Re: Fread problem

From	James Kuyper <jameskuyper@verizon.net>
Newsgroups	comp.lang.c
Subject	Re: Fread problem
Date	2011-07-16 18:29 -0400
Organization	A noiseless patient Spider
Message-ID	<ivt3ct$ukf$1@dont-email.me> (permalink)
References	(1 earlier) <ivqcap$eqo$1@reader1.panix.com> <ivrl7v$3gb$1@speranza.aioe.org> <87d3har2zu.fsf@temporary-address.org.uk> <ivss0p$ulb$1@speranza.aioe.org> <ivstbi$ou3$1@reader1.panix.com>

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On 07/16/2011 04:46 PM, John Gordon wrote:
> In <ivss0p$ulb$1@speranza.aioe.org> Jack <spamtrap@trashcan.invalid> writes:
> 
>> 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.
> 
> If that works, it's purely accidental and I wouldn't count on it.
> 
> Your code even has a bug: you're copying sizeof(FILE) bytes, but
> fp is a pointer-to-FILE which is probably not the same size.

If he wanted to copy the pointer, that would imply that he's got several
different errors, not just one. What you're thinking of would be written
as follows:

	FILE *fback;
	memcpy(&fback, &fp, sizeof fp);

Which is almost (but not quite) the same as the much simpler:

	FILE *fback = fp;

He's not copying fp, he's copying the FILE object that fp points at. It
can't be the correct solution to his problem, but it is done correctly.
-- 
James Kuyper

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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