Function FLValue_IsUnsigned 

pub unsafe extern "C" fn FLValue_IsUnsigned(arg1: FLValue) -> bool

Returns true if the value is non-NULL and represents an integer >= 2^63. Such a value can’t be represented in C as an int64_t, only a uint64_t, so you should access it by calling FLValueAsUnsigned, not FLValueAsInt, which would return an incorrect (negative) value.