Redirecting Standard I/O to Windows Console

Warning! Some information on this page is older than 3 years now. I keep it for reference, but it probably doesn't reflect my current knowledge and beliefs.

20:10
Sat
28
Nov 2009

Redirecting Standard I/O to Windows Console

Every process, no matter if under Windows or Linux, has standard text streams called input, output and error. Of course it makes most sense when you deal with command line applications, where you can directly interact with it using these streams. If you use Linux, you are probably familiar with redirecting streams to file using > operator or to another application using | operator. We use these streams in C++ code with standard C library (like printf and scanf functions) or standard C++ library (like std::cout and std::cin streams). But what if we write a GUI application in Windows (like a game), which has WinMain instead of main entry point and thus doesn't have console?

There is a simple WinAPI function called AllocConsole, which opens this black system console for your Win32 application. But today I've noticed that the console opened this way doesn't handle standard I/O! I've tried to do std::cout << "Hello World" and nothing happened. So after some googling I've found a hacky solution (or maybe rather a smart code snippet? :) in an article hosted at Andrew Tucker's Home Page, published in 1997 in Windows Developer Journal: Adding Console I/O to a Win32 GUI App. Here is my version of this code with all necessary includes:

#include <windows.h>
#include <ios>
#include <cstdio>
#include <io.h>
#include <fcntl.h>

void RedirectStandardIo()
{
  /* This clever code have been found at:
  Adding Console I/O to a Win32 GUI App
  Windows Developer Journal, December 1997
  http://dslweb.nwnexus.com/~ast/dload/guicon.htm
  Andrew Tucker's Home Page */

  // redirect unbuffered STDOUT to the console
  long lStdHandle = (long)GetStdHandle(STD_OUTPUT_HANDLE);
  int hConHandle = _open_osfhandle(lStdHandle, _O_TEXT);
  FILE *fp = _fdopen(hConHandle, "w");
  *stdout = *fp;
  setvbuf(stdout, NULL, _IONBF, 0);

  // redirect unbuffered STDIN to the console
  lStdHandle = (long)GetStdHandle(STD_INPUT_HANDLE);
  hConHandle = _open_osfhandle(lStdHandle, _O_TEXT);
  fp = _fdopen( hConHandle, "r" );
  *stdin = *fp;
  setvbuf(stdin, NULL, _IONBF, 0);

  // redirect unbuffered STDERR to the console
  lStdHandle = (long)GetStdHandle(STD_ERROR_HANDLE);
  hConHandle = _open_osfhandle(lStdHandle, _O_TEXT);
  fp = _fdopen( hConHandle, "w" );
  *stderr = *fp;
  setvbuf(stderr, NULL, _IONBF, 0);

  // make cout, wcout, cin, wcin, wcerr, cerr, wclog and clog point to console as well
  std::ios::sync_with_stdio();
}

Comments (0) | Tags: winapi | Author: Adam Sawicki | Share

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