How do I get console output in C++ with a Windows program?

Question

If I have a native C++ windows program (i.e. the entry point is WinMain) how do I view output from console functions like std::cout?

Answer 1

Check out Adding Console I/O to a Win32 GUI App. This may help you do what you want.

If you don't have, or can't modify the code, try the suggestions found here to redirect console output to a file.

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Edit: bit of thread necromancy here. I first answered this 9ish years ago, in the early days of SO, before the (good) policy of non-link-only answers came into effect. I'll repost the code from the original article in the hope to atone for my past sins.

guicon.cpp -- A console redirection function

#include <windows.h> #include <stdio.h> #include <fcntl.h> #include <io.h> #include <iostream> #include <fstream> #ifndef _USE_OLD_IOSTREAMS using namespace std; #endif // maximum mumber of lines the output console should have static const WORD MAX_CONSOLE_LINES = 500; #ifdef _DEBUG void RedirectIOToConsole() {     int hConHandle;     long lStdHandle;     CONSOLE_SCREEN_BUFFER_INFO coninfo;     FILE *fp;      // allocate a console for this app     AllocConsole();      // set the screen buffer to be big enough to let us scroll text     GetConsoleScreenBufferInfo(GetStdHandle(STD_OUTPUT_HANDLE), &coninfo);     coninfo.dwSize.Y = MAX_CONSOLE_LINES;     SetConsoleScreenBufferSize(GetStdHandle(STD_OUTPUT_HANDLE), coninfo.dwSize);      // 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 );      // 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     ios::sync_with_stdio(); }  #endif //End of File 

guicon.h -- Interface to console redirection function

#ifndef __GUICON_H__ #define __GUICON_H__ #ifdef _DEBUG  void RedirectIOToConsole();  #endif #endif  // End of File 

test.cpp -- Demonstrating console redirection

#include <windows.h> #include <iostream> #include <fstream> #include <conio.h> #include <stdio.h> #ifndef _USE_OLD_OSTREAMS using namespace std; #endif #include "guicon.h"   #include <crtdbg.h>  int APIENTRY WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPTSTR lpCmdLine, int nCmdShow) {     #ifdef _DEBUG     RedirectIOToConsole();     #endif     int iVar;      // test stdio     fprintf(stdout, "Test output to stdout\n");     fprintf(stderr, "Test output to stderr\n");     fprintf(stdout, "Enter an integer to test stdin: ");     scanf("%d", &iVar);     printf("You entered %d\n", iVar);      //test iostreams     cout << "Test output to cout" << endl;     cerr << "Test output to cerr" << endl;     clog << "Test output to clog" << endl;     cout << "Enter an integer to test cin: ";     cin >> iVar;     cout << "You entered " << iVar << endl;     #ifndef _USE_OLD_IOSTREAMS      // test wide iostreams     wcout << L"Test output to wcout" << endl;     wcerr << L"Test output to wcerr" << endl;     wclog << L"Test output to wclog" << endl;     wcout << L"Enter an integer to test wcin: ";     wcin >> iVar;     wcout << L"You entered " << iVar << endl;     #endif      // test CrtDbg output     _CrtSetReportMode( _CRT_ASSERT, _CRTDBG_MODE_FILE );     _CrtSetReportFile( _CRT_ASSERT, _CRTDBG_FILE_STDERR );     _CrtSetReportMode( _CRT_ERROR, _CRTDBG_MODE_FILE );     _CrtSetReportFile( _CRT_ERROR, _CRTDBG_FILE_STDERR);     _CrtSetReportMode( _CRT_WARN, _CRTDBG_MODE_FILE );     _CrtSetReportFile( _CRT_WARN, _CRTDBG_FILE_STDERR);     _RPT0(_CRT_WARN, "This is testing _CRT_WARN output\n");     _RPT0(_CRT_ERROR, "This is testing _CRT_ERROR output\n");     _ASSERT( 0 && "testing _ASSERT" );     _ASSERTE( 0 && "testing _ASSERTE" );     Sleep(2000);     return 0; }  //End of File 

Answer 2

The problem some of the other answers is that they unnecessarily create new FILE instances which are then leaked and can cause debug assertions in the CRT cleanup code.

freopen_s is all that is really needed:

FILE* fp = nullptr; freopen_s(&fp, "CONIN$", "r", stdin); freopen_s(&fp, "CONOUT$", "w", stdout); freopen_s(&fp, "CONOUT$", "w", stderr); 

You'll probably want to do a little error checking and cleanup as well. Below is the complete solution that I currently use.

Redirecting Console Standard IO:

bool RedirectConsoleIO() {     bool result = true;     FILE* fp;      // Redirect STDIN if the console has an input handle     if (GetStdHandle(STD_INPUT_HANDLE) != INVALID_HANDLE_VALUE)         if (freopen_s(&fp, "CONIN$", "r", stdin) != 0)             result = false;         else             setvbuf(stdin, NULL, _IONBF, 0);      // Redirect STDOUT if the console has an output handle     if (GetStdHandle(STD_OUTPUT_HANDLE) != INVALID_HANDLE_VALUE)         if (freopen_s(&fp, "CONOUT$", "w", stdout) != 0)             result = false;         else             setvbuf(stdout, NULL, _IONBF, 0);      // Redirect STDERR if the console has an error handle     if (GetStdHandle(STD_ERROR_HANDLE) != INVALID_HANDLE_VALUE)         if (freopen_s(&fp, "CONOUT$", "w", stderr) != 0)             result = false;         else             setvbuf(stderr, NULL, _IONBF, 0);      // Make C++ standard streams point to console as well.     ios::sync_with_stdio(true);      // Clear the error state for each of the C++ standard streams.     std::wcout.clear();     std::cout.clear();     std::wcerr.clear();     std::cerr.clear();     std::wcin.clear();     std::cin.clear();      return result; } 

Releasing a Console:

bool ReleaseConsole() {     bool result = true;     FILE* fp;      // Just to be safe, redirect standard IO to NUL before releasing.      // Redirect STDIN to NUL     if (freopen_s(&fp, "NUL:", "r", stdin) != 0)         result = false;     else         setvbuf(stdin, NULL, _IONBF, 0);      // Redirect STDOUT to NUL     if (freopen_s(&fp, "NUL:", "w", stdout) != 0)         result = false;     else         setvbuf(stdout, NULL, _IONBF, 0);      // Redirect STDERR to NUL     if (freopen_s(&fp, "NUL:", "w", stderr) != 0)         result = false;     else         setvbuf(stderr, NULL, _IONBF, 0);      // Detach from console     if (!FreeConsole())         result = false;      return result; } 

Resizing Console Buffer:

void AdjustConsoleBuffer(int16_t minLength) {     // Set the screen buffer to be big enough to scroll some text     CONSOLE_SCREEN_BUFFER_INFO conInfo;     GetConsoleScreenBufferInfo(GetStdHandle(STD_OUTPUT_HANDLE), &conInfo);     if (conInfo.dwSize.Y < minLength)         conInfo.dwSize.Y = minLength;     SetConsoleScreenBufferSize(GetStdHandle(STD_OUTPUT_HANDLE), conInfo.dwSize); } 

Allocating a New Console:

bool CreateNewConsole(int16_t minLength) {     bool result = false;      // Release any current console and redirect IO to NUL     ReleaseConsole();      // Attempt to create new console     if (AllocConsole())     {         AdjustConsoleBuffer(minLength);         result = RedirectConsoleIO();     }      return result; } 

Attaching to Parent's Console:

bool AttachParentConsole(int16_t minLength) {     bool result = false;      // Release any current console and redirect IO to NUL     ReleaseConsole();      // Attempt to attach to parent process's console     if (AttachConsole(ATTACH_PARENT_PROCESS))     {         AdjustConsoleBuffer(minLength);         result = RedirectConsoleIO();     }      return result; } 

Calling from WinMain:

Link with /SUBSYSTEM:Windows

int APIENTRY WinMain(     HINSTANCE /*hInstance*/,     HINSTANCE /*hPrevInstance*/,     LPTSTR    /*lpCmdLine*/,     int       /*cmdShow*/) {     if (CreateNewConsole(1024))     {         int i;          // test stdio         fprintf(stdout, "Test output to stdout\n");         fprintf(stderr, "Test output to stderr\n");         fprintf(stdout, "Enter an integer to test stdin: ");         scanf("%d", &i);         printf("You entered %d\n", i);          // test iostreams         std::cout << "Test output to std::cout" << std::endl;         std::cerr << "Test output to std::cerr" << std::endl;         std::clog << "Test output to std::clog" << std::endl;         std::cout << "Enter an integer to test std::cin: ";         std::cin >> i;         std::cout << "You entered " << i << std::endl;          std::cout << endl << "Press any key to continue..." << endl;         _getch();          ReleaseConsole();     }      return 0; };