chance it will
>> fail
>> /ungracefully/ - depending on your OS and your computer,
>
> For malloc, 1B to 1000MB might be a reasonable amount to allocate, while
> 100,000MB might be less reasonable given that available RAM is typically
> considerably less. (Actually, allocating 100GB fails immediately on my
> machine with 8GB physical RAM.)
>
> For VLAs, the reasonable range might be 1B to 1MB, some 3 magnitudes
> smaller than malloc. It would be a bad choice for sort implementations
> for data sizes with arbitrary upper limits. (1MB accommodates lists of
> only 128K strings, and merge-sorting may need multiple copies.)
>
> (VLAs implemented via calls to malloc and free might be better; you have
> the more convenient handling, without the limitations of stack memory
> (actually, the approach I use in dynamic implementations.) But I haven't
> seen evidence of that being done by gcc.
No. It would be possible to implement this, but fiddly - and it would
be unexpected. People use VLA's expecting them to be on the stack,
avoiding the overhead of malloc and free.
A key part to making VLA's on heap work is to have a destructor to be
called when the array goes out of scope. gcc has support for that (even
in C, as an extension), so the machinery is there. I guess it is just
not a feature people have been looking for.
>
> What is also possible is to conditionally use either VLA or heap
> allocations depending on size requested.)
>
>
> it might
>> happily allocate all your ram and start chewing up your harddisk for
>> virtual memory space, with your computer grinding to a crawl and being
>> so slow it is barely even possible to kill the program.
>>
>> uncheckedvla with the same parameter would die quickly and harmlessly.
>>
>>
>> If you do stupid things in your program, bad things happen.
>
> I'm talking about things being done inadvertently, or through some
> program bug.
That counts as stupid things. Doing stupid things doesn't mean the
programmer is stupid - smart people do stupid things regularly. They
just don't do them quite as regularly as stupid people.
I don't expect a programming language or a compiler to give me good
results when I do something stupid in my code. I like it when the tool
tells me I'm being stupid, but I don't expect it to turn my stupid code
into smart code.
> It doesn't help that VLAs are generally created silently so
> someone may not be aware that an is a VLA that can blow up with
> different parameters.
>
That makes no sense. If you have a function taking a parameter "n", and
you declare an array of size "n", how can you not know that it will be
bigger if "n" is bigger? People might unknowingly make a VLA (for
example, if you write "const int n = 10; int x[n];" it is a VLA in C,
but if you are familiar with C++ you might think it is a normal array).
But you should know about the array's size.
>
>> Anyone who
>> thinks malloc is safer than a VLA because it returns 0 on allocation
>> failure, is kidding themselves. They have completely misunderstood how
>> to write safe and reliable code.
>
> You think just ignoring stack limitations and letting a program crash is
> good way to program?
It is no worse than what people typically do with heap allocations.
But no, I don't think it is a good way to program. And I can't see how
you thought that from what I wrote. The good way to program is to check
any unknown incoming data for sanity before you do anything with it -
that includes allocating memory with malloc just as much as it includes
declaring a VLA. Calling malloc with a size that depends on unchecked
outside data is as bad as allocating a VLA with unchecked outside data.
> In a larger system, a failure to allocate memory
> can be detected and a /part/ of the program can fail under full control,
> but it still run.
>
> For example, while implementing one user command. Then it can report a
> failure ("Not enough to perform this task") but the data etc is still
> intact. With a VLA, it will just abort the whole thing with loss of data.
>
Yes, in theory - handling a failed malloc is done properly in a tiny
fraction of real programs. And if you have coded well, your mallocs
will never fail - nor will your VLA allocations.
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OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-12 15:55 -0500
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-12 21:20 +0000
Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-12 23:28 -0500
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-13 10:52 +0100
Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-13 10:16 -0500
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-13 16:53 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-13 11:18 +0000
Re: OT: improve this simple algorithm? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-12 23:30 +0000
Re: OT: improve this simple algorithm? Ian Collins <ian-news@hotmail.com> - 2019-11-15 05:12 +1300
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 16:34 +0000
Re: OT: improve this simple algorithm? Ian Collins <ian-news@hotmail.com> - 2019-11-15 10:07 +1300
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 22:25 +0000
Re: OT: improve this simple algorithm? Ian Collins <ian-news@hotmail.com> - 2019-11-15 12:02 +1300
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 23:27 +0000
Re: OT: improve this simple algorithm? Ian Collins <ian-news@hotmail.com> - 2019-11-15 19:06 +1300
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-15 10:45 +0000
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-15 13:28 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-15 12:58 +0000
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-15 15:39 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-15 15:04 +0000
Re: OT: improve this simple algorithm? Richard Damon <Richard@Damon-Family.org> - 2019-11-15 12:19 -0500
Re: OT: improve this simple algorithm? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-11-15 09:46 -0800
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-15 18:59 +0000
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-16 11:59 +0000
Re: OT: improve this simple algorithm? Ian Collins <ian-news@hotmail.com> - 2019-11-16 09:02 +1300
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-15 11:17 +0100
Re: OT: improve this simple algorithm? Jorgen Grahn <grahn+nntp@snipabacken.se> - 2019-11-15 21:38 +0000
Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-14 11:44 -0500
Re: OT: improve this simple algorithm? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-14 18:11 +0000
Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-14 13:33 -0500
Re: OT: improve this simple algorithm? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-15 01:49 +0000
Re: OT: improve this simple algorithm? Barry Schwarz <schwarzb@delq.com> - 2019-11-14 10:38 -0800
Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-14 12:56 -0500
Re: OT: improve this simple algorithm? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-14 13:50 -0500
Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-14 14:42 -0500
Re: OT: improve this simple algorithm? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-14 15:25 -0500
Re: OT: improve this simple algorithm? Barry Schwarz <schwarzb@delq.com> - 2019-11-14 11:03 -0800
Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-14 16:08 -0500
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-15 11:30 +0100
Re: OT: improve this simple algorithm? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-15 11:17 +0000
Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-15 17:35 -0500
Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-15 17:39 -0500
Re: OT: improve this simple algorithm? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-15 23:19 +0000
Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-15 18:53 -0500
Re: OT: improve this simple algorithm? Richard Damon <Richard@Damon-Family.org> - 2019-11-15 19:08 -0500
Re: OT: improve this simple algorithm? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-16 01:30 +0000
Re: OT: improve this simple algorithm? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-16 07:39 -0500
Re: OT: improve this simple algorithm? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-16 07:37 -0500
Re: OT: improve this simple algorithm? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-15 09:46 -0500
Re: OT: improve this simple algorithm? Richard Damon <Richard@Damon-Family.org> - 2019-11-15 12:38 -0500
Re: OT: improve this simple algorithm? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-15 23:07 +0000
Re: OT: improve this simple algorithm? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-28 16:54 -0800
Re: OT: improve this simple algorithm? Richard Damon <Richard@Damon-Family.org> - 2019-11-28 20:17 -0500
Re: OT: improve this simple algorithm? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-29 06:08 -0800
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 20:45 +0100
Re: OT: improve this simple algorithm? Keith Thompson <kst-u@mib.org> - 2019-11-14 10:10 -0800
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 20:19 +0100
Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-12 23:57 -0500
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-13 10:34 +0100
Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-13 23:57 -0500
Re: OT: improve this simple algorithm? Barry Schwarz <schwarzb@delq.com> - 2019-11-13 23:00 -0800
Re: OT: improve this simple algorithm? Ike Naar <ike@sdf.lonestar.org> - 2019-11-14 07:53 +0000
Re: OT: improve this simple algorithm? guinness.tony@gmail.com - 2019-11-14 02:34 -0800
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 10:01 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 11:27 +0000
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 12:46 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 12:01 +0000
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 14:05 +0100
Re: OT: improve this simple algorithm? Barry Schwarz <schwarzb@delq.com> - 2019-11-12 18:34 -0800
Re: OT: improve this simple algorithm? Keith Thompson <kst-u@mib.org> - 2019-11-12 19:01 -0800
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 17:08 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 17:06 +0000
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 19:11 +0000
Re: OT: improve this simple algorithm? Ian Collins <ian-news@hotmail.com> - 2019-11-15 05:15 +1300
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 18:13 +0000
Re: OT: improve this simple algorithm? Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 20:16 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 20:03 +0000
Re: OT: improve this simple algorithm? Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-15 04:51 +0100
Re: OT: improve this simple algorithm? Keith Thompson <kst-u@mib.org> - 2019-11-14 10:15 -0800
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 20:52 +0100
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-13 10:57 +0100
Re: OT: improve this simple algorithm? Ian Collins <ian-news@hotmail.com> - 2019-11-13 23:23 +1300
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-13 11:53 +0000
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-13 14:02 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-13 13:40 +0000
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-13 17:01 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-13 16:28 +0000
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-13 22:49 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-13 22:38 +0000
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-13 23:06 +0000
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 10:10 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 10:42 +0000
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 12:53 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 12:28 +0000
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 14:21 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 14:28 +0000
Re: OT: improve this simple algorithm? Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 15:38 +0100
Re: OT: improve this simple algorithm? Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 13:37 +0100
Re: OT: improve this simple algorithm? Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 13:38 +0100
Re: OT: improve this simple algorithm? Thiago Adams <thiago.adams@gmail.com> - 2019-11-13 05:21 -0800
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