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ASSERT(3) Library Functions Manual ASSERT(3)

assert —
expression verification macro

#include <assert.h>

assert(expression);

The assert() macro tests the given expression and if it is false, the calling process is terminated. A diagnostic message is written to stderr and the abort(3) function is called, effectively terminating the program.

If expression is true, the assert() macro does nothing.

The assert() macro may be removed at compile time with the cc(1) option -DNDEBUG.

The following diagnostic message is written to stderr if expression is false: 
"assertion \"%s\" failed: file \"%s\", line %d\n", \
		    "expression", __FILE__, __LINE__);

cc(1), abort(3)

The assert() macro conforms to ANSI X3.159-1989 (“ANSI C89”).

An assert() macro first appeared in Version 7 AT&T UNIX.
December 10, 2014 OpenBSD-current