Would you like signs with those chars?

Let's be completely precise here. It is not undefined, it is "implementation-defined or an implementation-defined signal is raised".

C99, 6.5.16.1 Simple assignment

(2) In simple assignment (=), the value of the right operand is converted to the type of the
assignment expression and replaces the value stored in the object designated by the left
operand.

and that "converted to the type of" is covered in

6.3 Conversions, 6.3.1.3 Signed and unsigned integers:

(1) When a value with integer type is converted to another integer type other than _Bool, if
the value can be represented by the new type, it is unchanged.
(2) Otherwise, if the new type is unsigned, the value is converted by repeatedly adding or
subtracting one more than the maximum value that can be represented in the new type
until the value is in the range of the new type.
(3) Otherwise, the new type is signed and the value cannot be represented in it; either the
result is implementation-defined or an implementation-defined signal is raised.

And in practice, any relevant compiler (at least when we're talking the linux kernel, but I'd be surprised if any non-academic compiler did otherwise) does as gcc:

* 'The result of, or the signal raised by, converting an integer to a
signed integer type when the value cannot be represented in an
object of that type (C90 6.2.1.2, C99 and C11 6.3.1.3).'

For conversion to a type of width N, the value is reduced modulo
2^N to be within range of the type; no signal is raised.