Exception Management

An exception is an abnormal condition that can occur during program execution. Common causes of exceptions include:

• I/O errors
  
• Protection violations
  
• Math errors
  
• Intervention by the user or by another process

• Trying to use memory that you do not have permission to access 
  Dividing by 0
  
  The user pressing Ctrl+Break
  Exceptions include both unexpected errors (such as a memory protection violation) and expected errors (such as guard-page exceptions). Exceptions can be a synchronous exception, that is, caused by an action of the executing thread, or an asynchronous exception, caused by an event external to the executing thread (such as the user pressing Ctrl+Break). When an exception is caused by the user pressing Ctrl+Break or Ctrl+C, or by another process issuing DosKillProcess for your process, the exception is called a signal exception.

  In most cases, the default action taken by OS/2 when an exception occurs is to terminate the application that caused the exception. Rather than having OS/2 default action occur, an application can register its own subroutine to handle exceptions. These routines are called exception handlers. Exception handlers enable an application to handle some errors itself, allowing the application to avoid termination (or at least to terminate gracefully).

  When exception handlers are registered, they are added to an exception handler chain. The chain starts empty and each new handler is added to the head of the chain. Exceptions are passed to the exception handlers in the chain in Last-In-First-Out order, so the last exception handler to be registered is the first one to get an opportunity to handle each exception.

  Exception handlers have the capability to complete critical code sections without being interrupted by other asynchronous exceptions; these critical code sections are called must-complete sections.

  Exception handlers can be removed from the exception handler chains with DosUnsetExceptionHandler. Another way that exception handlers can be removed from the chain is with an unwind operation. When unwinding an exception handler, the exception handler is first called, then removed from the exception handler chain.

  The following topics are related to the information in this chapter:

  • Memory 
    Program execution and control

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