Binary Output Files

Streams were originally designed for text, so the default output mode is text. In text mode, the newline character (hexadecimal 10) expands to a carriage return–linefeed (16-bit only). The expansion can cause problems, as shown here:

#include <fstream.h>
int iarray[2] = { 99, 10 };
void main()
{
    ofstream os( "test.dat" );
    os.write( (char *) iarray, sizeof( iarray ) );
}

You might expect this program to output the byte sequence { 99, 0, 10, 0 }; instead, it outputs { 99, 0, 13, 10, 0 }, which causes problems for a program expecting binary input. If you need true binary output, in which characters are written untranslated, you have several choices:

• Construct a stream as usual, then use the setmode member function, which changes the mode after the file is opened:

  ofstream ofs ( "test.dat" );
  ofs.setmode( filebuf::binary );
  ofs.write( char*iarray, 4 ); // Exactly 4 bytes written
  

• Specify binary output by using the ofstream constuctor mode argument:

  #include <fstream.h>
  #include <fcntl.h>
  #include <io.h>
  int iarray[2] = { 99, 10 };
  void main()
  {
      ofstream os( "test.dat", ios::binary );
      ofs.write( iarray, 4 ); // Exactly 4 bytes written
  }
  

• Use the binary manipulator instead of the setmode member function:

  ofs << binary;
  

  Use the text manipulator to switch the stream to text translation mode.

• Open the file using the run-time _open function with a binary mode flag:

  filedesc fd = _open( "test.dat", 
                _O_BINARY | _O_CREAT | _O_WRONLY );
  ofstream ofs( fd );
  ofs.write( ( char* ) iarray, 4 ); // Exactly 4 bytes written