assert.h

<h2 id="use">Use</h2>
The assert macro implements runtime assertion. If the expression within it is false, the macro will print a message to <a href="/facts/Stderr/r92M7Wd4">stderr</a> and call <a href="/facts/C_process_control/jyvQdka8">abort()</a>, defined in <a href="/facts/Stdlib.h/qdG1nW2c">stdlib.h</a>. The message includes the source filename and the source line number from the macros __FILE__ and __LINE__, respectively.<a class="footnote-ref" id="fnref:2" href="#fn:2">2</a> Since <a href="/facts/C99/o5uMWjTQ">C99</a>, the name of the function the assert statement is included as (__FUNC__) and the expression itself.<a class="footnote-ref" id="fnref:3" href="#fn:3">3</a> In ANSI C, the expression in the assert macro is defined as <a href="/facts/Signed_integer/zWabo5M9">signed integer</a>, although any expression that can be implicitly cast to a signed integer may be used. In C99, the assert macro explicitly allows any <a href="/facts/Scalar_(computing)/VoCKtwml">scalar</a> type.<a class="footnote-ref" id="fnref:4" href="#fn:4">4</a> Two common uses of the assert macro are to assert that a <a href="/facts/Pointer_(computer_programming)/fPoNzy9b">pointer</a> is not null and to ensure that an array index is in-bounds.<a class="footnote-ref" id="fnref:5" href="#fn:5">5</a>
Below is a program using the assert macro. This program will always evaluate pointer as false, as pointer is a <a href="/facts/Null_pointer/66zwVEfL">null pointer</a> and does not point to a valid <a href="/facts/Memory_location/7EsUmbea">memory location</a>:

#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),<a class="footnote-ref" id="fnref:6" href="#fn:6">6</a> 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 <a href="/facts/Side_effect_(computer_science)/AU3GlLzG">side effects</a>, as these statements are not evaluated.<a class="footnote-ref" id="fnref:7" href="#fn:7">7</a>
The assert macro does not include an <a href="/facts/Error_message/ko8elN1w">error message</a>. However the <a href="/facts/Comma_operator/RaKkDWwS">comma operator</a> can be used to add it to the printed expression, as in assert(("Not Orwellian", 2 + 2 == 5));.<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a>

<h3>static_assert</h3>
The static_assert macro, added in <a href="/facts/C%252B%252B11/fI3YCoKQ">C++11</a>, serves a similar purpose to the assert macro. Unlike the assert macro, static_assert runs at <a href="/facts/Compile-time/yCdI3oqn">compile-time</a> rather than at runtime.<a class="footnote-ref" id="fnref:9" href="#fn:9">9</a> The original implementation used template hacks. The static_assert macro takes in a constant expression that can be converted into a <a href="/facts/Boolean_data_type/0GWYLDoC">Boolean</a> and a string literal; if the expression fails, the string literal is returned, otherwise, the macro has no effect.<a class="footnote-ref" id="fnref:10" href="#fn:10">10</a> In <a href="/facts/C%252B%252B17/B3gKNeER">C++17</a>, this assertion failure message was made optional, and the subsequent message is omitted if not specified.<a class="footnote-ref" id="fnref:11" href="#fn:11">11</a>
In <a href="/facts/C11_(C_standard_revision)/SNelulWN">C11</a>, the functionally equivalent declaration _Static_assert was added. assert.h defines static_assert as an alias for _Static_assert to ensure parity with <a href="/facts/C%252B%252B/Et0F2qn9">C++</a>.<a class="footnote-ref" id="fnref:12" href="#fn:12">12</a> In <a href="/facts/C23_(C_standard_revision)/1VvQNwSb">C23</a>, _Static_assert was renamed to static_assert and the string literal argument was made optional.<a class="footnote-ref" id="fnref:13" href="#fn:13">13</a><a class="footnote-ref" id="fnref:14" href="#fn:14">14</a> <a href="/facts/Gnulib/TjwWUNIK">Gnulib</a> defines static_assert for platforms that do not use C11 and does not require assert.h to be included.<a class="footnote-ref" id="fnref:15" href="#fn:15">15</a>

<h3>Citations</h3>

<h3>Bibliography</h3>
<ul><li><a href="/facts/American_National_Standards_Institute/p3d1TQCb">American National Standards Institute</a> (1990). Rationale for the ANSI C Programming Language. Summit: Silicon Press. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 9780929306070.</li>
<li>Ballman, Aaron; Grammatech (July 6, 2018). Harmonizing static_assert with C++ (Report).</li>
<li>Binder, Robert (2000). Testing Object-oriented Systems: Models, Patterns, and Tools (2nd ed.). Boston: <a href="/facts/Addison-Wesley/A7UFATEG">Addison-Wesley</a>. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 9780201809381.</li>
<li>Gregoire, Marc (2021). Professional C++ (5th ed.). Hoboken: <a href="/facts/Wiley_(publisher)/53g3LqJc">Wiley</a>. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 9781119695455.</li>
<li>Gustedt, Jens (February 15, 2022). Revise spelling of keywords (Report).</li>
<li><a href="/facts/Brian_Kernighan/oW3I7FWJ">Kernighan, Brian</a>; <a href="/facts/Dennis_Ritchie/I1yu1UWU">Ritchie, Dennis</a> (1988). The C Programming Language (2nd ed.). Hoboken: Prentice Hall. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 9780131103627.</li>
<li>Lischner, Ray (2009). C++ In a Nutshell: A Desktop Quick Reference (2nd ed.). Sebastopol: <a href="/facts/O%2527Reilly_Media/Fr3Kv7sm">O'Reilly Media</a>. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 9781449378837.</li>
<li><a href="/facts/ISO%2fIEC_JTC_1%2fSC_22/lJPn05nl">ISO/IEC JTC 1/SC 22/WG14</a> (December 1999). ISO/IEC 9899:1999 (Report).{{cite report}}: CS1 maint: numeric names: authors list (link)</li>
<li><a href="/facts/ISO%2fIEC_JTC_1%2fSC_22/lJPn05nl">ISO/IEC JTC 1/SC 22/WG21</a> (January 2012). ISO/IEC 14882:2011 (Report).{{cite report}}: CS1 maint: numeric names: authors list (link)</li>
<li>Prata, Stephen (2013). C Primer Plus (6th ed.). London: <a href="/facts/Pearson_Education/UNq9rds8">Pearson Education</a>. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 9780133432381.</li>
<li>Swaminathan, Jeganathan (2017). Mastering C++ Programming. Birmingham: <a href="/facts/Packt/eT0RTSSK">Packt</a>. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 9781786461629.</li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">Binder 2000, p. 860. - Binder, Robert (2000). Testing Object-oriented Systems: Models, Patterns, and Tools (2nd ed.). Boston: Addison-Wesley. ISBN 9780201809381. <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">Kernighan & Ritchie 1988, p. 253-254. - Kernighan, Brian; Ritchie, Dennis (1988). The C Programming Language (2nd ed.). Hoboken: Prentice Hall. ISBN 9780131103627. <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">ISO/IEC JTC 1/SC 22/WG14 1999, p. 169. - ISO/IEC JTC 1/SC 22/WG14 (December 1999). ISO/IEC 9899:1999 (Report). <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">"Linux Programmer's Manual". August 25, 2002. Retrieved March 14, 2023. <a href="https://fog.misty.com/perry/osp/C99/man/assert.html" target="_blank">https://fog.misty.com/perry/osp/C99/man/assert.html</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">Reekie, John (December 7, 1995). "How to use assertions in C". University of California, Berkeley. Retrieved March 14, 2023. <a href="https://ptolemy.berkeley.edu/~johnr/tutorials/assertions.html" target="_blank">https://ptolemy.berkeley.edu/~johnr/tutorials/assertions.html</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">ISO/IEC JTC 1/SC 22/WG14 1999, p. 169. - ISO/IEC JTC 1/SC 22/WG14 (December 1999). ISO/IEC 9899:1999 (Report). <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">American National Standards Institute 1990, p. 76. - American National Standards Institute (1990). Rationale for the ANSI C Programming Language. Summit: Silicon Press. ISBN 9780929306070. <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">Gregoire 2021, p. 1058. - Gregoire, Marc (2021). Professional C++ (5th ed.). Hoboken: Wiley. ISBN 9781119695455. <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
<li id="fn:9">ISO/IEC JTC 1/SC 22/WG21 2012, p. 134. - ISO/IEC JTC 1/SC 22/WG21 (January 2012). ISO/IEC 14882:2011 (Report). <a href="#fnref:9" class="footnote-back-ref">↩</a></li>
<li id="fn:10">ISO/IEC JTC 1/SC 22/WG21 2012, p. 134. - ISO/IEC JTC 1/SC 22/WG21 (January 2012). ISO/IEC 14882:2011 (Report). <a href="#fnref:10" class="footnote-back-ref">↩</a></li>
<li id="fn:11">Swaminathan 2017, p. 13. - Swaminathan, Jeganathan (2017). Mastering C++ Programming. Birmingham: Packt. ISBN 9781786461629. <a href="#fnref:11" class="footnote-back-ref">↩</a></li>
<li id="fn:12">Prata 2013, p. 762-763. - Prata, Stephen (2013). C Primer Plus (6th ed.). London: Pearson Education. ISBN 9780133432381. <a href="#fnref:12" class="footnote-back-ref">↩</a></li>
<li id="fn:13">Gustedt 2022, p. 3. - Gustedt, Jens (February 15, 2022). Revise spelling of keywords (Report). <a href="#fnref:13" class="footnote-back-ref">↩</a></li>
<li id="fn:14">Ballman & Grammatech 2018, p. 1. - Ballman, Aaron; Grammatech (July 6, 2018). Harmonizing static_assert with C++ (Report). <a href="#fnref:14" class="footnote-back-ref">↩</a></li>
<li id="fn:15">"GNU Gnulib". Free Software Foundation. February 6, 2023. Retrieved March 14, 2023. <a href="https://www.gnu.org/software//gnulib/manual/html_node/static_005fassert.html" target="_blank">https://www.gnu.org/software//gnulib/manual/html_node/static_005fassert.html</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></li>
</ol>

assert.h open-in-new

assert.h