.\" Copyright (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
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.\" Modified Sat Jul 24 21:42:42 1993 by Rik Faith <faith@cs.unc.edu>
.\" Modified Tue Oct 22 23:44:11 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified Thu Jun  2 23:44:11 2016 by Nikos Mavrogiannopoulos <nmav@redhat.com>
.TH ASSERT 3  2016-07-17 "GNU" "Linux Programmer's Manual"
.SH NAME
assert \- abort the program if assertion is false
.SH SYNOPSIS
.nf
.B #include <assert.h>
.sp
.BI "void assert(scalar " expression );
.fi
.SH DESCRIPTION
This macro can help programmers find bugs in their programs,
or handle exceptional cases
via a crash that will produce limited debugging output.

If
.I expression
is false (i.e., compares equal to zero),
.BR assert ()
prints an error message to standard error
and terminates the program by calling
.BR abort (3).
The error message includes the name of the file and function containing the
.BR assert ()
call, the source code line number of the call, and the text of the argument;
something like:

    prog: some_file.c:16: some_func: Assertion `val == 0' failed.

If the macro
.B NDEBUG
is defined at the moment
.I <assert.h>
was last included, the macro
.BR assert ()
generates no code, and hence does nothing at all.
It is not recommended to define
.B NDEBUG
if using
.BR assert ()
to detect error conditions since the software
may behave non-deterministically.
.SH RETURN VALUE
No value is returned.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lb lb lb
l l l.
Interface	Attribute	Value
T{
.BR assert ()
T}	Thread safety	MT-Safe
.TE

.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008, C89, C99.
In C89,
.I expression
is required to be of type
.I int
and undefined behavior results if it is not, but in C99
it may have any scalar type.
.\" See Defect Report 107 for more details.
.SH BUGS
.BR assert ()
is implemented as a macro; if the expression tested has side-effects,
program behavior will be different depending on whether
.B NDEBUG
is defined.
This may create Heisenbugs which go away when debugging
is turned on.
.SH SEE ALSO
.BR abort (3),
.BR assert_perror (3),
.BR exit (3)