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Re: fread a file of unknown size

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glen herrmannsfeldt <gah@ugcs.caltech.edu> writes:
> Keith Thompson <kst-u@mib.org> wrote:
> (snip, I wrote)
>>> Yes, but does the C standard allow for a file to reach EOF, and then,
>>> on the next read, not be at EOF?
>  
>> I don't think so, but fseek() clears the end-of-file indicator.
>  
>>> I did once (OS/2 1.0 days) port gnu tail to OS/2, and it did work.
>>> I didn't try very hard to figure out how it worked, just that it did.
>  
>> I think tail (specifically `tail -f`) uses fseek(file, 0, SEEK_CUR).
>
> Okay, I was going to say that once fread() returned zero it would
> keep doing that, but then I remembered tail.  But then it was about 25
> years ago that I was looking at tail.  I did remember wondering if
> it would work the same way.
>
> The biggest problem I remember with gnu utilities is that they would
> use 0 as an argument to a function expecting (char *). When I complained
> to the gnu people, they had some comment about systems having different
> sized (int) and (char*) not being supported.

I don't suppose you have an example?

If there's a visible prototype, passing 0 to a non-variadic function
expecting a char* is not a problem.  It will be implicitly converted to
char*, and converting a constant 0 to a pointer type yields a null
pointer.  (Personally I'd use NULL or (char*)NULL just for the sake of
clarity.)

Passing 0 to a function expecting a char* either when there's no visible
prototype, or when it corresponds to the ", ..." in a variadic function,
does have undefined behavior.  It can break if char* and int are of
different sizes, if a null pointer is not represented as all-bits-zero,
if int and char* are passed as arguments using different mechanisms, or
for any other conceivable reason.

-- 
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

fread a file of unknown size ruben safir <ruben@mrbrklyn.com> - 2015-03-08 14:47 -0400
  Re: fread a file of unknown size Richard Heathfield <rjh@cpax.org.uk> - 2015-03-08 18:59 +0000
    Re: fread a file of unknown size James Kuyper <jameskuyper@verizon.net> - 2015-03-08 18:21 -0400
      Re: fread a file of unknown size Keith Thompson <kst-u@mib.org> - 2015-03-08 18:43 -0700
    Re: fread a file of unknown size fmassei@gmail.com - 2015-03-08 15:41 -0700
      Re: fread a file of unknown size raltbos@xs4all.nl (Richard Bos) - 2015-03-10 16:04 +0000
    Re: fread a file of unknown size Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2015-03-08 18:44 -0400
  Re: fread a file of unknown size jacob navia <jacob@spamsink.net> - 2015-03-08 20:32 +0100
  Re: fread a file of unknown size Keith Thompson <kst-u@mib.org> - 2015-03-08 12:48 -0700
  Re: fread a file of unknown size glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-03-08 23:39 +0000
    Re: fread a file of unknown size James Kuyper <jameskuyper@verizon.net> - 2015-03-08 20:40 -0400
      Re: fread a file of unknown size glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-03-09 01:24 +0000
        Re: fread a file of unknown size Keith Thompson <kst-u@mib.org> - 2015-03-09 07:53 -0700
          Re: fread a file of unknown size glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-03-09 17:36 +0000
            Re: fread a file of unknown size Keith Thompson <kst-u@mib.org> - 2015-03-09 10:44 -0700
              Re: fread a file of unknown size glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-03-09 20:07 +0000

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