View source for Unspecified behavior

{{short description|Differing behavior by identically-coded programs when compiled}}
{{distinguish|Undefined behavior}}

In [[computer programming]], '''unspecified behavior''' is behavior that may vary on different implementations of a [[programming language]].{{clarify|date=September 2012}}<!--very vague term here; is it behavior on the part of the compiler, on the part of the hardware running the program, or what?--> A [[computer program|program]] can be said to contain unspecified behavior when its [[source code]] may produce an [[executable]] that exhibits different behavior when compiled on a different [[compiler]], or on the same compiler with different settings, or indeed in different parts of the same executable. While the respective language standards or specifications may impose a range of possible behaviors, the exact behavior depends on the implementation and may not be completely determined upon examination of the program's source code.<ref name="pdtr">ISO/IEC (2009-05-29). ''ISO/IEC PDTR 24772.2: Guidance to Avoiding Vulnerabilities in Programming Languages through Language Selection and Use''</ref> Unspecified behavior will often not manifest itself in the resulting program's external behavior, but it may sometimes lead to differing outputs or results, potentially causing [[Software portability|portability]] problems.

==Definition==
To enable compilers to produce optimal code for their respective target platforms, programming language standards do not always impose a certain specific behavior for a given source code construct.<ref name="ddj_becker">{{cite magazine|url=http://www.ddj.com/cpp/187203727?pgno=2|title=Living By the Rules|last=Becker|first=Pete|date=2006-05-16|magazine=[[Dr. Dobb's Journal]]|access-date=26 November 2009}}</ref> Failing to explicitly define the exact behavior of every possible program is not considered an error or weakness in the language specification, and doing so would be infeasible.<ref name="pdtr"/> In the [[C (programming)|C]] and [[C++]] languages, such non-[[Software portability|portable]] constructs are generally grouped into three categories: Implementation-defined, unspecified, and [[undefined behavior]].<ref name="industrialcpp">{{cite book
|last1=Henricson
|first1=Mats
|last2=Nyquist
|first2=Erik
|title=Industrial Strength C++
|publisher=Prentice Hall
|year=1997
|isbn=0-13-120965-5
|url-access=registration
|url=https://archive.org/details/industrialstreng0000henr
}}
</ref>

The exact definition of unspecified behavior varies. In C++, it is defined as "behavior, for a well-formed program construct and correct data, that depends on the implementation."<ref name="C++03 1.3.13">[[International Organization for Standardization|ISO]]/[[International Electrotechnical Commission|IEC]] (2003). ''[[ISO/IEC 14882|ISO/IEC 14882:2003(E): Programming Languages - C++]] §1.3.13 unspecified behavior [defns.unspecified]''</ref> The C++ Standard also notes that the range of possible behaviors is usually provided.<ref name="C++03 1.3.13"/> Unlike implementation-defined behavior, there is no requirement for the implementation to document its behavior.<ref name="C++03 1.3.13"/> Similarly, the C Standard defines it as behavior for which the standard "provides two or more possibilities and imposes no further requirements on which is chosen in any instance".<ref name="c99_usb">[[International Organization for Standardization|ISO]]/[[International Electrotechnical Commission|IEC]] (1999). ''[[C99|ISO/IEC 9899:1999(E): Programming Languages - C]] §3.4.4 para 1''</ref>  Unspecified behavior is different from [[undefined behavior]]. The latter is typically a result of an erroneous program construct or data, and no requirements are placed on the translation or execution of such constructs.<ref name="C++03 1.3.12">[[International Organization for Standardization|ISO]]/[[International Electrotechnical Commission|IEC]] (2003). ''[[ISO/IEC 14882|ISO/IEC 14882:2003(E): Programming Languages - C++]] §1.3.12 undefined behavior [defns.undefined]''</ref>

== Implementation-defined behavior ==

C and C++ distinguish ''implementation-defined behavior'' from unspecified behavior. For implementation-defined behavior, the implementation must choose a particular behavior and document it. An example in C/C++ is the size of integer data types.  The choice of behavior must be consistent with the documented behavior within a given execution of the program.

==Examples==
===Order of evaluation of subexpressions===
Many programming languages do not specify the order of evaluation of the sub-expressions of a complete [[Expression (programming)|expression]]. This non-determinism can allow optimal implementations for specific platforms e.g. to utilise parallelism. If one or more of the sub-expressions has [[Side effect (computer science)|side effect]]s, then the result of evaluating the full-expression may be different depending on the order of evaluation of the sub-expressions.<ref name="pdtr"/> For example, given <syntaxhighlight lang="c">a = f(b) + g(b);</syntaxhighlight>, where <code>f</code> and <code>g</code> both modify <code>b</code>, the result stored in <code>a</code> may be different depending on whether <code>f(b)</code> or <code>g(b)</code> is evaluated first.<ref name="pdtr"/> In the C and C++ languages, this also applies to function arguments. Example:<ref name="ddj_becker"/>
<syntaxhighlight lang="cpp">
#include <iostream>
int f() {
  std::cout << "In f\n";
  return 3;
}

int g() {
  std::cout << "In g\n";
  return 4;
}

int sum(int i, int j) {
  return i + j;
}

int main() {
  return sum(f(), g()); 
}
</syntaxhighlight>
The resulting program will write its two lines of output in an unspecified order.<ref name="ddj_becker"/> In some other languages, such as [[Java (programming language)|Java]], the order of evaluation of operands and function arguments is explicitly defined.<ref name="jls">[[James Gosling]], [[Bill Joy]], [[Guy L. Steele, Jr.|Guy Steele]], and [[Gilad Bracha]] (2005). ''The Java Language Specification'', Third Edition. Addison-Wesley. {{ISBN|0-321-24678-0}}</ref>

==See also==
*[[Software portability]]
*[[Undefined behavior]]

==References==
{{reflist}}

[[Category:Articles with example C code]]
[[Category:Articles with example C++ code]]
[[Category:Computer programming]]
[[Category:Programming language implementation]]