assert - program verification
Assert is a macro that indicates expression is expected to be true
at this point in the program. It causes an exit(2) with a
diagnostic comment on the standard output when expression is false
(0). Compiling with the cc(1) option -DNDEBUG effectively deletes
assert from the program.
‘Assertion failed: file f line n.’ F is the source file and n the
source line number of the assert statement.