void __assert_fail(const char *assertion, const char *file, unsigned int line, const char *function);
It then aborts program execution via a call to abort. The exact form of the message is up to the implementation.
a.c:10: foobar: Assertion a == b failed.
If FUNCTION is NULL, then omit information about the function.
FILE, LINE, and ASSERTION must be non-NULL.
__assert_fail is not in the source standard; it is only in the binary standard.