How do I print to the debug output window in a Win32 app?

I've got a win32 project that I've loaded into Visual Studio 2005. I'd like to be able to print things to the Visual Studio output window, but I can't for the life of me work out how. I've tried 'printf' and 'cout <<' but my messages stay stubbornly unprinted.

Is there some sort of special way to print to the Visual Studio output window?

You can use OutputDebugString. OutputDebugString is a macro that depending on your build options either maps to OutputDebugStringA(char const*) or OutputDebugStringW(wchar_t const*). In the later case you will have to supply a wide character string to the function. To create a wide character literal you can use the L prefix:

OutputDebugStringW(L"My output string.");

Normally you will use the macro version together with the _T macro like this:

OutputDebugString(_T("My output string."));

If you project is configured to build for UNICODE it will expand into:

OutputDebugStringW(L"My output string.");

If you are not building for UNICODE it will expand into:

OutputDebugStringA("My output string.");

If the project is a GUI project, no console will appear. In order to change the project into a console one you need to go to the project properties panel and set:

• In "linker->System->SubSystem" the value "Console (/SUBSYSTEM:CONSOLE)"
• In "C/C++->Preprocessor->Preprocessor Definitions" add the "_CONSOLE" define

This solution works only if you had the classic "int main()" entry point.

But if you are like in my case (an openGL project), you don't need to edit the properties, as this works better:

AllocConsole();
freopen("CONIN$", "r",stdin);
freopen("CONOUT$", "w",stdout);
freopen("CONOUT$", "w",stderr);

printf and cout will work as usual.

If you call AllocConsole before the creation of a window, the console will appear behind the window, if you call it after, it will appear ahead.

Update

freopen is deprecated and may be unsafe. Use freopen_s instead:

FILE* fp;

AllocConsole();
freopen_s(&fp, "CONIN$", "r", stdin);
freopen_s(&fp, "CONOUT$", "w", stdout);
freopen_s(&fp, "CONOUT$", "w", stderr);

To print to the real console, you need to make it visible by using the linker flag /SUBSYSTEM:CONSOLE. The extra console window is annoying, but for debugging purposes it's very valuable.

OutputDebugString prints to the debugger output when running inside the debugger.

If you want to print decimal variables:

wchar_t text_buffer[20] = { 0 }; //temporary buffer
swprintf(text_buffer, _countof(text_buffer), L"%d", your.variable); // convert
OutputDebugString(text_buffer); // print

Consider using the VC++ runtime Macros for Reporting _RPTN() and _RPTFN()

> You can use the _RPTn, and _RPTFn macros, defined in CRTDBG.H, to > replace the use of printf statements for debugging. These macros > automatically disappear in your release build when _DEBUG is not > defined, so there is no need to enclose them in #ifdefs.

Example...

if (someVar > MAX_SOMEVAR) {
    _RPTF2(_CRT_WARN, "In NameOfThisFunc( )," 
         " someVar= %d, otherVar= %d\n", someVar, otherVar );
}

Or you can use the VC++ runtime functions _CrtDbgReport, _CrtDbgReportW directly.

> _CrtDbgReport and _CrtDbgReportW can send the debug report to three different destinations: a debug report file, a debug monitor (the > Visual Studio debugger), or a debug message window. > > _CrtDbgReport and _CrtDbgReportW create the user message for the debug report by substituting the argument[n] arguments into the format > string, using the same rules defined by the printf or wprintf > functions. These functions then generate the debug report and > determine the destination or destinations, based on the current report > modes and file defined for reportType. When the report is sent to a > debug message window, the filename, lineNumber, and moduleName are > included in the information displayed in the window.

If you need to see the output of an existing program that extensively used printf w/o changing the code (or with minimal changes) you can redefine printf as follows and add it to the common header (stdafx.h).

int print_log(const char* format, ...)
{
	static char s_printf_buf[1024];
	va_list args;
	va_start(args, format);
	_vsnprintf(s_printf_buf, sizeof(s_printf_buf), format, args);
	va_end(args);
	OutputDebugStringA(s_printf_buf);
	return 0;
}

#define printf(format, ...) \
		print_log(format, __VA_ARGS__)

Your Win32 project is likely a GUI project, not a console project. This causes a difference in the executable header. As a result, your GUI project will be responsible for opening its own window. That may be a console window, though. Call AllocConsole() to create it, and use the Win32 console functions to write to it.

I was looking for a way to do this myself and figured out a simple solution.

I'm assuming that you started a default Win32 Project (Windows application) in Visual Studio, which provides a "WinMain" function. By default, Visual Studio sets the entry point to "SUBSYSTEM:WINDOWS". You need to first change this by going to:

Project -> Properties -> Linker -> System -> Subsystem

And select "Console (/SUBSYSTEM:CONSOLE)" from the drop-down list.

Now, the program will not run, since a "main" function is needed instead of the "WinMain" function.

So now you can add a "main" function like you normally would in C++. After this, to start the GUI program, you can call the "WinMain" function from inside the "main" function.

The starting part of your program should now look something like this:

#include <iostream>

using namespace std;

// Main function for the console
int main(){

	// Calling the wWinMain function to start the GUI program
	// Parameters:
	// GetModuleHandle(NULL) - To get a handle to the current instance
	// NULL - Previous instance is not needed
	// NULL - Command line parameters are not needed
	// 1 - To show the window normally
	wWinMain(GetModuleHandle(NULL), NULL,NULL, 1); 

	system("pause");
	return 0;
}

// Function for entry into GUI program
int APIENTRY wWinMain(_In_ HINSTANCE hInstance,
                     _In_opt_ HINSTANCE hPrevInstance,
                     _In_ LPWSTR    lpCmdLine,
                     _In_ int       nCmdShow)
{
	// This will display "Hello World" in the console as soon as the GUI begins.
	cout << "Hello World" << endl;
.
.
.

Result of my implementation

Now you can use functions to output to the console in any part of your GUI program for debugging or other purposes.

You can also use WriteConsole method to print on console.

AllocConsole();
LPSTR lpBuff = "Hello Win32 API";
DWORD dwSize = 0;
WriteConsole(GetStdHandle(STD_OUTPUT_HANDLE), lpBuff, lstrlen(lpBuff), &dwSize, NULL);

This works for C++ under MSVC, and even for GUI applications when being run via Debugger. It also gets omitted entirely from release builds. It even uses a C++ stringstream for flexible input.

#include <iostream>

#ifdef _MSC_VER
#include "Windows.h"
#endif

#if !defined(NDEBUG) && defined(_MSC_VER)
#define LOG(args) {std::stringstream _ss; _ss << __FILE__ << "@" << __LINE__ << ": " \
    << args << std::endl; OutputDebugString(_ss.str().c_str());}
#else
#define LOG(args)
#endif

Use like:

LOG("some message " << someValue);