In case the negative value is -1.0 or lower, it invokes undefined behavior since the integral part then cannot be represented by an unsigned number. Otherwise, (as in the case of -0.1), if it can be represented by an integer type, it is well-defined behavior. See the C11 standard, ISO 9899:2011:
6.3.1.4
When a finite value of real floating type is converted to an integer
type other than _Bool, the fractional part is discarded (i.e., the
value is truncated toward zero). If the value of the integral part
cannot be represented by the integer type, the behavior is undefined. 61)
And then there is a non-normative foot note explaining the above text:
61) The remaindering operation performed when a value of integer type
is converted to unsigned type need not be performed when a value of
real floating type is converted to unsigned type. Thus, the range of
portable real floating values is (−1, Utype_MAX+1).
ISO/IEC 9899:1999 (C99) contains exactly the same text.
