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assert.h

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C standard library (libc)
General topics
Miscellaneous headers

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. Additionally, assert.h is available in C++.

Use

The assert macro implements runtime assertion. If the expression within fails, the assert macro will print a message to stderr and call the abort function.[note 1] The message printed includes the source filename and the source line number, defined as the string literal __FILE__ and decimal constant __LINE__, respectively.[1] Since C99, the message additionally includes the source function, of the identifier __func__, and the expression itself.[2] In ANSI C, the expression in the assert macro must be a signed integer; in C99, the expression may be of any scalar type.[3] Two common uses of the assert macro are to assert that a pointer is not null, otherwise known as a null pointer, and to ensure that indexes and size values are not negative and below a specified limit.[4]

The value 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:[2] 

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

The assert macro does not include an error message. However, a custom message can be provided using the comma operator, as seen below.[5] 

assert(("Orwellian statement", 2 + 2 == 5));

static_assert

The static_assert macro, added in C++11, serves a similar purpose to the assert macro. The static_assert macro takes in a constant expression that can be converted into a Boolean and a string literal; if the expression fails, the string literal is returned, otherwise, the macro has no effect.[6] In C++17, this assertion failure message was made optional, and the subsequent message is omitted if not specified.[7] Alternatively, the <cassert> header from the assert.h header may also be used to declare the assert macro.[8]

In C11, the _Static_assert declaration was added. Unlike the assert macro, _Static_assert is compile-time. As a result, _Static_assert results in a compilation error if its first argument evaluates to zero, or is false. assert.h defines static_assert as an alias for _Static_assert to ensure parity with C++.[9] In C23, _Static_assert was renamed to static_assert and the string literal argument was made optional.[10][11] Gnulib defines static_assert for platforms that do not use C11 and does not require assert.h to be included.[12]

Notes

  1. ^ As defined in stdlib.h

References

Citations

  1. ^ Kernighan & Ritchie 1988, p. 253-254.
  2. ^ a b ISO/IEC JTC 1/SC 22/WG14 1999, p. 169.
  3. ^ "Linux Programmer's Manual". August 25, 2002. Retrieved March 14, 2023.
  4. ^ Reekie, John (December 7, 1995). "How to use assertions in C". University of California, Berkeley. Retrieved March 14, 2023.
  5. ^ Gregoire 2021, p. 1058.
  6. ^ ISO/IEC JTC 1/SC 22/WG21 2011, p. 134.
  7. ^ Swaminathan 2017, p. 13.
  8. ^ Lischner 2009, p. 375.
  9. ^ Prata 2013, p. 762-763.
  10. ^ Gustedt 2022, p. 3.
  11. ^ Ballman & Grammatech 2018, p. 1.
  12. ^ "GNU Gnulib". Free Software Foundation. February 6, 2023. Retrieved March 14, 2023.

Bibliography

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