Programmatically press a button on another application (C, Windows)

1. Are you sure that "SaveButton" class name is valid? Do you get the button handle?
2. Try to send messages to ButtonHandle window (directly to the button).

Update: I believe this should work,

SendMessage(ButtonHandle, BM_CLICK, 0, 0);

maybe this can help: http://www.cplusplus.com/forum/beginner/8806/

See the following solution, also you can use

SendMessage(ButtonHandle, WM_LBUTTONDOWN, 0, 0);
SendMessage(ButtonHandle, WM_LBUTTONUP, 0, 0);

Or

SendMessage(ButtonHandle, BM_CLICK, 0, 0);

HWND buttonHandle = 0;

BOOL CALLBACK GetButtonHandle(HWND handle, LPARAM)
{
 char label[100];
 int size = GetWindowTextA(handle,label,sizeof(label));
 if(strcmp(label,"&Save") == 0)
 {
  buttonHandle = handle;
  return false;
 }
 return true;
}
void main()
{
 HWND windowHandle = FindWindowA(NULL,"Do you want to Save?");
 if(windowHandle != 0)
 {
  BOOL ret = EnumChildWindows(windowHandle,GetButtonHandle,0);

  if(buttonHandle != 0)
  {
   LRESULT res = SendMessage(buttonHandle,BM_CLICK,0,0);
   //SendMessage(buttonHandle,WM_LBUTTONDOWN,0,0); 
   //SendMessage(buttonHandle,WM_LBUTTONUP,0,0);
  }

 }



}

Note: Getting sure from the window text,button text (check if there is space at the end of the window title)

SendMessage(btnHandle, WM_LBUTTONDOWN, 0, 0);
SendMessage(btnHandle, WM_LBUTTONUP, 0, 0);
SendMessage(btnHandle, WM_LBUTTONDOWN, 0, 0);
SendMessage(btnHandle, WM_LBUTTONUP, 0, 0);

You have to send a button click twice. Not sure why (maybe the first click only activates the window of the button), but I'm using this code for a long time and it always worked for me.