  We contribute to the knowledge of linear codes from special polynomials and
functions, which have been studied intensively in the past few years. Such
codes have several applications in secret sharing, authentication codes,
association schemes and strongly regular graphs.
  This is the first work in which we study the dual codes in the framework of
the two generic constructions; in particular, we propose a Gram-Schmidt
(complexity of $\mathcal{O}(n^3)$) process to compute them explicitly. The
originality of this contribution is in the study of the existence or not of
defining sets $D'$, which can be used as ingredients to construct the dual code
$\mathcal{C}'$ for a given code $\mathcal{C}$ in the context of the second
generic construction. We also determine a necessary condition expressed by
employing the Walsh transform for a codeword of $\mathcal{C}$ to belong in the
dual. This achievement was done in general and when the involved functions are
weakly regularly bent. We shall give a novel description of the Hull code in
the framework of the two generic constructions. Our primary interest is
constructing linear codes of fixed Hull dimension and determining the (Hamming)
weight of the codewords in their duals.
