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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | HISTORY | BUGS | SEE ALSO | COLOPHON |
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assert(3) Library Functions Manual assert(3)
assert - abort the program if assertion is false
Standard C library (libc, -lc)
#include <assert.h>
void assert(bool expression);
This macro can help programmers find bugs in their programs, or
handle exceptional cases via a crash that will produce limited
debugging output.
If expression is false (i.e., compares equal to zero), assert()
prints an error message to standard error and terminates the
program by calling abort(3). The error message includes the name
of the file and function containing the 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 NDEBUG is defined at the moment <assert.h> was last
included, the macro assert() generates no code, and hence does
nothing at all. It is not recommended to define NDEBUG if using
assert() to detect error conditions since the software may behave
non-deterministically.
No value is returned.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ assert() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
C11, POSIX.1-2008.
C89, C99, POSIX.1-2001.
In C89, expression is required to be of type int and undefined
behavior results if it is not, but in C99 it may have any scalar
type.
assert() is implemented as a macro; if the expression tested has
side-effects, program behavior will be different depending on
whether NDEBUG is defined. This may create Heisenbugs which go
away when debugging is turned on.
abort(3), assert_perror(3), exit(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
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2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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Linux man-pages 6.15 2025-05-17 assert(3)
Pages that refer to this page: abort(3), assert_perror(3), static_assert(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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