Re: [Gurusamy Sarathy <gsar@activestate.com>] ferror() after fread() on a FILE* opened for write

[Andreas Jaeger <aj@arthur.rhein-neckar.de>]

>>>>> Andreas Schwab writes:

 > Andreas Jaeger <aj@arthur.rhein-neckar.de> writes:
 > |> Looking at the appended bug report, I guess we should check for
 > |> if (fp->_flags & _IO_NO_WRITES)
 > |> 
 > |> in the fread implementation.  But what's the right place and who
 > |> exactly should we change it?

 > I cannot find anything in the standard that requires setting the error
 > indicator when writing to a read-only stream or reading from a write-only
 > stream.  Currently we only check _IO_NO_{READS,WRITES} in the underflow
 > and overflow methods, so that getc/putc will fail (but getc doesn't set
 > the error indicator yet; I'm fixing that).  Neither xsgetn nor xsputn do
Thanks for the fix.  Should this go also into glibc 2.1.2?
 > any explicit checking, so they will fail only if the underlying descriptor
 > is truely write-only or read-only, resp. (but the standard descriptors are
 > read/write).

I've modified the program to use fopen:

#include <stdio.h>
int
main(int argc, char **argv)
{
  char buf[500];
  FILE *f;
  int r;

  f = fopen ("/tmp/foo", "a");
  r = fread(buf, 1, 1, f);
  printf("%d ferror=%d feof=%d\n", r, ferror(f), feof(f));
  return 0;
}


The output of the program is:
$ ./ferror 
0 ferror=0 feof=0

The ISO C9x draft I've got here, mentions as return value for fread:

      [#3] The fread  function  returns  the  number  of  elements
       successfully  read,  which  may be less than nmemb if a read
       error or end-of-file is encountered.  If size  or  nmemb  is
       zero,  fread  returns zero and the contents of the array and
       the state of the stream remain unchanged.

fread returned 0 which is less than 1 - therefore either a read error
or end-of-file is encountered.  But feof and ferror tell me that
neither is encountered.

Is this a valid interpretation of the standard?

Andreas
-- 
 Andreas Jaeger   aj@arthur.rhein-neckar.de    jaeger@informatik.uni-kl.de
  for pgp-key finger ajaeger@aixd1.rhrk.uni-kl.de