assert.h

Short description: Header file for C programs

assert.h is a header file in the C standard library. It defines the C preprocessor macro

assert

and implements runtime assertion in C. assert.h is defined in ANSI C as part of the C standard library. In the C++ programming language, assert.h and

<cassert>

are available; both are functionally equivalent.

Use

The

assert

macro implements runtime assertion. If the expression within it is false, the macro will print a message to stderr and call abort(), defined in stdlib.h. The message includes the source filename and the source line number from the macros __FILE__ and __LINE__, respectively. Since C99, the name of the function the assert statement is included as (__FUNC__) and the expression itself. In ANSI C, the expression in the

assert

macro is defined as signed integer, although any expression that can be implicitly cast to a signed integer may be used. In C99, the

assert

macro explicitly allows any scalar type. Two common uses of the

assert

macro are to assert that a pointer is not null and to ensure that an array index is in-bounds. Below is a program using the

assert

macro. This program will always evaluate

pointer

as false, as

pointer

is a null pointer and does not point to a valid memory location:

#include <assert.h>
int main()
{
    void* pointer = 0;
    assert(pointer);
    return 0;
}

Upon compiling the program and running it, a message similar to the following will be output:

program: source.c:5: main: Assertion 'pointer' failed.
Aborted (core dumped)

The definition of the

assert

macro changes depending on the definition of another macro,

NDEBUG

. If

NDEBUG

is defined as a macro name, the

assert

macro is defined as

#define assert(ignore) ((void)0)

, thus resulting in the macro not evaluating the expression. The use of

NDEBUG

may affect the overall behavior of a program if one or more

assert

statements contain side effects, as these statements are not evaluated. The

assert

macro does not include an error message. However the comma operator can be used to add it to the printed expression, as in

assert(("Not Orwellian", 2 + 2 == 5));

.

static_assert

In C11, the functionally equivalent declaration _Static_assert was added. assert.h defines static_assert as an alias for _Static_assert to ensure parity with C++. In C23, _Static_assert was renamed to static_assert and the string literal argument was made optional. Gnulib defines static_assert for platforms that do not use C11 and does not require assert.h to be included.

References

Bibliography

• American National Standards Institute (1990). Rationale for the ANSI C Programming Language. Summit: Silicon Press. ISBN 9780929306070. 
• Ballman, Aaron; Grammatech (July 6, 2018). Harmonizing static_assert with C++ (Report). 
• Binder, Robert (2000). Testing Object-oriented Systems: Models, Patterns, and Tools (2nd ed.). Boston: Addison-Wesley. ISBN 9780201809381. 
• Gregoire, Marc (2021). Professional C++ (5th ed.). Hoboken: Wiley. ISBN 9781119695455. 
• Gustedt, Jens (February 15, 2022). Revise spelling of keywords (Report). 
• Kernighan, Brian; Ritchie, Dennis (1988). The C Programming Language (2nd ed.). Hoboken: Prentice Hall. ISBN 9780131103627. 
• Lischner, Ray (2009). C++ In a Nutshell: A Desktop Quick Reference (2nd ed.). Sebastopol: O'Reilly Media. ISBN 9781449378837. 
• ISO/IEC JTC 1/SC 22/WG14 (December 1999). ISO/IEC 9899:1999 (Report). 
• ISO/IEC JTC 1/SC 22/WG21 (January 2012). ISO/IEC 14882:2011 (Report). 
• Prata, Stephen (2013). C Primer Plus (6th ed.). London: Pearson Education. ISBN 9780133432381. 
• Swaminathan, Jeganathan (2017). Mastering C++ Programming. Birmingham: Packt. ISBN 9781786461629.