Обработка исключений в программировании

Programming language construct for special conditions

computing
In computing and computer programming, exception handling is the process of responding to the occurrence of exceptions – anomalous or exceptional conditions requiring special processing – during the execution of a program. In general, an exception breaks the normal flow of execution and executes a pre registered exception handler; the details of how this is done depend on whether it is a hardware or software exception and how the software exception is implemented. Exceptions are defined by different layers of a computer system, and the typical layers are CPU defined interrupts, operating system (OS) defined signals, programming language defined exceptions. Each layer requires different ways of exception handling although they may be interrelated, e. g. a CPU interrupt could be turned into an OS signal. Some exceptions, especially hardware ones, may be handled so gracefully that execution can resume where it was interrupted.

The definition of an exception is based on the observation that each procedure has a precondition, a set of circumstances for which it will terminate "normally". An exception handling mechanism allows the procedure to raise an exception if this precondition is violated, an unavailable resource (like a missing file, a network drive error, or out of memory errors), or that the routine has detected a normal condition that requires special handling, e. g., attention, end of file. Social pressure is a major influence on the scope of exceptions and use of exception handling mechanisms, i. e. "examples of use, typically found in core libraries, and code examples in technical books, magazine articles, and online discussion forums, and in an organization’s code standards". Exception handling solves the semipredicate problem, in that the mechanism distinguishes normal return values from erroneous ones. In languages without built in exception handling such as C, routines would need to signal the error in some other way, such as the common return code and errno pattern. The term "exception" is preferred to "error" because it does not imply that anything is wrong a condition viewed as an error by one procedure or programmer may not be viewed that way by another. when in fact raising the exception may be a normal and usual situation in the program. For example, suppose a lookup function for an associative array throws an exception if the key has no value associated. Depending on context, this "key absent" exception may occur much more often than a successful lookup.

The first hardware exception handling was found in the UNIVAC I from 1951. Arithmetic overflow executed two instructions at address 0 which could transfer control or fix up the result. Software exception handling developed in the 1960s and 1970s. Exception handling was subsequently widely adopted by many programming languages from the 1980s onward.

There is no clear consensus as to the exact meaning of an exception with respect to hardware. From the implementation point of view, it is handled identically to an interrupt: the processor halts execution of the current program, looks up the interrupt handler in the interrupt vector table for that exception or interrupt condition, saves state, and switches control.