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Unspecified behavior

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In computer programming, unspecified behavior is behavior that may vary on different implementations of a programming language.[clarification needed] A 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.[1] 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 portability problems.

Definition

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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.[2] 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.[1] In the C and C++ languages, such non-portable constructs are generally grouped into three categories: Implementation-defined, unspecified, and undefined behavior.[3]

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."[4] The C++ Standard also notes that the range of possible behaviors is usually provided.[4] Unlike implementation-defined behavior, there is no requirement for the implementation to document its behavior.[4] 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".[5] 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.[6]

Implementation-defined behavior

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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

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Order of evaluation of subexpressions

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Many programming languages do not specify the order of evaluation of the sub-expressions of a complete 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 effects, then the result of evaluating the full-expression may be different depending on the order of evaluation of the sub-expressions.[1] For example, given

a = f(b) + g(b);

, where f and g both modify b, the result stored in a may be different depending on whether f(b) or g(b) is evaluated first.[1] In the C and C++ languages, this also applies to function arguments. Example:[2]

#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()); 
}

The resulting program will write its two lines of output in an unspecified order.[2] In some other languages, such as Java, the order of evaluation of operands and function arguments is explicitly defined.[7]

See also

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References

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  1. ^ a b c d ISO/IEC (2009-05-29). ISO/IEC PDTR 24772.2: Guidance to Avoiding Vulnerabilities in Programming Languages through Language Selection and Use
  2. ^ a b c Becker, Pete (2006-05-16). "Living By the Rules". Dr. Dobb's Journal. Retrieved 26 November 2009.
  3. ^ Henricson, Mats; Nyquist, Erik (1997). Industrial Strength C++. Prentice Hall. ISBN 0-13-120965-5.
  4. ^ a b c ISO/IEC (2003). ISO/IEC 14882:2003(E): Programming Languages - C++ §1.3.13 unspecified behavior [defns.unspecified]
  5. ^ ISO/IEC (1999). ISO/IEC 9899:1999(E): Programming Languages - C §3.4.4 para 1
  6. ^ ISO/IEC (2003). ISO/IEC 14882:2003(E): Programming Languages - C++ §1.3.12 undefined behavior [defns.undefined]
  7. ^ James Gosling, Bill Joy, Guy Steele, and Gilad Bracha (2005). The Java Language Specification, Third Edition. Addison-Wesley. ISBN 0-321-24678-0