List of Implementation Defined Behaviours

VLAs are unsupported
_Complex is unsupported
(J.3.4): Multi-byte character constants are truncated to the first char
(J.3.3): The number of significant initial characters in an identifier is unlimited
(J.3.5): There are no extended integer types
(J.3.4/6.2.5.0.15): char behaves like signed char
(J.3.13): The value of the result of sizeof and _Alignof is of type unsigned int
(J.3.1/5.1.1.2.0.1.3): Sequences of whitespace are retained in translation phase 3
(J.3.4/5.2.2.0.3): Escape sequences produce the relevant ASCII codes
(6.10.2.0.4): Macro replaced #include directives are resolved using diagnostic information for the given pp-tokens
(J.3.3/6.4.2.1.0.3): Additional characters are the ‘$’ character, and all unicode characters of category LOWERCASE_LETTER or UPPERCASE_LETTER

List of Undefined Behaviours

(6.5.7.0.3) << and >> pass all values as they are to the assembler
Empty character constants '' have the value 0
(6.4.5.0.7) String literals are stored in the .data section, so what happens is up to the assembler/linker/etc
(6.5.2.2) The evaluation order of function call arguments is the reverse order of declaration (by default, certain things can influence it)
(6.10.1.0.4) When the defined unary operator is used incorrectly, we print an error diagnostic, and the entire expression has value 0
(6.3.2.1.0.3) Lvalue conversion on an array object with register storage class is ignored, address is taken
(J.2/6.2.4) The value of objects with automatic storage duration when used while it is indeterminate is the value in the first register of the correct type (for example, on X64 that means rax for ints and xmm0 for floats)

(6.4.5.0.7) The arrays are not distinct if they have the same encoding and the same contents