Sending a key to a window that’s not in focus (C program)

In a previous post, I showed how a C program can simulate a keystroke in Windows using SendInput. I also posted a subsequent example showing how to simulate a keypress with the Control key included (e.g. Ctrl+V). In this post I’m revisting SendInput, but this time an out-of-focus window is the target for the simulated keystroke(s).

The following C program sends a keypress to a specified window that does not need to be in focus. The keystroke to send is specified as the first command line argument. The second command line argument is a “target” word that appears in the title of the target window. The target word can be anything that appears in the title of the taregt window, but it makes sense to pick something that definitely won’t appear in the title of any other windows. The search word is case sensitive and the simulated keystroke is sent to the first matching window.

//
// sendkey.c - Send a key press to a specific window
// Written by Ted Burke
// Last updated 13-2-2013
//
// To compile using gcc:
//
//		gcc -o sendkey.exe sendkey.c
//
// To send the letter 'a' to a WordPad window:
//
//		sendkey.exe a WordPad
//

#define WINVER 0x0500
#include <windows.h>
#include <stdio.h>

int main(int argc, char* argv[])
{
	// Check number of command line arguments
	if (argc < 3)
	{
		fprintf(stderr, "Too few command line arguments\n");
		fprintf(stderr, "Usage: sendkey.exe KEY_TO_SEND");
		fprintf(stderr, " WORD_FROM_TARGET_WINDOW_TITLE\n");
		return 1;
	}
	
	// Get the character to send from the first command
	// line argument
	char char_to_send = toupper(argv[1][0]);
	
	// Get first window on desktop
	HWND firstwindow = FindWindowEx(NULL, NULL, NULL, NULL);
	HWND window = firstwindow;
	TCHAR windowtext[MAX_PATH];
	
	// We need to get the console title in case we
	// accidentally match the search word with it
	// instead of the intended target window.
	TCHAR consoletitle[MAX_PATH];
	GetConsoleTitle(consoletitle, MAX_PATH);
	
	while(1)
	{
		fprintf(stderr, ".");
		
		// Check window title for a match
		GetWindowText(window, windowtext, MAX_PATH);
		if (strstr(windowtext, argv[2]) != NULL &&
			strcmp(windowtext, consoletitle) != 0) break;
		
		// Get next window
		window = FindWindowEx(NULL, window, NULL, NULL);
		if (window == NULL || window == firstwindow)
		{
			fprintf(stderr, "Window not found\n");
			return 1;
		}
	}
	fprintf(stderr, "Window found: %s\n", windowtext);
	
	// Bring specified window into focus
	SetForegroundWindow(window);

	// Create the desired keyboard event
	INPUT ip;
	ip.type = INPUT_KEYBOARD;
	ip.ki.wVk = char_to_send;
	ip.ki.wScan = 0;
	ip.ki.dwFlags = 0;
	ip.ki.time = 0;
	ip.ki.dwExtraInfo = 0;
	
	// Send the keyboard event to the specified window
	SendInput(1, &ip, sizeof(INPUT));
	
	// Exit normally
	return 0;
}

This is what it looked like in the console where I compiled and ran sendkey.exe:

This screenshot shows the WordPad window that received the ‘A’ key press: