Will result always be the floor of the division? What is the defined behavior?

Not quite. It rounds toward 0, rather than flooring.

6.5.5 Multiplicative operators

6When integers are divided, the result of the / operator is the algebraic quotient with any fractional part discarded.^{88)}If the quotient a/b is representable, the expression (a/b)*b + a%b shall equal a.

and the corresponding footnote:

- This is often called ‘‘truncation toward zero’’.

Of course two points to note are:

3The usual arithmetic conversions are performed on the operands.

and:

5The result of the / operator is the quotient from the division of the first operand by the second; the result of the % operator is the remainder.In both operations, if the value of the second operand is zero, the behavior is undefined.

[Note: Emphasis mine]