Exceptions and Exception Handling

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Exceptions and Exception Handling

• Yaodong Bi

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Requirements for Exception Handling

• Simple to understand and use
• The code for exception handling should not
  obscure understanding of normal operations
• Run-time overheads are incurred only when
  handling an exception
• Uniform treatment of exceptions
• Recovery actions can be programmed

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Exception handling in old languages

• Use unused return value
• If (function() ERROR)
• // error handling
• else
• // normal operation
• Forced branch mostly done at assembly level
• Jsr pc, PRINT_CHAR
• Jmp IO_ERROR // error handling
• Jmp DEVICE_NOT_READY // error handling
• //normal processing
• Subroutine PRINT_CHAR would return to a different
  location by manipulating returning address.

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Modern Exception Handling

• Domain of an exception handler
• The region of computation with which, if an
  exception occurs, the handler will be invoked
• Exception propagation
• Regard the absence of a handler as a programmer
  error, or the function must specify what
  exceptions it may raise handled by compiler
• When no local handler can be found, the system
  looks for handlers up the chain of invokers at
  run-time propagation employed in Ada, C, and
  Java

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Modern Exception Handling

• Resumption and termination
• Resumption
• A handler is invoked as an implicit procedure
• Figure 6.1
• Termination
• Control does not return to the point where the
  exception occurred.
• The block/procedure containing the handler is
  terminated, and control returned back to the
  calling block/procedure
• Figure 6.2