Assert.h

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<assert.h> is a header file in the C standard library. It defines the C preprocessor macro assert and implements runtime assertion in C.

Contents

<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. [1]

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. [2] Since C99, the name of the function the assert statement is included as (__FUNC__) and the expression itself. [3] 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. [4] 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. [5]

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>#include<stddef.h>intmain(){void*ptr=NULL;assert(ptr);return0;}

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

program: source.c:5: main: Assertion 'ptr' 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), [3] 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. [6]

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));. [7]

static_assert

The static_assert keyword, added in C++11, serves a similar purpose to the assert macro. Unlike the assert macro, static_assert runs at compile-time rather than at runtime. [8] The original implementation used template hacks.[ citation needed ] The static_assert keyword 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 assertion has no effect. [8] In C++17, this assertion failure message was made optional, and the subsequent message is omitted if not specified. [9]

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++. [10] In C23, _Static_assert was renamed to static_assert and the string literal argument was made optional. [11] [12] Gnulib defines static_assert for platforms that do not use C11 and does not require <assert.h> to be included. [13]

Other languages

In Java, assert is a keyword.

In C#, there is no assertion macro or keyword, but instead classes System.Diagnostics.Debug and System.Diagnostics.Trace which provide Assert() methods.

In Rust, there is an assert!() macro.

References

Citations

  1. ↑ Binder 2000, p. 860.
  2. ↑ Kernighan & Ritchie 1988, p. 253-254.
  3. 1 2 ISO/IEC JTC 1/SC 22/WG14 1999, p. 169.
  4. ↑ "Linux Programmer's Manual". August 25, 2002. Retrieved March 14, 2023.
  5. ↑ Reekie, John (December 7, 1995). "How to use assertions in C". University of California, Berkeley . Retrieved March 14, 2023.
  6. ↑ American National Standards Institute 1990, p. 76.
  7. ↑ Gregoire 2021, p. 1058.
  8. 1 2 ISO/IEC JTC 1/SC 22/WG21 2012, p. 134.
  9. ↑ Swaminathan 2017, p. 13.
  10. ↑ Prata 2013, p. 762-763.
  11. ↑ Gustedt 2022, p. 3.
  12. ↑ Ballman & Grammatech 2018, p. 1.
  13. ↑ "GNU Gnulib". Free Software Foundation. February 6, 2023. Retrieved March 14, 2023.

Bibliography

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