# Undefined value > Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Undefined_value > Markdown URL: https://mediated.wiki/source/Undefined_value.md > Source: https://en.wikipedia.org/wiki/Undefined_value > Source revision: 1328981578 > License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) {{short description|In computing, a condition where an expression does not have a correct value}} {{Other uses|Undefined (disambiguation){{!}}Undefined}} {{Refimprove|date=March 2010}} {{Use dmy dates|date=December 2021|cs1-dates=y}} In computing (particularly, in [programming](/source/Computer_programming)), '''undefined value''' is a condition where an [expression](/source/expression_(programming)) in a [programming language](/source/programming_language) does not have a [semantically](/source/semantics_(computer_science)) correct [value](/source/value_(computer_science)) in the language, even though it is [syntactically](/source/syntax_(programming_languages)) correct. An undefined value must not be confused with [empty string](/source/empty_string), [Boolean](/source/Boolean_data_type) "false" or other "empty" (but defined) values. Depending on circumstances, evaluation to an undefined value may lead to [exception](/source/exception_(programming)) or [undefined behaviour](/source/undefined_behaviour), but in some [programming language](/source/programming_language)s undefined values can occur during a normal, predictable course of [program](/source/program_(computing)) execution. [Dynamically typed](/source/Dynamic_typing) languages usually treat undefined values explicitly when possible. For instance, [Perl](/source/Perl) has undef operator which can "assign" such value to a variable. In other type systems an undefined value can mean an unknown, unpredictable value, or merely a program failure on attempt of its evaluation. Nullable types offer an intermediate approach; see below. == Handling == The value of a [partial function](/source/partial_function) is undefined when its argument is out of its [domain of definition](/source/domain_of_definition). This include numerous [arithmetic](/source/arithmetic)al cases such as [division by zero](/source/division_by_zero), [square root](/source/square_root) or [logarithm](/source/logarithm) of a [negative number](/source/negative_number) etc. Another common example is accessing an [array](/source/array_data_structure) with an index which is out of bounds, as is the value in an [associative array](/source/associative_array) for a key which it does not contain. There are various ways that these situations are handled in practice: === Reserved value === {{main|Nullable type|Sentinel value|NaN}} In applications where undefined values must be handled gracefully, it is common to reserve a special null value which is distinguishable from normal values. This resolves the difficulty by creating a defined value to represent the formerly undefined case. There are many examples of this: * The C [standard I/O library](/source/C_file_input%2Foutput) reserves the special value [EOF](/source/End-of-file) to indicate that no more input is available. The getchar() function returns the next available input character, or EOF if there is no more available. (The [ASCII](/source/ASCII) character code defines a [null character](/source/null_character) for this purpose, but the standard I/O library wishes to be able to send and receive null characters, so it defines a separate EOF value.) * The [IEEE 754](/source/IEEE_754) floating-point arithmetic standard defines a special "[not a number](/source/not_a_number)" value which is returned when an [arithmetic](/source/arithmetic) operation has no defined value. Examples are [division by zero](/source/division_by_zero), or the [square root](/source/square_root) or [logarithm](/source/logarithm) of a [negative number](/source/negative_number). * [Structured Query Language](/source/Structured_Query_Language) has a special [NULL](/source/Null_(SQL)) value to indicate missing data. * The Perl language lets the definedness of an expression be checked via the defined() predicate. * Many programming languages support the concept of a [null pointer](/source/null_pointer) distinct from any valid pointer, and often used as an error return. * Some languages allow most types to be nullable, for example [C#](/source/C_Sharp_(programming_language)). * Most [Unix](/source/Unix) [system call](/source/system_call)s return the special value −1 to indicate failure. While [dynamically typed](/source/dynamically_typed) languages often ensure that [uninitialized variable](/source/uninitialized_variable)s default to a null value, statically typed values often do not, and distinguish null values (which are well-defined) from uninitialized values (which are not).{{r|Carr_2006}} === Exception handling === Some programming languages have a concept of [exception handling](/source/exception_handling) for dealing with failure to return a value. The function returns in a defined way, but it does not return a value, so there is no need to invent a special value to return. A variation on this is [signal handling](/source/signal_handling), which is done at the operating system level and not integrated into a programming language. Signal handlers can attempt some forms of recovery, such as terminating part of a computation, but without as much flexibility as fully integrated exception handling. === Non-returning functions === {{main|Bottom type}} A function which never returns has an undefined value because the value can never be observed. Such functions are formally assigned the [bottom type](/source/bottom_type), which has no values. Examples fall into two categories: * Functions which [loop forever](/source/infinite_loop). This may arise deliberately, or as a result of a search for something which will never be found. (For example, in the case of failed [μ operator](/source/%CE%BC_operator) in a [partial recursive function](/source/partial_recursive_function).) * Functions which terminate the computation, such as the [exit system call](/source/Exit_(system_call)). From within the program, this is indistinguishable from the preceding case, but it makes a difference to the invoker of the program. === Undefined behaviour === {{main|Undefined behaviour}} All of the preceding methods of handling undefined values require that the undefinedness be detected. That is, the called function determines that it cannot return a normal result and takes some action to notify the caller. At the other end of the spectrum, ''undefined behaviour'' places the onus on the caller to avoid calling a function with arguments outside of its domain. There is no limit on what might happen. At best, an easily detectable [crash](/source/Crash_(computing)); at worst, a subtle error in a seemingly unrelated computation. (The formal definition of "undefined behaviour" includes even more extreme possibilities, including things like "[halt and catch fire](/source/Halt_and_Catch_Fire_(computing))" and "make demons fly out of your nose".) The classic example is a [dangling pointer](/source/dangling_pointer) reference. It is very fast to [dereference](/source/dereference) a valid [pointer](/source/pointer_(computer_programming)), but can be very complex to determine if a pointer is valid. Therefore, computer hardware and low-level languages such as [C](/source/C_(programming_language)) do not attempt to validate pointers before dereferencing them, instead passing responsibility to the programmer. This offers speed at the expense of safety. === Undefined value ''sensu stricto'' === The strict definition of an undefined value is a superficially valid (non-null) output which is meaningless but does not trigger undefined behaviour. For example, passing a negative number to the [fast inverse square root](/source/fast_inverse_square_root) function will produce a number. Not a very useful number, but the computation will complete and return ''something''. Undefined values occur particularly often in hardware. If a wire is not carrying useful information, it still exists and has some voltage level. The voltage should not be abnormal (e.g. not a damaging [overvoltage](/source/overvoltage)), but the particular [logic level](/source/logic_level) is unimportant. The same situation occurs in software when a [data buffer](/source/data_buffer) is provided but not completely filled. For example, the C library [strftime](/source/strftime) function converts a timestamp to human-readable form in a supplied output buffer. If the output buffer is not large enough to hold the result, an error is returned and the buffer's contents are undefined. In the other direction, the [open system call](/source/open_(system_call)) in [POSIX](/source/POSIX) takes three arguments: a file name, some flags, and a file mode. The file mode is only used if the flags include O_CREAT. It is common to use a two-argument form of open, which provides an undefined value for the file mode, when O_CREAT is omitted. Sometimes it is useful to work with such undefined values in a limited way. The overall computation can still be well-defined if the undefined value is later ignored. As an example of this, the C language permits converting a pointer to an integer, although the numerical value of that integer is undefined. It may still be useful for debugging, for comparing two pointers for equality, or for creating an [XOR linked list](/source/XOR_linked_list). Safely handling undefined values is important in [optimistic concurrency control](/source/optimistic_concurrency_control) systems, which detect [race condition](/source/race_condition)s after the fact. For example, reading a [shared variable](/source/shared_variable) protected by [seqlock](/source/seqlock) will produce an undefined value ''before'' determining that a race condition happened. It will then discard the undefined data and retry the operation. This produces a defined result as long as the operations performed on the undefined values do not produce full-fledged undefined behaviour. Other examples of undefined values being useful are [random number generator](/source/random_number_generator)s and [hash function](/source/hash_function)s. The specific values returned are undefined, but they have well-defined properties and may be used without error. == Notation == In [computability theory](/source/computability_theory), undefinedness of an expression is denoted as ''expr''↑, and definedness as ''expr''↓. == See also == {{columns-list| * [Defined and undefined](/source/Defined_and_undefined) (mathematics) * [Null (SQL)](/source/Null_(SQL)) * [Type theory](/source/Type_theory) * [Unit type](/source/Unit_type) }} == References == {{Reflist|refs= {{cite web |title=undef |date=2009-09-25 |work=Perl 5 documentation |url=http://perldoc.perl.org/functions/undef.html |access-date=2010-03-26}} {{cite web |title=defined |date=2009-09-25 |work=Perl 5 documentation |url=http://perldoc.perl.org/functions/defined.html |access-date=2010-03-26}} {{cite web |title=C# Nullable Numeric Data Types |author-first=Richard |author-last=Carr |date=2006-10-01 |work=C# Fundamentals tutorial |url=http://www.blackwasp.co.uk/CSharpNullableDataTypes.aspx |archive-url=https://web.archive.org/web/20070121094123/http://www.blackwasp.co.uk/CSharpNullableDataTypes.aspx |url-status=usurped |archive-date=21 January 2007 |access-date=2010-03-27}} {{cite web |title=Nasal demons |website=[Jargon File](/source/Jargon_File) |url=http://catb.org/jargon/html/N/nasal-demons.html |access-date=2014-06-12}} }} Category:Software anomalies Category:Theory of computation --- Adapted from the Wikipedia article [Undefined value](https://en.wikipedia.org/wiki/Undefined_value) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Undefined_value?action=history)). 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