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.Dd April 20, 2021 29.Dt ASSERT 3 30.Os 31.Sh NAME 32.Nm assert , 33.Nm static_assert 34.Nd expression verification macro 35.Sh SYNOPSIS 36.In assert.h 37.Fn assert expression 38.Fn static_assert expression 39.Fn static_assert expression message 40.Sh DESCRIPTION 41The 42.Fn assert 43macro tests the given 44.Ar expression 45and if it is false, 46the calling process is terminated. 47A diagnostic message is written to 48.Dv stderr 49and the function 50.Xr abort 3 51is called, effectively terminating the program. 52.Pp 53If 54.Ar expression 55is true, 56the 57.Fn assert 58macro does nothing. 59.Pp 60The 61.Fn assert 62macro 63may be removed at compile time by defining 64.Dv NDEBUG 65as a macro 66(e.g., by using the 67.Xr cc 1 68option 69.Fl D Ns Dv NDEBUG ) . 70Unlike most other include files, 71.In assert.h 72may be included multiple times. 73Each time whether or not 74.Dv NDEBUG 75is defined determines the behavior of assert from that point forward 76until the end of the unit or another include of 77.In assert.h . 78.Pp 79The 80.Fn assert 81macro should only be used for ensuring the developer's expectations 82hold true. 83It is not appropriate for regular run-time error detection. 84.Pp 85The 86.Fn static_assert 87macro expands to 88.Fn _Static_assert , 89and, contrarily to 90.Fn assert , 91makes assertions at compile-time. 92Once the constraint is violated, the compiler produces a diagnostic 93message including the string literal message, if provided. 94The initial form of the 95.Fn _Static_assert 96containing a string literal message was introduced in C11 standard, and 97the other form with no string literal is to be implemented by C2x and 98some compilers may lack its adoption at present. 99.Sh EXAMPLES 100The assertion: 101.Dl "assert(1 == 0);" 102generates a diagnostic message similar to the following: 103.Dl "Assertion failed: (1 == 0), function main, file main.c, line 100." 104.Pp 105The following assert tries to assert there was no partial read: 106.Dl "assert(read(fd, buf, nbytes) == nbytes);" 107However, there are two problems. 108First, it checks for normal conditions, rather than conditions that 109indicate a bug. 110Second, the code will disappear if 111.Dv NDEBUG 112is defined, changing the semantics of the program. 113.Pp 114The following asserts that the size of the S structure is 16. 115Otherwise, it produces a diagnostic message which points at the 116constraint and includes the provided string literal: 117.Dl "static_assert(sizeof(struct S) == 16, ""size mismatch"");" 118If none is provided, it only points at the constraint. 119.Sh SEE ALSO 120.Xr abort2 2 , 121.Xr abort 3 122.Sh STANDARDS 123The 124.Fn assert 125macro conforms to 126.St -isoC-99 . 127.Pp 128The 129.Fn static_assert 130macro conforms to 131.St -isoC-2011 . 132.Sh HISTORY 133An 134.Nm 135macro appeared in 136.At v7 . 137