Exception handling is a programming language construct or computer hardware mechanism designed to handle the occurrence of exceptions, special conditions that change the normal flow of program execution.
Programming languages differ considerably in their support for exception handling (as distinct from error checking, which is normal program flow that codes for responses to adverse contingencies such as invalid state changes or the unsuccessful termination of invoked operations.) In some programming languages there are functions which cannot be safely called on invalid input data or functions which return values which cannot be distinguished from exceptions. For example, in C the atoi (ASCII to integer conversion) function may return 0 (zero) for any input that cannot be parsed into a valid value. In such languages the programmer must either perform error checking (possibly through some auxiliary global variable such as C's errno) or input validation (perhaps using regular expressions).
The degree to which such explicit validation and error checking is necessary is in contrast to exception handling support provided by any given programming environment. Hardware exception handling differs somewhat from the support provided by software tools, but similar concepts and terminology are prevalent.
In general, an exception is handled (resolved) by saving the current state of execution in a predefined place and switching the execution to a specific subroutine known as an exception handler. Depending on the situation, the handler may later resume the execution at the original location using the saved information. For example, a page fault will usually allow the program to be resumed, while a division by zero might not be resolvable transparently.
From the processing point of view, hardware interrupts are similar to resume-able exceptions, though they are typically unrelated to the user's program flow.
From the point of view of the author of a routine, raising an exception is a useful way to signal that a routine could not execute normally. For example, when an input argument is invalid (e.g. a zero denominator in division) or when a resource it relies on is unavailable (like a missing file, or a hard disk error). In systems without exceptions, routines would need to return some special error code. However, this is sometimes complicated by the semipredicate problem, in which users of the routine need to write extra code to distinguish normal return values from erroneous ones.
In runtime engine environments such as Java or .NET, there exist tools that attach to the runtime engine and every time that an exception of interest occurs, they record debugging information that existed in memory at the time the exception was thrown (call stack and heap values). These tools are called automated exception handling or error interception tools and provide 'root-cause' information for exceptions.
Contemporary applications face many design challenges when considering exception handling strategies. Particularly in modern enterprise level applications, exceptions must often cross process boundaries and machine boundaries. Part of designing a solid exception handling strategy is recognizing when a process has failed to the point where it cannot be economically handled by the software portion of the process.
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