Late Loading

Windows 95/98 improves system performance by supporting late loading for thunk DLLs. In a 16-bit to 32-bit thunk, the system loads the 32-bit DLL when the application calls the first 16-bit to 32-bit thunk. It does not load the 32-bit DLL when the 16-bit DLL is loaded.

Late loading has the following implications:

• The performance and working set size are improved for 16-bit applications that use only the 16-bit portions of thunk DLLs. These processes do not load the 32-bit DLL.
• The 16-bit DLL must not depend on any action taken by the 32-bit DLL's initialization code until the application has called at least one 16-bit to 32-bit thunk.
• The system cannot detect missing 32-bit DLLs or failure to load a 32-bit DLL until the application makes the first call to a 16-bit to 32-bit thunk. If the 32-bit DLL does not load or fails to initialize, the 16-bit to 32-bit thunk call returns a value of zero. You can change this error code on a thunk-by-thunk basis by including the faulterrorcode keyword in the function prototype in the thunk script. For example, the following function instructs the thunk subsystem to return a – 1 from the thunk call if it is the first thunk call and the 32-bit DLL cannot finish loading.

  int Sample(void) 
  {
      faulterrorcode = -1;
  }

Although late loading is a valuable optimization for a 16-bit DLL that can execute autonomously from its 32-bit counterpart, it does complicate error recovery. You can disable late loading by including the following line in the thunk script:

preload32 = true;

If you use this option, the 16-bit subsystem will be released during the loading of your 16-bit DLL, possibly causing other 16-bit code to be reentered. For this reason, the preload32 keyword is not available if your thunk script requires the win31compat keyword. For more information about the win31compat keyword, see Compatibility with Existing 16-Bit DLLs.

Although the thunk compiler supports a preload16 keyword, late loading of 16-bit DLLs is neither currently supported nor planned for a future version of Windows.