SetConsoleCtrlHandler

Excerpt:

Registering a Control Handler Function

 

This is an example of the SetConsoleCtrlHandler function that is used to install a control handler.

When a CTRL+C signal is received, the control handler returns TRUE, indicating that it has handled the signal. Doing this prevents other control handlers from being called.

When a CTRL_CLOSE_EVENT signal is received, the control handler returns TRUE and the process terminates.

When a CTRL_BREAK_EVENT, CTRL_LOGOFF_EVENT, or CTRL_SHUTDOWN_EVENT signal is received, the control handler returns FALSE. Doing this causes the signal to be passed to the next control handler function. If no other control handlers have been registered or none of the registered handlers returns TRUE, the default handler will be used, resulting in the process being terminated.

C++

 

#include <windows.h> 
#include <stdio.h> 
 
BOOL CtrlHandler( DWORD fdwCtrlType ) 
{ 
  switch( fdwCtrlType ) 
  { 
    // Handle the CTRL-C signal. 
    case CTRL_C_EVENT: 
      printf( "Ctrl-C event

" );
      Beep( 750, 300 ); 
      return( TRUE );
 
    // CTRL-CLOSE: confirm that the user wants to exit. 
    case CTRL_CLOSE_EVENT: 
      Beep( 600, 200 ); 
      printf( "Ctrl-Close event

" );
      return( TRUE ); 
 
    // Pass other signals to the next handler. 
    case CTRL_BREAK_EVENT: 
      Beep( 900, 200 ); 
      printf( "Ctrl-Break event

" );
      return FALSE; 
 
    case CTRL_LOGOFF_EVENT: 
      Beep( 1000, 200 ); 
      printf( "Ctrl-Logoff event

" );
      return FALSE; 
 
    case CTRL_SHUTDOWN_EVENT: 
      Beep( 750, 500 ); 
      printf( "Ctrl-Shutdown event

" );
      return FALSE; 
 
    default: 
      return FALSE; 
  } 
} 
 
int main( void ) 
{ 
  if( SetConsoleCtrlHandler( (PHANDLER_ROUTINE) CtrlHandler, TRUE ) ) 
  { 
    printf( "
The Control Handler is installed.
" ); 
    printf( "
 -- Now try pressing Ctrl+C or Ctrl+Break, or" ); 
    printf( "
    try logging off or closing the console...
" ); 
    printf( "
(...waiting in a loop for events...)

" ); 
 
    while( 1 ){ } 
  } 
  else 
  {
    printf( "
ERROR: Could not set control handler"); 
    return 1;
  }
return 0;
}

HandlerRoutine callback function

 

An application-defined function used with the SetConsoleCtrlHandler function. A console process uses this function to handle control signals received by the process. When the signal is received, the system creates a new thread in the process to execute the function.

The PHANDLER_ROUTINE type defines a pointer to this callback function. HandlerRoutine is a placeholder for the application-defined function name.

Syntax

C++

 

BOOL WINAPI HandlerRoutine(
  _In_  DWORD dwCtrlType
);

Parameters

dwCtrlType [in]

The type of control signal received by the handler. This parameter can be one of the following values.

Value	Meaning
CTRL_C_EVENT 0	A CTRL+C signal was received, either from keyboard input or from a signal generated by the GenerateConsoleCtrlEvent function.
CTRL_BREAK_EVENT 1	A CTRL+BREAK signal was received, either from keyboard input or from a signal generated by GenerateConsoleCtrlEvent.
CTRL_CLOSE_EVENT 2	A signal that the system sends to all processes attached to a console when the user closes the console (either by clicking Close on the console window's window menu, or by clicking the End Task button command from Task Manager  (Test Failed)).
CTRL_LOGOFF_EVENT 5	A signal that the system sends to all console processes when a user is logging off. This signal does not indicate which user is logging off, so no assumptions can be made. Note that this signal is received only by services. Interactive applications are terminated at logoff, so they are not present when the system sends this signal.
CTRL_SHUTDOWN_EVENT 6	A signal that the system sends when the system is shutting down. Interactive applications are not present by the time the system sends this signal, therefore it can be received only be services in this situation. Services also have their own notification mechanism for shutdown events. For more information, see Handler. This signal can also be generated by an application usingGenerateConsoleCtrlEvent.

Return value

If the function handles the control signal, it should return TRUE. If it returns FALSE, the next handler function in the list of handlers for this process is used.

Remarks

Because the system creates a new thread in the process to execute the handler function, it is possible that the handler function will be terminated by another thread in the process. Be sure to synchronize threads in the process with the thread for the handler function.

Each console process has its own list of HandlerRoutine functions. Initially, this list contains only a default handler function that calls ExitProcess. A console process adds or removes additional handler functions by calling the SetConsoleCtrlHandler function, which does not affect the list of handler functions for other processes. When a console process receives any of the control signals, its handler functions are called on a last-registered, first-called basis until one of the handlers returns TRUE. If none of the handlers returns TRUE, the default handler is called.

The CTRL_CLOSE_EVENT, CTRL_LOGOFF_EVENT, and CTRL_SHUTDOWN_EVENT signals give the process an opportunity to clean up before termination. A HandlerRoutine can perform any necessary cleanup, then take one of the following actions:

• Call the ExitProcess function to terminate the process.
• Return FALSE. If none of the registered handler functions returns TRUE, the default handler terminates the process.
• Return TRUE. In this case, no other handler functions are called and the system terminates the process.

A process can use the SetProcessShutdownParameters function to prevent the system from displaying a dialog box to the user during logoff or shutdown. In this case, the system terminates the process whenHandlerRoutine returns TRUE or when the time-out period elapses.

When a console application is run as a service, it receives a modified default console control handler. This modified handler does not call ExitProcess when processing the CTRL_LOGOFF_EVENT andCTRL_SHUTDOWN_EVENT signals. This allows the service to continue running after the user logs off. If the service installs its own console control handler, this handler is called before the default handler. If the installed handler calls ExitProcess when processing the CTRL_LOGOFF_EVENT signal, the service exits when the user logs off.

Note that a third-party library or DLL can install a console control handler for your application. If it does, this handler overrides the default handler, and can cause the application to exit when the user logs off.

 http://msdn.microsoft.com/en-us/library/windows/desktop/ms685049(v=vs.85).aspx

http://msdn.microsoft.com/en-us/library/windows/desktop/ms683242(v=vs.85).aspx