Waiting for Multiple Objects

The following example uses the CreateEvent function to create two event objects and the CreateThread function to create a thread. It then uses the WaitForMultipleObjects function to wait for the thread to set the state of one of the objects to signaled using the SetEvent function.

For an example that waits for a single object, see Using Mutex Objects.

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

HANDLE ghEvents[2];

DWORD WINAPI ThreadProc( LPVOID );

int main( void )
{
 HANDLE hThread; 
 DWORD i, dwEvent, dwThreadID; 

 // Create two event objects

 for (i = 0; i < 2; i++) 
 { 
 ghEvents[i] = CreateEvent( 
 NULL, // default security attributes
 FALSE, // auto-reset event object
 FALSE, // initial state is nonsignaled
 NULL); // unnamed object

 if (ghEvents[i] == NULL) 
 { 
 printf("CreateEvent error: %d\n", GetLastError() ); 
 ExitProcess(0); 
 } 
 } 

 // Create a thread

 hThread = CreateThread( 
 NULL, // default security attributes
 0, // default stack size
 (LPTHREAD_START_ROUTINE) ThreadProc, 
 NULL, // no thread function arguments
 0, // default creation flags
 &dwThreadID); // receive thread identifier

 if( hThread == NULL )
 {
 printf("CreateThread error: %d\n", GetLastError());
 return 1;
 }

 // Wait for the thread to signal one of the event objects

 dwEvent = WaitForMultipleObjects( 
 2, // number of objects in array
 ghEvents, // array of objects
 FALSE, // wait for any object
 5000); // five-second wait

 // The return value indicates which event is signaled

 switch (dwEvent) 
 { 
 // ghEvents[0] was signaled
 case WAIT_OBJECT_0 + 0: 
 // TODO: Perform tasks required by this event
 printf("First event was signaled.\n");
 break; 

 // ghEvents[1] was signaled
 case WAIT_OBJECT_0 + 1: 
 // TODO: Perform tasks required by this event
 printf("Second event was signaled.\n");
 break; 

 case WAIT_TIMEOUT:
 printf("Wait timed out.\n");
 break;

 // Return value is invalid.
 default: 
 printf("Wait error: %d\n", GetLastError()); 
 ExitProcess(0); 
 }

 // Close event handles

 for (i = 0; i < 2; i++) 
 CloseHandle(ghEvents[i]); 
 
 return 0; 
}

DWORD WINAPI ThreadProc( LPVOID lpParam )
{

 // lpParam not used in this example
 UNREFERENCED_PARAMETER( lpParam);

 // Set one event to the signaled state

 if ( !SetEvent(ghEvents[0]) ) 
 {
 printf("SetEvent failed (%d)\n", GetLastError());
 return 1;
 }
 return 0;
}