Re: Fread problem

From	Jack <spamtrap@trashcan.invalid>
Newsgroups	comp.lang.c
Subject	Re: Fread problem
Date	2011-07-16 20:23 +0000
Organization	Aioe.org NNTP Server
Message-ID	<ivss0p$ulb$1@speranza.aioe.org> (permalink)
References	<ivq9i7$841$1@speranza.aioe.org> <ivqcap$eqo$1@reader1.panix.com> <ivrl7v$3gb$1@speranza.aioe.org> <87d3har2zu.fsf@temporary-address.org.uk>

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On Sat, 16 Jul 2011 10:25:41 +0100, Dr Nick wrote:

> Jack <spamtrap@trashcan.invalid> writes:
> 
>> On Fri, 15 Jul 2011 21:43:53 +0000, John Gordon wrote:
>>> In <ivq9i7$841$1@speranza.aioe.org> Jack <spamtrap@trashcan.invalid>
>>> writes:
>>> 
>>>> I got a strange problem with Fread:
>>> 
>>>> It crashes my program from time to time, in one of its internal
>>>> memcpy calls.
>>> 
>>> When things like this happen it is nearly always because of a memory
>>> corruption bug elsewhere in your program, not because of a bug in a
>>> library function like fread().
>>> 
>>> C is, unfortunately, a fragile language.  If you copy more to a buffer
>>> than it can hold, or call free() on a bad value, or call free() twice
>>> on the same value, you've just corrupted the memory space of your
>>> program and anything can happen from that point onward, including
>>> crashes in library functions such as fread().
>>> 
>>> If your program is a simple one, go over it again and try to identify
>>> any areas where you might have committed the above errors.  If it's a
>>> complex program, you might use a memory analyzer such as efence or
>>> valgrind.
>>
>> Yes...
>> Actually, I think it's a memory error somewhere that causes really
>> indirectly this bug.
>> But I find it strange that even a memory error makes a fread with
>> proper parameters ( I do check this ) fail.
> 
> Somewhere the library and the operating system have to store things like
> the details of the underlying file, the position in it etc.  When you
> trample over memory you can overwrite this.
> 
> In particular, many systems get the buffers for the stdio streams from
> the same memory pool as you get with malloc.  So do something wrong with
> a block you've got from malloc and you can really mess the streams up.
> 
> And vice versa.  I once managed to get malloc to return "null" by
> previously fclosing the same file pointer twice.
> 
>> I've heard somewhere one can play with the file buffer, so I tried it,
>> and put MY file buffer, with the Setbuff() function. And it didn't
>> crash anymore, but the read couldn't read all the files. Some bytes in
>> the middle were missing (I think it is when I resume the reading, after
>> having processed the first part)...
> 
> That's not incompatible with what I wrote above.

Thanks for your help. I think I've now found a solution.

After the file is first Fopen'd, I make a backup of the internal buffers.
fp=Fopen(...);
FILE fback;
memcpy(&fback,fp,sizeof(FILE));

then before the fread I restore this buffer,
memcpy(fp,&fback,sizeof(FILE));
Fread(...,fp);

Now there is no segmentation fault, job done.

Thread

Fread problem Jack <spamtrap@trashcan.invalid> - 2011-07-15 20:56 +0000
  Re: Fread problem Ian Collins <ian-news@hotmail.com> - 2011-07-16 09:05 +1200
    Re: Fread problem Noob <root@127.0.0.1> - 2011-07-19 13:41 +0200
  Re: Fread problem John Gordon <gordon@panix.com> - 2011-07-15 21:43 +0000
    Re: Fread problem Jack <spamtrap@trashcan.invalid> - 2011-07-16 09:22 +0000
      Re: Fread problem Dr Nick <3-nospam@temporary-address.org.uk> - 2011-07-16 10:25 +0100
        Re: Fread problem Jack <spamtrap@trashcan.invalid> - 2011-07-16 20:23 +0000
          Re: Fread problem Ian Collins <ian-news@hotmail.com> - 2011-07-17 08:37 +1200
          Re: Fread problem John Gordon <gordon@panix.com> - 2011-07-16 20:46 +0000
            Re: Fread problem James Kuyper <jameskuyper@verizon.net> - 2011-07-16 18:29 -0400
              Re: Fread problem John Gordon <gordon@panix.com> - 2011-07-16 23:14 +0000
          Re: Fread problem Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-07-16 16:55 -0400
            Re: Fread problem blmblm@myrealbox.com <blmblm.myrealbox@gmail.com> - 2011-07-19 17:17 +0000
              [OT] Re: Fread problem Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-07-19 20:11 -0400
                Re: [OT] Re: Fread problem Seebs <usenet-nospam@seebs.net> - 2011-07-20 02:27 +0000
          Re: Fread problem Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2011-07-16 14:57 -0600
            Re: Fread problem Jack <spamtrap@trashcan.invalid> - 2011-07-17 20:45 +0000
              Re: Fread problem Ian Collins <ian-news@hotmail.com> - 2011-07-18 08:50 +1200
              Re: Fread problem James Kuyper <jameskuyper@verizon.net> - 2011-07-17 17:06 -0400
                Re: Fread problem Ian Collins <ian-news@hotmail.com> - 2011-07-18 09:09 +1200
                Re: Fread problem Hans Vlems <hvlems@freenet.de> - 2011-07-18 13:01 -0700
              Re: Fread problem Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2011-07-17 16:06 -0600
              Re: Fread problem Geoff <geoff@invalid.invalid> - 2011-07-17 17:56 -0700
                Re: Fread problem Seebs <usenet-nospam@seebs.net> - 2011-07-18 04:08 +0000
                Re: Fread problem Geoff <geoff@invalid.invalid> - 2011-07-17 22:08 -0700
                Re: Fread problem Jack <spamtrap@trashcan.invalid> - 2011-07-18 20:20 +0000
                Re: Fread problem Ian Collins <ian-news@hotmail.com> - 2011-07-19 08:50 +1200
                Re: Fread problem Cat 22 <cat22@invalid.org> - 2011-07-18 18:34 -0700
                Re: Fread problem James Kuyper <jameskuyper@verizon.net> - 2011-07-19 07:19 -0400
                Re: Fread problem Stephen Sprunk <stephen@sprunk.org> - 2011-07-19 09:31 -0500
              Re: Fread problem Todd Carnes <toddcarnes@gmail.com> - 2011-07-18 04:23 +0000
              Re: Fread problem Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-07-18 01:15 -0700
              Re: Fread problem Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-07-18 07:22 -0400
                Re: Fread problem Kenneth Brody <kenbrody@spamcop.net> - 2011-07-18 13:56 -0400
              Re: Fread problem Kenneth Brody <kenbrody@spamcop.net> - 2011-07-18 13:49 -0400
                Re: Fread problem Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-07-18 20:31 -0400
                Re: Fread problem Squeamizh <squeamz@hotmail.com> - 2011-07-18 21:11 -0700
              Re: Fread problem blmblm@myrealbox.com <blmblm.myrealbox@gmail.com> - 2011-07-19 17:19 +0000
                Re: Fread problem Kenneth Brody <kenbrody@spamcop.net> - 2011-07-19 14:18 -0400
                Re: Fread problem pete <pfiland@mindspring.com> - 2011-07-19 15:19 -0400
                Re: Fread problem blmblm@myrealbox.com <blmblm.myrealbox@gmail.com> - 2011-07-19 21:33 +0000
          Re: Fread problem Geoff <geoff@invalid.invalid> - 2011-07-16 14:27 -0700
            Re: Fread problem John Gordon <gordon@panix.com> - 2011-07-16 22:22 +0000
          Re: Fread problem "Kleuskes & Moos" <kleuske@xs4all.nl> - 2011-07-16 23:28 -0700
          Re: Fread problem "J. J. Farrell" <jjf@bcs.org.uk> - 2011-07-18 04:58 +0100
          Re: Fread problem Keith Thompson <kst-u@mib.org> - 2011-07-18 08:53 -0700
            Re: Fread problem Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-07-18 18:07 +0100
              Re: Fread problem Keith Thompson <kst-u@mib.org> - 2011-07-18 12:50 -0700
            Re: Fread problem Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2011-07-18 12:18 -0600
            Re: Fread problem John Gordon <gordon@panix.com> - 2011-07-18 20:18 +0000
              Re: Fread problem Keith Thompson <kst-u@mib.org> - 2011-07-18 13:28 -0700
                Re: Fread problem Lew Pitcher <lpitcher@teksavvy.com> - 2011-07-18 16:55 -0400
              Re: Fread problem Anders Wegge Keller <wegge@wegge.dk> - 2011-07-18 22:51 +0200
            Re: Fread problem blmblm@myrealbox.com <blmblm.myrealbox@gmail.com> - 2011-07-19 17:21 +0000
          Re: Fread problem Kenneth Brody <kenbrody@spamcop.net> - 2011-07-18 13:44 -0400
      Re: Fread problem Ian Collins <ian-news@hotmail.com> - 2011-07-16 21:39 +1200
      Re: Fread problem Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2011-07-16 12:18 -0600
      Re: Fread problem John Gordon <gordon@panix.com> - 2011-07-16 19:36 +0000

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