C Standard Library: 6 Diagnostics: <assert.h>

The assert macro is used to add diagnostics to programs:
void assert(int expression)
If expression is zero when
assert(expression)
is executed, the assert macro will print on stderr a message, such as
Assertion failed: expression, file filename, line nnn
It then calls abort to terminate execution. The source filename and line number come from the
preprocessor macros __FILE__ and __LINE__.
If NDEBUG is defined at the time <assert.h> is included, the assert macro is ignored.