Re: Nasty consequence of fclose followed by ferror

[Andreas Jaeger]

>>>>> Garance A Drosihn writes:

 > Hi.
 > I was porting code which is used on several OS's (freebsd, sunOS,
 > solaris, aix, irix) to redhat linux 7.0, and ran into a number
 > of odd bugs which were time-consuming to track down.

 > I eventually found that the basic cause for all those bugs was
 > in the code I was porting.  It has:
 >      fclose(fp);
 >      if (ferror(fp)) { ... do stuff ... }

 > Please note I am quite happy to admit this code is wrong.  Also
 > note I did not write this code, I am just porting it from other
 > places.  I do not need any lecture on why this code is bad.  I
 > can read the "Single Unix Specification" as well as anyone else.
 > I would be perfectly happy if that call to ferror would cause a
 > SEGV or some other disaster -- as long as the disaster is at
 > THAT call and not far-far-away from it.

 > The trick is that the call to ferror with the freed FILE pointer
 > causes random memory to be overwritten.  When tracking down
 > this problem, I first had calls to inet_aton() which would
 > fail.  I also had some calls to syslog which were generating
 > garbage messages.  When I commented out the call to inet_aton(),
 > I had failures in some other system routine (which I forget
 > now).  Debugging was complicated by the fact that sometimes
 > everything would work fine, and other times the process in
 > question would just disappear on me (in the middle of one
 > of these system routines).

 > Eventually I commented out enough things that the failing
 > system call was at another fopen, which at least got me on
 > the right track.  Eventually I used that to trace the
 > problem back to the bad code above.  As I sit here writing
 > this message, I have already fixed this code I am porting,
 > and it is working fine (and on all the above platforms).

 > However, for the sanity of other people who might run into
 > this problem, I was wondering if there was some simple and
 > inexpensive change which could be made to fclose or ferror
 > which would make debugging this much less painful.

No such change is possible - it would lead to memory leaks.

Andreas
-- 
 Andreas Jaeger
  SuSE Labs address@hidden
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    http://www.suse.de/~aj