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Re: Why is this outputting alternate blank lines?

From Keith Thompson <kst-u@mib.org>
Newsgroups comp.lang.c
Subject Re: Why is this outputting alternate blank lines?
Date 2016-12-14 12:28 -0800
Organization None to speak of
Message-ID <lnmvfymf0a.fsf@kst-u.example.com> (permalink)
References (1 earlier) <o2r7cs$6l1$1@dont-email.me> <o2s0sb$tb$1@dont-email.me> <o2s1i8$3qt$1@dont-email.me> <ln4m26nv0o.fsf@kst-u.example.com> <o2s8ir$uk5$1@dont-email.me>

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Richard Heathfield <rjh@cpax.org.uk> writes:
> On 14/12/16 19:57, Keith Thompson wrote:
>> Richard Heathfield <rjh@cpax.org.uk> writes:
>>> On 14/12/16 17:53, DFS wrote:
>> [...]
>>>> Any idea why it crashes above 1 million?
>>>
>>> Do you remember when I suggested checking the return value of malloc?
>>>
>>> I again suggest that you check the return value of malloc.
>>>
>>> The malloc function returns NULL when it fails to allocate memory. Since
>>> you're not checking for this, if ever it /does/ return NULL you will be
>>> scribbling all over memory you don't own, hence the segfault.
>>
>> On the other hand, on some systems malloc() generally doesn't report
>> failure; rather if you allocate too much memory, a crash occurs when you
>> try to access it.  (This is arguably non-conforming.)
>
> Arguably? I'd have said it's absolutely definitely non-conforming. The 
> malloc function is /required/ to report failure if the memory can't be 
> allocated.
[snip]
> What intrigues me, though, is why you think this is arguable. I suspect 
> that you probably share my take on this. If you were to play Devil's 
> Advocate, though, what arguments would you muster for such behaviour 
> being conforming?

I agree that it's non-conforming.  It's also much less useful than
having malloc() properly report failures.

I've seen a counterargument (which I can't necessarily summarize
accurately, but I'll try) that malloc() succeeded in allocating a
chunk of virtual memory.  The later failure to use that virtual
memory is a capacity failure, and the standard does not specify
"the size or complexity of a program and its data that will exceed
the capacity of any specific data-processing system or the capacity
of a particular processor".

Furthermore, it's possible that this:

    ptr = malloc(big_value);
    if (ptr != NULL) memset(ptr, 42, big_value);

could fail on the memset call, but this:

    ptr = malloc(big_value);
    do_some_other_stuff();
    if (ptr != NULL) memset(ptr, 42, big_value);

might succeed.  do_some_other_stuff() might free some memory,
or it might even be something that happens in other processes,
depending on whether the shortage applies to the current process
or the entire system.

I suppose I should mention the background.  This can happen,
as I understand it, on Linux-based systems.  If the attempt to
access the "allocated" memory fails, it invokes something called
the "OOM killer" (OOM is Out Of Memory), which will kill *some*
chosen process(es), not necessarily the one that caused the error.

It's entirely possible that I'm not describing this correctly, and
that there are valid reasons for this behavior.  Again, be aware
that I'm summarizing arguments that I don't necessarily agree with
or know very well, so take all this with a large grain of salt.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Thread

Why is this outputting alternate blank lines? DFS <nospam@dfs.com> - 2016-12-13 23:18 -0500
  Re: Why is this outputting alternate blank lines? Melzzzzz <mel@zzzzz.com> - 2016-12-14 05:30 +0100
  Re: Why is this outputting alternate blank lines? mark.bluemel@gmail.com - 2016-12-14 00:59 -0800
  Re: Why is this outputting alternate blank lines? Richard Heathfield <rjh@cpax.org.uk> - 2016-12-14 10:38 +0000
    Re: Why is this outputting alternate blank lines? DFS <nospam@dfs.com> - 2016-12-14 12:53 -0500
      Re: Why is this outputting alternate blank lines? Richard Heathfield <rjh@cpax.org.uk> - 2016-12-14 18:05 +0000
        Re: Why is this outputting alternate blank lines? Keith Thompson <kst-u@mib.org> - 2016-12-14 11:57 -0800
          Re: Why is this outputting alternate blank lines? Richard Heathfield <rjh@cpax.org.uk> - 2016-12-14 20:05 +0000
            Re: Why is this outputting alternate blank lines? "James R. Kuyper" <jameskuyper@verizon.net> - 2016-12-14 15:21 -0500
            Re: Why is this outputting alternate blank lines? Keith Thompson <kst-u@mib.org> - 2016-12-14 12:28 -0800
              Re: Why is this outputting alternate blank lines? supercat@casperkitty.com - 2016-12-14 13:09 -0800
              Re: Why is this outputting alternate blank lines? Jean-Marc Bourguet <jm@bourguet.org> - 2016-12-14 22:53 +0100
                Re: Why is this outputting alternate blank lines? <william@wilbur.25thandClement.com> - 2016-12-14 15:47 -0800
            Re: Why is this outputting alternate blank lines? Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2016-12-14 15:41 -0700
              Re: Why is this outputting alternate blank lines? Richard Heathfield <rjh@cpax.org.uk> - 2016-12-14 23:26 +0000
                Re: Why is this outputting alternate blank lines? Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2016-12-14 17:33 -0700
                Re: Why is this outputting alternate blank lines? Richard Heathfield <rjh@cpax.org.uk> - 2016-12-15 00:42 +0000
                Re: Why is this outputting alternate blank lines? supercat@casperkitty.com - 2016-12-14 17:05 -0800
                Re: Why is this outputting alternate blank lines? Richard Heathfield <rjh@cpax.org.uk> - 2016-12-15 08:21 +0000
                Re: Why is this outputting alternate blank lines? supercat@casperkitty.com - 2016-12-15 09:15 -0800
                Re: Why is this outputting alternate blank lines? Richard Heathfield <rjh@cpax.org.uk> - 2016-12-15 17:50 +0000
                Re: Why is this outputting alternate blank lines? supercat@casperkitty.com - 2016-12-15 12:15 -0800
                Re: Why is this outputting alternate blank lines? Richard Heathfield <rjh@cpax.org.uk> - 2016-12-15 20:37 +0000
                Re: Why is this outputting alternate blank lines? raltbos@xs4all.nl (Richard Bos) - 2016-12-15 20:08 +0000
                Re: Why is this outputting alternate blank lines? Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2016-12-14 20:40 -0700
                Re: Why is this outputting alternate blank lines? Richard Heathfield <rjh@cpax.org.uk> - 2016-12-15 08:36 +0000
                Re: Why is this outputting alternate blank lines? Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2016-12-15 17:43 -0700
                Re: Why is this outputting alternate blank lines? Richard Heathfield <rjh@cpax.org.uk> - 2016-12-16 01:05 +0000
                Re: Why is this outputting alternate blank lines? Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2016-12-15 18:34 -0700
                Re: Why is this outputting alternate blank lines? <william@wilbur.25thandClement.com> - 2016-12-15 10:14 -0800
                Re: Why is this outputting alternate blank lines? raltbos@xs4all.nl (Richard Bos) - 2016-12-15 20:15 +0000
            Re: Why is this outputting alternate blank lines? Tim Rentsch <txr@alumni.caltech.edu> - 2016-12-15 01:51 -0800
              Re: Why is this outputting alternate blank lines? asetofsymbols@gmail.com - 2016-12-15 06:09 -0800
                Re: Why is this outputting alternate blank lines? Tim Rentsch <txr@alumni.caltech.edu> - 2016-12-15 17:47 -0800
          Re: Why is this outputting alternate blank lines? Keith Thompson <kst-u@mib.org> - 2016-12-14 12:15 -0800
            Re: Why is this outputting alternate blank lines? <william@wilbur.25thandClement.com> - 2016-12-15 10:24 -0800
          Re: overcommit is the plague Noob <root@127.0.0.1> - 2016-12-17 01:17 +0100
      Re: Why is this outputting alternate blank lines? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-12-14 20:19 +0000
        Re: Why is this outputting alternate blank lines? DFS <nospam@dfs.com> - 2016-12-14 17:44 -0500
          Re: Why is this outputting alternate blank lines? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-12-15 00:21 +0000
            Re: Why is this outputting alternate blank lines? mark.bluemel@gmail.com - 2016-12-15 00:56 -0800
              Re: Why is this outputting alternate blank lines? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-12-15 22:08 +0000
      Re: Why is this outputting alternate blank lines? Geoff <geoff@invalid.invalid> - 2016-12-14 13:39 -0800
      Re: Why is this outputting alternate blank lines? Ike Naar <ike@iceland.freeshell.org> - 2016-12-16 07:12 +0000

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