From: "Richard L.F. van Paasen" <R DOT L DOT F DOT v DOT Paasen AT stud DOT tue DOT nl>
Newsgroups: comp.os.msdos.djgpp
Subject: fstream in R/W mode fails
Date: Mon, 30 Sep 1996 23:05:17 -0700
Organization: University of Technology Eindhoven
Lines: 93
Message-ID: <3250B49D.4474@stud.tue.nl>
Reply-To: R DOT L DOT F DOT v DOT Paasen AT stud DOT tue DOT nl
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To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

Sorry about the long message, but this problem is somewhat complicated.
I have problems with fstream, in read/write mode, when I try to seek
in a file and read and write within one open ... close session. I have
included the source of a test program, which is documented to show the
problem.
The problem, however only exists when the program is compiled by DJGPP.

// Source code starts on this line
// This source is tested with Borland C++ and DJGPP compilers

// Is there something wrong with the fstream class in DJGPP ??
// This program below should run successfully, and produce
// the output (to cout):

//   line 1a
//   line 2
//   line 1b
//   line 2

// But instead it outputs "error 2" when compiled with DJGPP ! (see
program)
// Compiling with Borland C++ does produce the correct output.

// What's wrong? The program first writes 2 strings in binary mode,
// then re-opens the file and replaces the first string by another.
// Both times it reads the two lines in the file, to check the
// results. The tricky part is that the program seeks, read and writes
// in file while it is in read/write mode. I cannot figure out
// what the problem is. Anyone? Bug in libiostr? In gcc? Or in my
program?
// Any suggestions for code that produces the wanted result?

#include <iostream.h>
#include <string.h>
#include <fstream.h>
#include <stdio.h>
#include <stdlib.h>

#define FNAME "temp.tmp"

void error(int i) // "custom" error
{ cout << "error " << i << "\n";
  exit(1);
}

int main(void)
{ fstream f;
  char L1a[10]="line 1a";
  char L1b[10]="line 1b";
  char L2[10]= "line 2";
  char S[10]=  "";

  remove("temp.tmp");

// THE NEXT CODE DOES THE FOLLOWING:
// - CREATE FILE AND WRITE "line 1a" AND "line 2"
// - OPEN FILE IN R/W MODE AND READ THE 2 LINES, DUMP THEM ON SCREEN
// - THEN REPLACE THE FIRST LINE "line 1a" BY "line1b"
// - THEN READ THE LINES AGAIN AND DUMP THEM ON SCREEN

  f.open(FNAME, ios::out | ios::binary);          // create file
  f.write(L1a,strlen(L1a)+1);                     // write line 1a
  f.write(L2,strlen(L2)+1);                       // write line 2
  if (f.fail()) error(1);                         // error ?
  f.close();                                      // close

  f.open(FNAME,ios::in | ios::out | ios::nocreate | ios::binary);
  f.read(S,strlen(L1a)+1); cout << S << "\n";     // reopen, read line
1a
  f.read(S,strlen(L2)+1);  cout << S << "\n";     // read line 2
  if (f.fail()) error(2);                         // error ?
  // --------------------------------------------
  // DJGPP STOPS HERE, AND TEMP.TMP IS 0 BYTES !!
  // f.open TRUNCATED THE FILE ???
  // NOTE: adding ios::ate in the open function
  //       and do a f.seekg(0,ios::beg) to rewind
  //       is NOT a solution!
  // --------------------------------------------

  f.seekp(0,ios::beg);                            // back to beginning
  f.write(L1b,strlen(L1b)+1);                     // replace line 1a by
1b

  f.seekg(0,ios::beg);                            // back to beginning
  f.read(S,strlen(L1b)+1); cout << S << "\n";     // read line 1b
  f.read(S,strlen(L2)+1);  cout << S << "\n";     // read line 2
  if (f.fail()) error(3);                         // error ?
  f.close();                                      // close

  return 0;
}

// Richard van Paasen.