Assertion (computing)
   HOME

TheInfoList



OR:

In
computer programming Computer programming is the process of performing a particular computation (or more generally, accomplishing a specific computing result), usually by designing and building an executable computer program. Programming involves tasks such as ana ...
, specifically when using the
imperative programming In computer science, imperative programming is a programming paradigm of software that uses statements that change a program's state. In much the same way that the imperative mood in natural languages expresses commands, an imperative program c ...
paradigm, an assertion is a
predicate Predicate or predication may refer to: * Predicate (grammar), in linguistics * Predication (philosophy) * several closely related uses in mathematics and formal logic: **Predicate (mathematical logic) **Propositional function **Finitary relation, o ...
(a
Boolean-valued function A Boolean-valued function (sometimes called a predicate or a proposition) is a function of the type f : X → B, where X is an arbitrary set and where B is a Boolean domain, i.e. a generic two-element set, (for example B = ), whose elements are i ...
over the
state space A state space is the set of all possible configurations of a system. It is a useful abstraction for reasoning about the behavior of a given system and is widely used in the fields of artificial intelligence and game theory. For instance, the toy ...
, usually expressed as a
logical proposition In logic and linguistics, a proposition is the meaning of a declarative sentence. In philosophy, " meaning" is understood to be a non-linguistic entity which is shared by all sentences with the same meaning. Equivalently, a proposition is the no ...
using the
variable Variable may refer to: * Variable (computer science), a symbolic name associated with a value and whose associated value may be changed * Variable (mathematics), a symbol that represents a quantity in a mathematical expression, as used in many ...
s of a program) connected to a point in the program, that always should evaluate to true at that point in code execution. Assertions can help a programmer read the code, help a
compiler In computing, a compiler is a computer program that translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs that ...
compile it, or help the program detect its own defects. For the latter, some programs check assertions by actually evaluating the predicate as they run. Then, if it is not in fact true – an assertion failure – the program considers itself to be broken and typically deliberately crashes or throws an assertion failure exception.


Details

The following code contains two assertions, x > 0 and x > 1, and they are indeed true at the indicated points during execution: x = 1; assert x > 0; x++; assert x > 1; Programmers can use assertions to help specify programs and to reason about program correctness. For example, a
precondition In computer programming, a precondition is a condition or predicate that must always be true just prior to the execution of some section of code or before an operation in a formal specification. If a precondition is violated, the effect of the s ...
—an assertion placed at the beginning of a section of code—determines the set of states under which the programmer expects the code to execute. A
postcondition In computer programming, a postcondition is a condition or predicate that must always be true just after the execution of some section of code or after an operation in a formal specification. Postconditions are sometimes tested using assertions wit ...
—placed at the end—describes the expected state at the end of execution. For example: x > 0 x > 1. The example above uses the notation for including assertions used by C. A. R. Hoare in his 1969 article. That notation cannot be used in existing mainstream programming languages. However, programmers can include unchecked assertions using the comment feature of their programming language. For example, in C: x = 5; x = x + 1; // The braces included in the comment help distinguish this use of a comment from other uses.
Libraries A library is a collection of Document, materials, books or media that are accessible for use and not just for display purposes. A library provides physical (hard copies) or electronic media, digital access (soft copies) materials, and may be a ...
may provide assertion features as well. For example, in C using
glibc The GNU C Library, commonly known as glibc, is the GNU Project's implementation of the C standard library. Despite its name, it now also directly supports C++ (and, indirectly, other programming languages). It was started in the 1980s by ...
with C99 support: #include int f(void) Several modern programming languages include checked assertions –
statements Statement or statements may refer to: Common uses *Statement (computer science), the smallest standalone element of an imperative programming language *Statement (logic), declarative sentence that is either true or false *Statement, a declarative ...
that are checked at runtime or sometimes statically. If an assertion evaluates to false at runtime, an assertion failure results, which typically causes execution to abort. This draws attention to the location at which the logical inconsistency is detected and can be preferable to the behaviour that would otherwise result. The use of assertions helps the programmer design, develop, and reason about a program.


Usage

In languages such as
Eiffel Eiffel may refer to: Places * Eiffel Peak, a summit in Alberta, Canada * Champ de Mars – Tour Eiffel station, Paris, France; a transit station Structures * Eiffel Tower, in Paris, France, designed by Gustave Eiffel * Eiffel Bridge, Ungheni, M ...
, assertions form part of the design process; other languages, such as C and
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...
, use them only to check assumptions at runtime. In both cases, they can be checked for validity at runtime but can usually also be suppressed.


Assertions in design by contract

Assertions can function as a form of documentation: they can describe the state the code expects to find before it runs (its
precondition In computer programming, a precondition is a condition or predicate that must always be true just prior to the execution of some section of code or before an operation in a formal specification. If a precondition is violated, the effect of the s ...
s), and the state the code expects to result in when it is finished running (
postcondition In computer programming, a postcondition is a condition or predicate that must always be true just after the execution of some section of code or after an operation in a formal specification. Postconditions are sometimes tested using assertions wit ...
s); they can also specify invariants of a
class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differentl ...
.
Eiffel Eiffel may refer to: Places * Eiffel Peak, a summit in Alberta, Canada * Champ de Mars – Tour Eiffel station, Paris, France; a transit station Structures * Eiffel Tower, in Paris, France, designed by Gustave Eiffel * Eiffel Bridge, Ungheni, M ...
integrates such assertions into the language and automatically extracts them to document the class. This forms an important part of the method of design by contract. This approach is also useful in languages that do not explicitly support it: the advantage of using assertion statements rather than assertions in comments is that the program can check the assertions every time it runs; if the assertion no longer holds, an error can be reported. This prevents the code from getting out of sync with the assertions.


Assertions for run-time checking

An assertion may be used to verify that an assumption made by the programmer during the implementation of the program remains valid when the program is executed. For example, consider the following
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...
code: int total = countNumberOfUsers(); if (total % 2

0) else
In
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...
, % is the ''
remainder In mathematics, the remainder is the amount "left over" after performing some computation. In arithmetic, the remainder is the integer "left over" after dividing one integer by another to produce an integer quotient ( integer division). In algeb ...
'' operator ('' modulo''), and in Java, if its first operand is negative, the result can also be negative (unlike the modulo used in mathematics). Here, the programmer has assumed that total is non-negative, so that the remainder of a division with 2 will always be 0 or 1. The assertion makes this assumption explicit: if countNumberOfUsers does return a negative value, the program may have a bug. A major advantage of this technique is that when an error does occur it is detected immediately and directly, rather than later through often obscure effects. Since an assertion failure usually reports the code location, one can often pin-point the error without further debugging. Assertions are also sometimes placed at points the execution is not supposed to reach. For example, assertions could be placed at the default clause of the switch statement in languages such as C,
C++ C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...
, and
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...
. Any case which the programmer does not handle intentionally will raise an error and the program will abort rather than silently continuing in an erroneous state. In D such an assertion is added automatically when a switch statement doesn't contain a default clause. In
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...
, assertions have been a part of the language since version 1.4. Assertion failures result in raising an AssertionError when the program is run with the appropriate flags, without which the assert statements are ignored. In C, they are added on by the standard header
assert.h assert.h is a header file in the standard library of the C programming language that defines the C preprocessor macro assert().International Standard for Programming Language C (C99), ISO/IEC 9899:1999, p. 169 In C++ it is also available thro ...
defining assert (''assertion'') as a macro that signals an error in the case of failure, usually terminating the program. In
C++ C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...
, both assert.h and cassert headers provide the assert macro. The danger of assertions is that they may cause side effects either by changing memory data or by changing thread timing. Assertions should be implemented carefully so they cause no side effects on program code. Assertion constructs in a language allow for easy
test-driven development Test-driven development (TDD) is a software development process relying on software requirements being converted to test cases before software is fully developed, and tracking all software development by repeatedly testing the software against al ...
(TDD) without the use of a third-party library.


Assertions during the development cycle

During the
development cycle In software engineering, a software development process is a process of dividing software development work into smaller, parallel, or sequential steps or sub-processes to improve design, product management. It is also known as a software devel ...
, the programmer will typically run the program with assertions enabled. When an assertion failure occurs, the programmer is immediately notified of the problem. Many assertion implementations will also halt the program's execution: this is useful, since if the program continued to run after an assertion violation occurred, it might corrupt its state and make the cause of the problem more difficult to locate. Using the information provided by the assertion failure (such as the location of the failure and perhaps a stack trace, or even the full program state if the environment supports
core dump In computing, a core dump, memory dump, crash dump, storage dump, system dump, or ABEND dump consists of the recorded state of the working memory of a computer program at a specific time, generally when the program has crashed or otherwise terminat ...
s or if the program is running in a
debugger A debugger or debugging tool is a computer program used to test and debug other programs (the "target" program). The main use of a debugger is to run the target program under controlled conditions that permit the programmer to track its executi ...
), the programmer can usually fix the problem. Thus assertions provide a very powerful tool in debugging.


Assertions in production environment

When a program is deployed to
production Production may refer to: Economics and business * Production (economics) * Production, the act of manufacturing goods * Production, in the outline of industrial organization, the act of making products (goods and services) * Production as a stati ...
, assertions are typically turned off, to avoid any overhead or side effects they may have. In some cases assertions are completely absent from deployed code, such as in C/C++ assertions via macros. In other cases, such as Java, assertions are present in the deployed code, and can be turned on in the field for debugging. Assertions may also be used to promise the compiler that a given edge condition is not actually reachable, thereby permitting certain optimizations that would not otherwise be possible. In this case, disabling the assertions could actually reduce performance.


Static assertions

Assertions that are checked at compile time are called static assertions. Static assertions are particularly useful in compile time
template metaprogramming Template metaprogramming (TMP) is a metaprogramming technique in which templates are used by a compiler to generate temporary source code, which is merged by the compiler with the rest of the source code and then compiled. The output of these t ...
, but can also be used in low-level languages like C by introducing illegal code if (and only if) the assertion fails.
C11 C11, C.XI, C-11 or C.11 may refer to: Transport * C-11 Fleetster, a 1920s American light transport aircraft for use of the United States Assistant Secretary of War * Fokker C.XI, a 1935 Dutch reconnaissance seaplane * LET C-11, a license-build var ...
and
C++11 C++11 is a version of the ISO/ IEC 14882 standard for the C++ programming language. C++11 replaced the prior version of the C++ standard, called C++03, and was later replaced by C++14. The name follows the tradition of naming language versions b ...
support static assertions directly through static_assert. In earlier C versions, a static assertion can be implemented, for example, like this: #define SASSERT(pred) switch(0) SASSERT( BOOLEAN CONDITION ); If the (BOOLEAN CONDITION) part evaluates to false then the above code will not compile because the compiler will not allow two case labels with the same constant. The boolean expression must be a compile-time constant value, for example (
sizeof sizeof is a unary operator in the programming languages C and C++. It generates the storage size of an expression or a data type, measured in the number of ''char''-sized units. Consequently, the construct ''sizeof (char)'' is guaranteed to be ' ...
(int)

4)
would be a valid expression in that context. This construct does not work at file scope (i.e. not inside a function), and so it must be wrapped inside a function. Another popular way of implementing assertions in C is: static char const static_assertion (BOOLEAN CONDITION) ? 1 : -1 = ; If the (BOOLEAN CONDITION) part evaluates to false then the above code will not compile because arrays may not have a negative length. If in fact the compiler allows a negative length then the initialization byte (the '!' part) should cause even such over-lenient compilers to complain. The boolean expression must be a compile-time constant value, for example (sizeof(int)

4)
would be a valid expression in that context. Both of these methods require a method of constructing unique names. Modern compilers support a __COUNTER__ preprocessor define that facilitates the construction of unique names, by returning monotonically increasing numbers for each compilation unit. D provides static assertions through the use of static assert.


Disabling assertions

Most languages allow assertions to be enabled or disabled globally, and sometimes independently. Assertions are often enabled during development and disabled during final testing and on release to the customer. Not checking assertions avoids the cost of evaluating the assertions while (assuming the assertions are free of
side effects In medicine, a side effect is an effect, whether therapeutic or adverse, that is secondary to the one intended; although the term is predominantly employed to describe adverse effects, it can also apply to beneficial, but unintended, consequence ...
) still producing the same result under normal conditions. Under abnormal conditions, disabling assertion checking can mean that a program that would have aborted will continue to run. This is sometimes preferable. Some languages, including C and
C++ C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...
, can completely remove assertions at compile time using the
preprocessor In computer science, a preprocessor (or precompiler) is a program that processes its input data to produce output that is used as input in another program. The output is said to be a preprocessed form of the input data, which is often used by so ...
. Similarly, launching the
Python Python may refer to: Snakes * Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia ** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia * Python (mythology), a mythical serpent Computing * Python (pro ...
interpreter with "-O" (for "optimize") as an argument will cause the Python code generator to not emit any bytecode for asserts. Java requires an option to be passed to the run-time engine in order to ''enable'' assertions. Absent the option, assertions are bypassed, but they always remain in the code unless optimised away by a JIT compiler at run-time or excluded at compile time via the programmer manually placing each assertion behind an if (false) clause. Programmers can build checks into their code that are always active by bypassing or manipulating the language's normal assertion-checking mechanisms.


Comparison with error handling

Assertions are distinct from routine error-handling. Assertions document logically impossible situations and discover programming errors: if the impossible occurs, then something fundamental is clearly wrong with the program. This is distinct from error handling: most error conditions are possible, although some may be extremely unlikely to occur in practice. Using assertions as a general-purpose error handling mechanism is unwise: assertions do not allow for recovery from errors; an assertion failure will normally halt the program's execution abruptly; and assertions are often disabled in production code. Assertions also do not display a user-friendly error message. Consider the following example of using an assertion to handle an error: int *ptr = malloc(sizeof(int) * 10); assert(ptr); // use ptr ... Here, the programmer is aware that
malloc C dynamic memory allocation refers to performing manual memory management for dynamic memory allocation in the C programming language via a group of functions in the C standard library, namely , , , and . The C++ programming language includes t ...
will return a NULL pointer if memory is not allocated. This is possible: the operating system does not guarantee that every call to malloc will succeed. If an out of memory error occurs the program will immediately abort. Without the assertion, the program would continue running until ptr was dereferenced, and possibly longer, depending on the specific hardware being used. So long as assertions are not disabled, an immediate exit is assured. But if a graceful failure is desired, the program has to handle the failure. For example, a server may have multiple clients, or may hold resources that will not be released cleanly, or it may have uncommitted changes to write to a datastore. In such cases it is better to fail a single transaction than to abort abruptly. Another error is to rely on side effects of expressions used as arguments of an assertion. One should always keep in mind that assertions might not be executed at all, since their sole purpose is to verify that a condition which should always be true does in fact hold true. Consequently, if the program is considered to be error-free and released, assertions may be disabled and will no longer be evaluated. Consider another version of the previous example: int *ptr; // Statement below fails if malloc() returns NULL, // but is not executed at all when compiling with -NDEBUG! assert(ptr = malloc(sizeof(int) * 10)); // use ptr: ptr isn't initialised when compiling with -NDEBUG! ... This might look like a smart way to assign the return value of malloc to ptr and check if it is NULL in one step, but the malloc call and the assignment to ptr is a side effect of evaluating the expression that forms the assert condition. When the NDEBUG parameter is passed to the compiler, as when the program is considered to be error-free and released, the assert() statement is removed, so malloc() isn't called, rendering ptr uninitialised. This could potentially result in a
segmentation fault In computing, a segmentation fault (often shortened to segfault) or access violation is a fault, or failure condition, raised by hardware with memory protection, notifying an operating system (OS) the software has attempted to access a restrict ...
or similar null pointer error much further down the line in program execution, causing bugs that may be sporadic and/or difficult to track down. Programmers sometimes use a similar VERIFY(X) define to alleviate this problem. Modern compilers may issue a warning when encountering the above code.


History

In 1947 reports by
von Neumann Von Neumann may refer to: * John von Neumann (1903–1957), a Hungarian American mathematician * Von Neumann family * Von Neumann (surname), a German surname * Von Neumann (crater), a lunar impact crater See also * Von Neumann algebra * Von Ne ...
and Goldstine on their design for the
IAS machine The IAS machine was the first electronic computer built at the Institute for Advanced Study (IAS) in Princeton, New Jersey. It is sometimes called the von Neumann machine, since the paper describing its design was edited by John von Neumann, a ...
, they described algorithms using an early version of
flow charts A flowchart is a type of diagram that represents a workflow or process. A flowchart can also be defined as a diagrammatic representation of an algorithm, a step-by-step approach to solving a task. The flowchart shows the steps as boxes of v ...
, in which they included assertions: "It may be true, that whenever C actually reaches a certain point in the flow diagram, one or more bound variables will necessarily possess certain specified values, or possess certain properties, or satisfy certain properties with each other. Furthermore, we may, at such a point, indicate the validity of these limitations. For this reason we will denote each area in which the validity of such limitations is being asserted, by a special box, which we call an assertion box." The assertional method for proving correctness of programs was advocated by
Alan Turing Alan Mathison Turing (; 23 June 1912 – 7 June 1954) was an English mathematician, computer scientist, logician, cryptanalyst, philosopher, and theoretical biologist. Turing was highly influential in the development of theoretical co ...
. In a talk "Checking a Large Routine" at Cambridge, June 24, 1949 Turing suggested: "How can one check a large routine in the sense of making sure that it's right? In order that the man who checks may not have too difficult a task, the programmer should make a number of definite ''assertions'' which can be checked individually, and from which the correctness of the whole program easily follows".Alan Turing
Checking a Large Routine
1949; quoted in C. A. R. Hoare, "The Emperor's Old Clothes", 1980 Turing Award lecture.


See also

* Assertion definition language * Design by contract *
Exception handling In computing and computer programming, exception handling is the process of responding to the occurrence of ''exceptions'' – anomalous or exceptional conditions requiring special processing – during the execution of a program. In general, an ...
*
Hoare logic Hoare logic (also known as Floyd–Hoare logic or Hoare rules) is a formal system with a set of logical rules for reasoning rigorously about the correctness of computer programs. It was proposed in 1969 by the British computer scientist and log ...
*
Static code analysis In computer science, static program analysis (or static analysis) is the analysis of computer programs performed without executing them, in contrast with dynamic program analysis, which is performed on programs during their execution. The term ...
*
Java Modeling Language The Java Modeling Language (JML) is a specification language for Java programs, using Hoare style pre- and postconditions and invariants, that follows the design by contract paradigm. Specifications are written as Java annotation comments to th ...
*
Invariant (computer science) In mathematics, an invariant is a property of a mathematical object (or a class of mathematical objects) which remains unchanged after operations or transformations of a certain type are applied to the objects. The particular class of objects ...


References

{{reflist


External links

*
A historical perspective on runtime assertion checking in software development
' by Lori A. Clarke, David S. Rosenblum in: ACM SIGSOFT Software Engineering Notes 31(3):25-37, 2006 *
Assertions: a personal perspective
' by C.A.R. Hoare in: IEEE Annals of the History of Computing, Volume: 25, Issue: 2 (2003), Page(s): 14 - 25 *
My Compiler Does Not Understand Me
' by Poul-Henning Kamp in: ACM Queue 10(5), May 2012 *
Use of Assertions
' by John Regehr Formal methods Logic in computer science Conditional constructs Debugging