Computer programming is the process of designing, writing, testing, debugging, and maintaining the source code of computer programs. This source code is written in one or more programming languages. The purpose of programming is to create a program that performs specific operations or exhibits a...
term used to describe behavior that may vary on different implementations of a programming language
Programming language
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...
can be said to contain unspecified behavior when its source code
Source code
In computer science, source code is text written using the format and syntax of the programming language that it is being written in. Such a language is specially designed to facilitate the work of computer programmers, who specify the actions to be performed by a computer mostly by writing source...
In computing, an executable file causes a computer "to perform indicated tasks according to encoded instructions," as opposed to a data file that must be parsed by a program to be meaningful. These instructions are traditionally machine code instructions for a physical CPU...
that exhibits different behavior when compiled on a different compiler
Compiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...
, or on the same compiler with different settings. 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. Unspecified behavior will often not manifest itself in the resulting program's external behavior, but it may sometimes lead to differing outputs or results, causing portability
Software portability
Portability in high-level computer programming is the usability of the same software in different environments. The prerequirement for portability is the generalized abstraction between the application logic and system interfaces...
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. 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. In the C and C++
C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
Portability in high-level computer programming is the usability of the same software in different environments. The prerequirement for portability is the generalized abstraction between the application logic and system interfaces...
constructs are generally grouped into three categories: Implementation-defined, unspecified, and undefined behavior.
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." Unlike implementation-defined behavior, there is no requirement for the implementation to document its behavior. 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". The C++ Standard also notes that the range of possible behaviors is usually provided. 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.
Order of evaluation of subexpressions
Many programming languages do not specify the order of evaluation of the sub-expressions of a complete expression
Expression (programming)
An expression in a programming language is a combination of explicit values, constants, variables, operators, and functions that are interpreted according to the particular rules of precedence and of association for a particular programming language, which computes and then produces another value...
. If one or more of the sub-expressions has side effect
Side effect (computer science)
In computer science, a function or expression is said to have a side effect if, in addition to returning a value, it also modifies some state or has an observable interaction with calling functions or the outside world...
s, then the result of evaluating the full-expression may be different depending on the order of evaluation of the sub-expressions. 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. In the C and C++ languages, this also applies to function arguments. Example:
include
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. In other languages, such as Java
Java (programming language)
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
, the order of evaluation of operands and function arguments is explicitly defined.
Pointer comparisons
In C and C++, the comparison of pointers to objects is only strictly defined if the pointers point to members of the same object, or elements of the same array. Example:
int main(void)
{
int a = 0;
int b = 0;
return &a < &b; /* unspecified behavior in C++, undefined in C */
}
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