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Groups > comp.lang.c > #147670
| From | James Kuyper <jameskuyper@alumni.caltech.edu> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: One works, the other doesn't? |
| Date | 2019-11-06 09:33 -0500 |
| Organization | A noiseless patient Spider |
| Message-ID | <qpulkn$htn$1@dont-email.me> (permalink) |
| References | <OHIvF.6838$u_3.620@fx29.iad> <qppdj2$52q$1@dont-email.me> <0EgwF.8082$kt1.6599@fx46.iad> <qpscqt$g9n$1@dont-email.me> <iKnwF.65564$P37.23863@fx33.iad> |
On 11/5/19 6:35 PM, DFS wrote:
> On 11/5/2019 12:51 PM, James Kuyper wrote:
>> On 11/5/19 10:34 AM, DFS wrote:
>>> On 11/4/2019 9:45 AM, James Kuyper wrote:
>>>> On 11/3/19 5:39 PM, DFS wrote:
...
> But rather than grow that last array as each element is added, you
> pointed out it's better to allocate bigger chunks. Then realloc(arr,0)
> will free extra-allocated memory.
No, that's not what I was suggesting. That frees all of the memory, not
just the excess (or, to place the blame more precisely, that call
falsely tells realloc() that ALL of the memory is excess). What I was
suggesting corresponds to:
temp = realloc(arr, cnt*sizeof *arr);
If the new space requested is sufficiently small compared to the last
request, then this call to realloc() may release some of the excess
space - but it's not guaranteed how much it will release. realloc() will
probably round the total amount requested up to the next multiple of a
fixed block size, and that number might not be any smaller than it was
for the previous call to malloc() or realloc(). It's not even guaranteed
that it will release any memory at all.
...
> https://www.thegeekstuff.com/2010/10/linux-error-codes/
>
> Looking there, it seems ENOMEM (out of memory) is the only error that
> would be returned by malloc() or realloc(). If so, would you typically
> end the program, or would you try to free unused memory from elsewhere?
In most cases, I would end the program. However, if it's important that
the program keeps running (for instance, a heart monitor), and if there
was an alternative approach that used less memory (presumably at the
cost of being slower), I would try to back up and redo the process using
the alternative approach. That situation has never come up in any code
I've ever written, but other people do run into it occasionally.
...
>> A common strategy
>> is to initially call malloc() to allocate significantly more than one
>> object, and to only call realloc() when you reach the end of the current
>> allocation and need to add more to it.
>
> Using that strategy, I have this (seems to work, and very quickly):
>
>
> int cnt=0, reallocated = 0;
> char **finalarr = malloc(sizeof(char*) * (arrsize/2));
> for(i=0;i<arrsize;i++) {
> while (i<arrsize-1 && strcasecmp(randoms3[i],randoms3[i+1])==0) {i++;}
> finalarr[cnt] = randoms3[i];
> cnt++;
> if(cnt>=(arrsize/2) && reallocated==0) {
> finalarr = realloc(finalarr, sizeof(char*) * (arrsize));
> printf("reallocated at cnt = %d\n",cnt);
> reallocated = 1;
> }
> }
> realloc(finalarr, 0);
> printf("%.4fs to remove %d dupes\n",elapsedtime(start),arrsize-cnt);
>
>
> It mallocs finalarr to 1/2 the size of the source array, then when
> finalarr gets full it reallocs the remaining half.
That's not quite what I was thinking of. Unless you feel the need to
efficiently handle the case where there's lots of duplicates, arrsize
itself will only be a little more than the number of elements you need
in your array, so you should simply allocate that entire space in one
call to malloc():
finalarr = malloc(arrsize * sizeof *finalarr);
Then realloc() at the end if you really need to reduce the allocation.
> But it's getting clunky with the 'reallocated' flag. Got any other
> methods to know when it's time to reallocate?
Yes. In cases where you don't start with a good estimate of the amount
of space needed, the approach I was describing would be written
something like this.
size_t current_allocation = 32; // Judgment call.
finalarr = malloc(current_allocation * sizeof *finalarr);
if(!finalarr)
{
// Error handling
}
Inside the loop:
if(cnt>=current_allocation)
{
current_allocation *= 2;
// If there's any danger of overflow, you
// should check for it here.
void* temp = realloc(finalarr,
current_allocation * sizeof *finalarr);
if(!temp)
{
// Error handling
}
finalarr = temp;
|
[Re: final realloc()]
> Is there an easy way to measure the amt of memory freed by that command?
There is no portable way; some libraries provide tools for inspecting
the heap.
>> Refinement 1: on many systems, allocating memory in blocks whose size is
>> an integer multiple of some particular power of 2 is more efficient than
>> allocating slightly more or slightly less than that amount. Which power
>> of two is relevant varies from one system to another. On some systems,
>> there's multiple different powers of 2 which have corresponding
>> efficiency advantages. If your initial allocation is for a power of 2
>> elements, and your growth factor is 2, you're virtually guaranteed to
>> hit sizes that are particularly efficient.
>
>
> When you say 'growth factor of 2' do you mean malloc N elements, then
> continue to realloc 2N elements as needed?
No, I mean current_allocation *=2, as in the example I provided above.
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One works, the other doesn't? DFS <nospam@dfs.com> - 2019-11-03 17:39 -0500
Re: One works, the other doesn't? Bart <bc@freeuk.com> - 2019-11-04 00:20 +0000
Re: One works, the other doesn't? Jorgen Grahn <grahn+nntp@snipabacken.se> - 2019-11-04 06:53 +0000
Re: One works, the other doesn't? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-04 09:45 -0500
Re: One works, the other doesn't? DFS <nospam@dfs.com> - 2019-11-05 10:34 -0500
Re: One works, the other doesn't? mark.bluemel@gmail.com - 2019-11-05 07:45 -0800
Re: One works, the other doesn't? DFS <nospam@dfs.com> - 2019-11-05 10:53 -0500
Re: One works, the other doesn't? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-11-05 08:04 -0800
Re: One works, the other doesn't? DFS <nospam@dfs.com> - 2019-11-05 11:29 -0500
Re: One works, the other doesn't? David Brown <david.brown@hesbynett.no> - 2019-11-05 23:24 +0100
Re: One works, the other doesn't? Bart <bc@freeuk.com> - 2019-11-05 23:06 +0000
Re: One works, the other doesn't? Ian Collins <ian-news@hotmail.com> - 2019-11-06 12:31 +1300
Re: One works, the other doesn't? Bart <bc@freeuk.com> - 2019-11-05 23:43 +0000
Re: One works, the other doesn't? Ian Collins <ian-news@hotmail.com> - 2019-11-06 12:54 +1300
Re: One works, the other doesn't? Bart <bc@freeuk.com> - 2019-11-06 01:35 +0000
Re: One works, the other doesn't? Ian Collins <ian-news@hotmail.com> - 2019-11-06 15:16 +1300
Re: One works, the other doesn't? Bart <bc@freeuk.com> - 2019-11-06 11:11 +0000
Re: One works, the other doesn't? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-11-06 01:59 -0800
Re: One works, the other doesn't? David Brown <david.brown@hesbynett.no> - 2019-11-06 11:35 +0100
Re: One works, the other doesn't? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-11-06 08:57 -0800
Re: One works, the other doesn't? Ian Collins <ian-news@hotmail.com> - 2019-11-07 07:24 +1300
Re: One works, the other doesn't? David Brown <david.brown@hesbynett.no> - 2019-11-06 22:59 +0100
Re: One works, the other doesn't? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-11-06 14:37 -0800
Re: One works, the other doesn't? David Brown <david.brown@hesbynett.no> - 2019-11-07 00:58 +0100
Re: One works, the other doesn't? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-11-07 02:08 -0800
Re: One works, the other doesn't? David Brown <david.brown@hesbynett.no> - 2019-11-07 15:43 +0100
Re: One works, the other doesn't? Robert Wessel <robertwessel2@yahoo.com> - 2019-11-07 16:15 -0600
Re: One works, the other doesn't? Jorgen Grahn <grahn+nntp@snipabacken.se> - 2019-11-10 20:01 +0000
Re: One works, the other doesn't? Ian Collins <ian-news@hotmail.com> - 2019-11-11 15:26 +1300
Re: One works, the other doesn't? David Brown <david.brown@hesbynett.no> - 2019-11-11 12:37 +0100
Re: One works, the other doesn't? Ian Collins <ian-news@hotmail.com> - 2019-11-07 07:21 +1300
Re: One works, the other doesn't? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-11-06 10:39 -0800
Re: One works, the other doesn't? Ian Collins <ian-news@hotmail.com> - 2019-11-07 07:47 +1300
Re: One works, the other doesn't? Ian Collins <ian-news@hotmail.com> - 2019-11-07 08:14 +1300
Re: One works, the other doesn't? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-11-06 12:30 -0800
Re: One works, the other doesn't? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-08 21:04 -0800
Re: One works, the other doesn't? "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-09 01:37 -0800
Re: One works, the other doesn't? "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-09 01:38 -0800
Re: One works, the other doesn't? Ian Collins <ian-news@hotmail.com> - 2019-11-10 08:32 +1300
Re: One works, the other doesn't? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-28 16:09 -0800
Re: One works, the other doesn't? David Brown <david.brown@hesbynett.no> - 2019-11-29 11:33 +0100
Re: One works, the other doesn't? Öö Tiib <ootiib@hot.ee> - 2019-11-30 14:20 -0800
Re: One works, the other doesn't? Ian Collins <ian-news@hotmail.com> - 2019-12-01 12:15 +1300
Re: One works, the other doesn't? David Brown <david.brown@hesbynett.no> - 2019-11-06 10:07 +0100
Re: One works, the other doesn't? Bart <bc@freeuk.com> - 2019-11-06 10:58 +0000
Re: One works, the other doesn't? David Brown <david.brown@hesbynett.no> - 2019-11-07 10:05 +0100
Re: One works, the other doesn't? David Brown <david.brown@hesbynett.no> - 2019-11-06 09:52 +0100
Re: One works, the other doesn't? "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-06 00:56 -0800
Re: One works, the other doesn't? "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-06 01:00 -0800
Re: One works, the other doesn't? Bart <bc@freeuk.com> - 2019-11-06 11:32 +0000
Re: One works, the other doesn't? David Brown <david.brown@hesbynett.no> - 2019-11-06 23:11 +0100
Re: One works, the other doesn't? Bart <bc@freeuk.com> - 2019-11-06 23:22 +0000
Re: One works, the other doesn't? David Brown <david.brown@hesbynett.no> - 2019-11-07 01:04 +0100
Re: One works, the other doesn't? Bart <bc@freeuk.com> - 2019-11-07 01:13 +0000
Re: One works, the other doesn't? David Brown <david.brown@hesbynett.no> - 2019-11-07 10:22 +0100
Re: One works, the other doesn't? Bart <bc@freeuk.com> - 2019-11-07 11:51 +0000
Re: One works, the other doesn't? David Brown <david.brown@hesbynett.no> - 2019-11-07 15:58 +0100
Re: One works, the other doesn't? Bart <bc@freeuk.com> - 2019-11-07 16:34 +0000
Re: One works, the other doesn't? DFS <nospam@dfs.com> - 2019-11-07 12:40 -0500
Re: One works, the other doesn't? Bart <bc@freeuk.com> - 2019-11-07 17:57 +0000
Re: One works, the other doesn't? Keith Thompson <kst-u@mib.org> - 2019-11-07 21:14 -0800
Re: One works, the other doesn't? Bart <bc@freeuk.com> - 2019-11-08 09:45 +0000
Re: One works, the other doesn't? Keith Thompson <kst-u@mib.org> - 2019-11-08 09:56 -0800
Re: One works, the other doesn't? Bart <bc@freeuk.com> - 2019-11-08 19:47 +0000
Re: One works, the other doesn't? Ian Collins <ian-news@hotmail.com> - 2019-11-09 09:14 +1300
Re: One works, the other doesn't? Keith Thompson <kst-u@mib.org> - 2019-11-08 19:10 -0800
Re: One works, the other doesn't? Bart <bc@freeuk.com> - 2019-11-09 12:24 +0000
Like an old married couple... (Was: One works, the other doesn't?) gazelle@shell.xmission.com (Kenny McCormack) - 2019-11-08 20:25 +0000
Re: One works, the other doesn't? David Brown <david.brown@hesbynett.no> - 2019-11-07 19:58 +0100
Re: One works, the other doesn't? Bart <bc@freeuk.com> - 2019-11-07 19:31 +0000
Re: One works, the other doesn't? Ian Collins <ian-news@hotmail.com> - 2019-11-08 08:56 +1300
Re: One works, the other doesn't? Bart <bc@freeuk.com> - 2019-11-07 20:06 +0000
Re: One works, the other doesn't? Ian Collins <ian-news@hotmail.com> - 2019-11-08 09:21 +1300
Re: One works, the other doesn't? Bart <bc@freeuk.com> - 2019-11-07 21:20 +0000
Re: One works, the other doesn't? Ian Collins <ian-news@hotmail.com> - 2019-11-08 10:28 +1300
Re: One works, the other doesn't? Bart <bc@freeuk.com> - 2019-11-07 22:35 +0000
Re: One works, the other doesn't? Ian Collins <ian-news@hotmail.com> - 2019-11-09 09:22 +1300
Re: One works, the other doesn't? jadill33@gmail.com - 2019-11-07 08:37 -0800
Re: One works, the other doesn't? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-11-07 08:52 -0800
Re: One works, the other doesn't? jadill33@gmail.com - 2019-11-07 09:52 -0800
Re: One works, the other doesn't? David Brown <david.brown@hesbynett.no> - 2019-11-07 20:04 +0100
Re: One works, the other doesn't? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-08 19:50 -0800
Re: One works, the other doesn't? Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-05 18:05 +0100
Re: One works, the other doesn't? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-11-05 09:28 -0800
Re: One works, the other doesn't? Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-05 19:02 +0100
Re: One works, the other doesn't? David Brown <david.brown@hesbynett.no> - 2019-11-05 23:21 +0100
Re: One works, the other doesn't? Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-06 18:36 +0100
Re: One works, the other doesn't? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-05 12:51 -0500
Re: One works, the other doesn't? DFS <nospam@dfs.com> - 2019-11-05 18:35 -0500
Re: One works, the other doesn't? Barry Schwarz <schwarzb@delq.com> - 2019-11-05 17:53 -0800
Re: One works, the other doesn't? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-06 09:33 -0500
Re: One works, the other doesn't? Barry Schwarz <schwarzb@delq.com> - 2019-11-05 13:36 -0800
Re: One works, the other doesn't? jak <nospam@please.ty> - 2019-11-07 10:17 +0100
Re: One works, the other doesn't? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-07 12:19 +0000
Re: One works, the other doesn't? jak <nospam@please.ty> - 2019-11-07 18:59 +0100
Re: One works, the other doesn't? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-08 00:02 +0000
Re: One works, the other doesn't? jak <nospam@please.ty> - 2019-11-20 10:54 +0100
Re: One works, the other doesn't? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-20 11:14 +0000
Re: One works, the other doesn't? jak <nospam@please.ty> - 2019-11-20 13:22 +0100
Re: One works, the other doesn't? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-28 15:54 -0800
Re: One works, the other doesn't? Keith Thompson <kst-u@mib.org> - 2019-11-20 08:42 -0800
Re: One works, the other doesn't? Manfred <noname@add.invalid> - 2019-11-21 18:56 +0100
Re: One works, the other doesn't? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-22 01:07 +0000
Re: One works, the other doesn't? Manfred <noname@add.invalid> - 2019-11-22 15:27 +0100
Re: One works, the other doesn't? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-28 15:47 -0800
Re: One works, the other doesn't? DFS <nospam@dfs.com> - 2019-11-07 18:41 -0500
Re: One works, the other doesn't? Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-05 17:39 +0100
Re: One works, the other doesn't? DFS <nospam@dfs.com> - 2019-11-05 11:47 -0500
Re: One works, the other doesn't? Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-05 17:54 +0100
Re: One works, the other doesn't? luser droog <luser.droog@gmail.com> - 2019-11-05 09:00 -0800
Re: One works, the other doesn't? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-08 21:02 -0800
Re: One works, the other doesn't? DFS <nospam@dfs.com> - 2019-11-09 02:09 -0500
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