1.\" Copyright (c) 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 3. Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)assert.3 8.1 (Berkeley) 6/9/93 29.\" 30.Dd April 20, 2021 31.Dt ASSERT 3 32.Os 33.Sh NAME 34.Nm assert , 35.Nm static_assert 36.Nd expression verification macro 37.Sh SYNOPSIS 38.In assert.h 39.Fn assert expression 40.Fn static_assert expression 41.Fn static_assert expression message 42.Sh DESCRIPTION 43The 44.Fn assert 45macro tests the given 46.Ar expression 47and if it is false, 48the calling process is terminated. 49A diagnostic message is written to 50.Dv stderr 51and the function 52.Xr abort 3 53is called, effectively terminating the program. 54.Pp 55If 56.Ar expression 57is true, 58the 59.Fn assert 60macro does nothing. 61.Pp 62The 63.Fn assert 64macro 65may be removed at compile time by defining 66.Dv NDEBUG 67as a macro 68(e.g., by using the 69.Xr cc 1 70option 71.Fl D Ns Dv NDEBUG ) . 72Unlike most other include files, 73.In assert.h 74may be included multiple times. 75Each time whether or not 76.Dv NDEBUG 77is defined determines the behavior of assert from that point forward 78until the end of the unit or another include of 79.In assert.h . 80.Pp 81The 82.Fn assert 83macro should only be used for ensuring the developer's expectations 84hold true. 85It is not appropriate for regular run-time error detection. 86.Pp 87The 88.Fn static_assert 89macro expands to 90.Fn _Static_assert , 91and, contrarily to 92.Fn assert , 93makes assertions at compile-time. 94Once the constraint is violated, the compiler produces a diagnostic 95message including the string literal message, if provided. 96The initial form of the 97.Fn _Static_assert 98containing a string literal message was introduced in C11 standard, and 99the other form with no string literal is to be implemented by C2x and 100some compilers may lack its adoption at present. 101.Sh EXAMPLES 102The assertion: 103.Dl "assert(1 == 0);" 104generates a diagnostic message similar to the following: 105.Dl "Assertion failed: (1 == 0), function main, file main.c, line 100." 106.Pp 107The following assert tries to assert there was no partial read: 108.Dl "assert(read(fd, buf, nbytes) == nbytes);" 109However, there are two problems. 110First, it checks for normal conditions, rather than conditions that 111indicate a bug. 112Second, the code will disappear if 113.Dv NDEBUG 114is defined, changing the semantics of the program. 115.Pp 116The following asserts that the size of the S structure is 16. 117Otherwise, it produces a diagnostic message which points at the 118constraint and includes the provided string literal: 119.Dl "static_assert(sizeof(struct S) == 16, ""size mismatch"");" 120If none is provided, it only points at the constraint. 121.Sh SEE ALSO 122.Xr abort2 2 , 123.Xr abort 3 124.Sh STANDARDS 125.Rs 126The 127.Fn assert 128macro conforms to 129.St -isoC-99 . 130.Re 131.Pp 132.Rs 133The 134.Fn static_assert 135macro conforms to 136.St -isoC-2011 . 137.Re 138.Sh HISTORY 139An 140.Nm 141macro appeared in 142.At v7 . 143