Users and applications
Formal description
Use in data structures
Architectural roots
Making pointers safer
Simulation using an array index
Project partners & contact details
Exception handling
Exception handling is the process of responding to the occurrence, during computation, of exceptions anomalous or exceptional conditions requiring special processing often disrupting the normal flow of program execution. It is provided by specialized programming language constructs, computer hardware mechanisms like interrupts or operating system IPC facilities like signals.
Alternative approaches to exception handling in software are error checking, which maintains normal program flow with later explicit checks for contingencies reported using special return values or some auxiliary global variable such as C's errno or floating point status flags; or input validation to preemptively filter exceptional cases.
Exception handling in the IEEE 754 floating point hardware standard refers in general to exceptional conditions and defines an exception as "an event that occurs when an operation on some particular operands has no outcome suitable for every reasonable application. That operation might signal one or more exceptions by invoking the default or, if explicitly requested, a language-defined alternate handling."
The operating system may provide facilities for handling exceptions in programs via IPC. Typically, interrupts caused by the execution of a process are handled by the interrupt service routines of the operating system, and the operating system may then send a signal to that process, which may have asked the operating system to register a signal handler to be called when the signal is raised, or let the operating system execute a default action (like terminating the program). Typical examples are SIGSEGV, SIGBUS, SIGILL and SIGFPE.
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. value is outside of the domain of a function) or when a resource it relies on is unavailable (like a missing file, a hard disk error, or out-of-memory errors). 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.
Excluding minor syntactic differences, there are only a couple of exception handling styles in use. In the most popular style, an exception is initiated by a special statement (throw or raise) with an exception object (e.g. with Java or Object Pascal) or a value of a special extendable enumerated type (e.g. with Ada or SML). The scope for exception handlers starts with a marker clause (try or the language's block starter such as begin) and ends in the start of the first handler clause (catch, except, rescue). Several handler clauses can follow, and each can specify which exception types it handles and what name it uses for the exception object.
Recent front-end web frameworks, such as React and Vue, have introduced error handling mechanisms where errors propagate up the UI component hierarchy, in a way that is analogous to how errors propagate up the call stack in executing code. Here the error boundary mechanism serves as an analogue to the typical try-catch mechanism. Thus a component can ensure that errors from it's child components are caught and handled, and not propagated up to parent components.
The second scheme, and the one implemented in many production-quality C++ compilers, is a table-driven approach. This creates static tables at compile time and link time that relate ranges of the program counter to the program state with respect to exception handling. Then, if an exception is thrown, the runtime system looks up the current instruction location in the tables and determines what handlers are in play and what needs to be done. This approach minimizes executive overhead for the case where an exception is not thrown. This happens at the cost of some space, but this space can be allocated into read-only, special-purpose data sections that are not loaded or relocated until an exception is actually thrown. This second approach is also superior in terms of achieving thread safety.
This approach has the merit of defining clearly what "normal" and "abnormal" cases are: an abnormal case, causing an exception, is one in which the routine is unable to fulfill its contract. It defines a clear distribution of roles: the do clause (normal body) is in charge of achieving, or attempting to achieve, the routine's contract; the rescue clause is in charge of reestablishing the context and restarting the process, if this has a chance of succeeding, but not of performing any actual computation.
Note that even though an uncaught exception may result in the program terminating abnormally (the program may not be correct if an exception is not caught, notably by not rolling back partially completed transactions, or not releasing resources), the process terminates normally (assuming the runtime works correctly), as the runtime (which is controlling execution of the program) can ensure orderly shutdown of the process.