How to view printf output in a Win32 application on Visual Studio 2010?

Question

How can you view printf output in a Win32 application (entering with a WinMain) in Visual Studio 2010?

Answer 1

Edit 2021, Visual Studio 2019

To write debug messages to the Output window use the OutputDebugStringA from debugapi.h (include windows.h)

test.c

#include <windows.h>
#include <stdio.h>

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdShow, int nCmdShow)
{
    int number = 10;
    char str[256];
    sprintf_s(str, sizeof(str), "It works! - number: %d \n", number);

    OutputDebugStringA(str);

    return 0;
}

Tested on Visual Studio 2019, Debug / x64.

Or alternatively utilize my drop-in header file.

Answer 2

You'll need a console window. By far the easiest way to get one is to change a linker option: Project + Properties, Linker, System, SubSystem = Console. Add a main() method:

int main() {
    return _tWinMain(GetModuleHandle(NULL), NULL, GetCommandLine(), SW_SHOW);
}

Answer 3

I know that I have done this in the past using the AllocConsole function, but I also recall that it was just a little trickier than I expected.

A quick Google search on AllocConsole yields what is apparently a Windows Developer Journal article that seems relevant. From there, the following seems similar to what I recall, vague as it is.

void SetStdOutToNewConsole()
{
    int hConHandle;
    long lStdHandle;
    FILE *fp;

    // Allocate a console for this app
    AllocConsole();

    // Redirect unbuffered STDOUT to the console
    lStdHandle = (long)GetStdHandle(STD_OUTPUT_HANDLE);
    hConHandle = _open_osfhandle(lStdHandle, _O_TEXT);
    fp = _fdopen(hConHandle, "w");
    *stdout = *fp;

    setvbuf(stdout, NULL, _IONBF, 0);
}

Answer 4

Another way which wouldn't require changing existing printf's and also print to VS output window would go something like this:

#define printf printf2

int __cdecl printf2(const char *format, ...)
{
    char str[1024];

    va_list argptr;
    va_start(argptr, format);
    int ret = vsnprintf(str, sizeof(str), format, argptr);
    va_end(argptr);

    OutputDebugStringA(str);

    return ret;
}

...

printf("remains %s", "the same");

Answer 5

Thanks torak for your answer. It helped me a lot.

I needed a bigger scroll back buffer so made a few additions after taking a look at the API functions. Shared here in case it helps anybody else:

void SetStdOutToNewConsole()
{
    // allocate a console for this app
    AllocConsole();

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

    // give the console window a nicer title
    SetConsoleTitle(L"Debug Output");

    // give the console window a bigger buffer size
    CONSOLE_SCREEN_BUFFER_INFO csbi;
    if ( GetConsoleScreenBufferInfo(consoleHandle, &csbi) )
    {
        COORD bufferSize;
        bufferSize.X = csbi.dwSize.X;
        bufferSize.Y = 9999;
        SetConsoleScreenBufferSize(consoleHandle, bufferSize);
    }
}

This increases the scroll back (screen buffer) height to 9999 lines.

Tested on Windows XP and Windows 7.

Answer 6

Here is a page that will tell you how to do this, including sample code.

You must create a console window using AllocConsole(), then associate the C standard file handles to the HANDLEs of the new console window.

Answer 7

For MinGW use "_A_SYSTEM" instead "_O_TEXT". So ported Quintin Willison answer is as follows:

#include <io.h>
void SetStdOutToNewConsole()
{
  // allocate a console for this app
  AllocConsole();
  // redirect unbuffered STDOUT to the console
  HANDLE consoleHandle = GetStdHandle(STD_OUTPUT_HANDLE);
  int fileDescriptor = _open_osfhandle((intptr_t)consoleHandle, _A_SYSTEM);
  FILE *fp = _fdopen( fileDescriptor, "w" );
  *stdout = *fp;
  setvbuf( stdout, NULL, _IONBF, 0 );
  // give the console window a nicer title
  SetConsoleTitle(L"Debug Output");
  // give the console window a bigger buffer size
  CONSOLE_SCREEN_BUFFER_INFO csbi;
  if ( GetConsoleScreenBufferInfo(consoleHandle, &csbi) )
  {
    COORD bufferSize;
    bufferSize.X = csbi.dwSize.X;
    bufferSize.Y = 9999;
    SetConsoleScreenBufferSize(consoleHandle, bufferSize);
  }
}

Answer 8

Following is a working code

FILE* fpFile;
AllocConsole(); // or AttachConsole(ATTACH_PARENT_PROCESS); // if parent has one
freopen_s(&fpFile,"CONOUT$", "w", stdout); // redirect stdout to console
freopen_s(&fpFile,"CONOUT$", "w", stderr); // redirect stderr to console
freopen_s(&fpFile,"CONIN$", "r", stdin); // redirect stdin to console