Nested Exceptions

A nested exception is an exception that occurs while another exception is being handled.

OS/2 supports nested exceptions because an unhandled exception that occurs in an exception handler should be handled at a higher level-that is, by an ancestor of the procedure that registered the offending handler.

When a nested exception occurs, the EH_NESTED_CALL flag is set in the exception structure to indicate that a nested function call is being made. The normal convention then is for the handler to return immediately without handling the exception if the EH_NESTED_CALL flag is set. Without this flag, it would be easy to create an infinitely recursive situation.

For example, suppose we have the following scenario:

• Procedure main calls procedure PA, which establishes exception handler HA.
  
• Procedure PA calls procedure PB, which establishes exception handler HB.
  
• Procedure PB calls procedure PC, which establishes exception handler HC.
  
• Procedure PC calls procedure PD.

Now suppose that procedure PD causes an exception. The system refers to the current thread's chain of exception handlers.

Because procedure PD has no handler, the system calls HC, the handler for procedure PC, with the EH_NESTED_CALL flag clear. If handler HC returns CONTINUE_SEARCH, the system calls the next handler in the chain, handler HB, again with the EH_NESTED_CALL flag clear.

Now suppose that exception handler HB causes an exception while it is processing the original exception. The call frames for the procedures are arranged in the following order on the stack:

• Procedure main 2.
  Procedure PA
  3.
  Procedure PB
  4.
  Procedure PC
  5.
  Procedure PD
  6.
  OS/2's exception dispatcher
  7.
  Procedure HB, which is the exception handler procedure
  8.
  OS/2's exception dispatcher

  The system will now start traversing the exception handler chain again. Exception handler HB could have registered an exception handler, which would be the first handler in the chain. If it had registered a handler, it would be called with the EH_NESTED_CALL flag clear.

  The range of the nested exception is exception handlers HC and HB. The end of this range can be determined by the fact that exception handler HB is the currently active handler.

  These exception handlers have already been given a chance to handle the original exception. They are now about to be called again in a nested range. Therefore, when handlers HC and HB are called again, they will be called with the EH_NESTED_CALL flag set. If they do not handle the exception, then exception handler HA will be called with the EH_NESTED_CALL flag clear, because it is outside the nested range.

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