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From: gdr@eddore.myrias.com (Devin Reade)
Newsgroups: comp.sys.apple2.programmer
Subject: problem with fread (Orca/C v2.1)
Date: 10 Nov 1996 14:43:48 -0700
Organization: Myrias Research Corporation
Lines: 161
Message-ID: <565iak$2ph@eddore.myrias.com>
NNTP-Posting-Host: eddore.myrias.com

Provided for your information:

This problem was first noticed by Norman Dodge.  Very briefly, it
seems that fread under Orca/C v2.1 reuses it's buffer contents on
the second invocation.

I've already emailed both Mike and Soenke on this.

I just noticed that I should have followed the first two perror() calls
with an exit().  Oh well ...

=========== cut here ===========

/*
 * This file demonstrates two problems with Orca/C v2.1 that were
 * not present in Orca/C v2.0.3.  One is with ferror(3), the other
 * is with fread(3).  (Actually, the symptoms occur in fread.  Since
 * there are zero diffs between the the Orca fread sources between
 * v2.0.3 and v2.1, it's probably elsewhere -- unless my sources that
 * date from teh v2.0.3 period don't correspond to the v2.0.3 libraries.)
 *
 * In order to run this program, you need an input data file, with
 * arbitrary contents.  The one I used was 2979 bytes long.
 *
 * See the description below.  These problems were first noted by
 * Norman Dodge <norman.dodge@techall.wa.com>.  This test was written
 * by Devin Reade <gdr@eddore.myrias.com>.  I have not yet examined the
 * library sources as to the cause of the error.
 */

#pragma keep "stdiotest"
#pragma lint -1
#pragma debug 25

#include <stdio.h>

#define INFILE  "data.in"
#define OUTFILE "data.out"
#define BUFFERSIZE 10

/*
 * Using a bare Orca/C v2.0.3 system, we get the following results for
 * a 2979 byte file (no resource fork), as expected:
 *
 *   feof  (data.in) = 0
 *   ferror(data.in) = 0
 *   done 298 loops (2979 bytes)
 *   feof  (data.in) = 128
 *   ferror(data.in) = 0
 *
 * When METHOD is 0, we get (on a sample text file of 2979 bytes, no
 * resource fork), we get:
 *
 *   feof  (data.in) = 0
 *   ferror(data.in) = 0
 *   done 1 loops (10 bytes)
 *   feof  (data.in) = 0
 *   ferror(data.in) = 256
 *
 * This shows that ferror is getting set prematurely.
 *
 * When METHOD is 1 (same input file), we get:
 *
 *   feof  (data.in) = 0
 *   ferror(data.in) = 0
 *   done 401 loops (4003 bytes)
 *   feof  (data.in) = 128
 *   ferror(data.in) = 256
 *
 * Note that the latter is exactly 1024 bytes more than is expected, which
 * is the size of the default stream buffer.
 *
 * This latter test makes it appear that it appears that fread(3) is
 * initially reusing the contents of its internal buffer.  In other words,
 * instead of:
 *
 *   do until eof
 *     fill buffer
 *     while (stuff left in buffer)
 *       provide user with bytes requested
 *     end while
 *   enddo
 *
 * it appears that we get:
 *
 *   count = 1
 *   fill buffer
 *   do until eof
 *     while (stuff left in buffer)
 *       provide user with bytes requested
 *     end while
 *     if (count > 1) {               <-- this should be non-conditional
 *       fill buffer
 *     }
 *     count = count + 1
 *   enddo
 */

#define METHOD 1

int main (int argc, char **argv) {

  FILE *fpin, *fpout;
  static char buffer[BUFFERSIZE];
  size_t totalWritten, loopCount, count;

  totalWritten = 0;
  loopCount = 0;
  if ((fpin = fopen(INFILE, "r")) == NULL) {
    perror("couldn't open input file");
  }
  if ((fpout = fopen(OUTFILE, "w")) == NULL) {
    perror("couldn't open output file");
  }

  /* both of these should return zero */
  printf("feof  (%s)\t= %d\n", INFILE, feof(fpin));
  printf("ferror(%s)\t= %d\n", INFILE, ferror(fpin));

#if METHOD == 0
  while (!feof(fpin) && !ferror(fpin)) {
    count = fread (buffer, sizeof(char), BUFFERSIZE, fpin);
    if (count > 0) {
      fwrite (buffer, sizeof(char), count, fpout);
    }
    totalWritten += count;
    loopCount++;
  }
#elif METHOD == 1
  for(;;) {
    int done = 0;

    count = fread(buffer, sizeof(char), BUFFERSIZE, fpin);
    if (count < BUFFERSIZE) {
      if (ferror(fpin)) {
	perror("error on input file");
      }
      done = 1;
    }
    fwrite (buffer, sizeof(char), count, fpout);
    totalWritten += count;
    loopCount++;
    if (done) {           
      break;
    }
  }
#else
#error unknown method
#endif
  printf("done %ld loops (%ld bytes)\n", loopCount, totalWritten);

  /* the first of these should return 1, the second zero */
  printf("feof  (%s)\t= %d\n", INFILE, feof(fpin));
  printf("ferror(%s)\t= %d\n", INFILE, ferror(fpin));

  fclose(fpin);
  fclose(fpout);
  return 0;
}
-- 
	Devin Reade	gdr@eddore.myrias.ab.ca