INFO: fstream's File Pointers Are Not Independent (104634)

SUMMARY

The Microsoft "iostream Class Library Reference" in the Books Online included with Visual C++ 32-bit Edition, version 4.0, contains the following as part of the description for class filebuf:public iostream:

The reserve area, put area, and get area are introduced in the streambuf class description. The put area and the get area are always the same for filebuf objects. Also, the get pointer and put pointers are tied; when one moves, so does the other.

Previous versions of the Microsoft "iostream Class Library Reference" for class filebuf:public iostream state the following:

Although the filebuf object's get and put pointers are theoretically independent, the get area and the put area cannot both be active at the same time.

This statement can lead to some confusion as to whether the get and put pointers are independent of each other. In the Microsoft iostream library implementation of fstream, these pointers are not independent of each other. If the get pointer moves, so does the put pointer. The source code listed below demonstrates this behavior.

MORE INFORMATION

Sample Code

/* Compile options needed: None. Build as a console .EXE for Windows NT
*/ 

#include <fstream.h>
#include <strstrea.h>
#include <assert.h>
#undef NDEBUG    // Make sure assert works.

void main()
{
  fstream stream("test",ios::in | ios::out | ios::binary);
  int temp;
  char input;

  cout << "\n\nOpened binary file test" << endl;
  cout << "Get pointer is " << hex << stream.tellg() << endl;
  cout << "Put pointer is " << hex << stream.tellp() << endl;
  cout << "Now writing 256 bytes..." << endl;
  for(temp = 0;temp < 256;temp++)
  {
    stream.put((char)temp);
  }
  cout << "Get pointer is " << hex << stream.tellg() << endl;
  cout << "Put pointer is " << hex << stream.tellp() << endl;

  cout << "\nNow setting the put pointer to hex 50" << endl;
  stream.seekp(0x50);
  cout << "Get pointer is " << hex << stream.tellg() << endl;
  cout << "Put pointer is " << hex << stream.tellp() << endl;

  cout << "\nNow setting the get pointer to hex 40" << endl;
  stream.seekg(0x40);
  cout << "Get pointer is " << hex << stream.tellg() << endl;
  cout << "Put pointer is " << hex << stream.tellp() << endl;

  cout << "\nNow writing one character" << endl;
  stream.put('a');
  cout << "Get pointer is " << hex << stream.tellg() << endl;
  cout << "Put pointer is " << hex << stream.tellp() << endl;

  cout << "\nNow reading one character" << endl;
  stream.get(input);
  cout << "Get pointer is " << hex << stream.tellg() << endl;
  cout << "Put pointer is " << hex << stream.tellp() << endl;
  stream.close();
}