The argument principle and holomorphic extendibility
  Let D be a bounded domain in the complex plane whose boundary consists of finitely many pairwise disjoint simple closed curves. Give bD the standard orientation and let A(D) be the algebra of all continuous functions on the closure of D which are holomorphic on D. In the paper we prove that a continuous function f on bD extends to a function in A(D) if and only if for each g in A(D) such that f+g has no zero on bD the change of argument of f+g along bD is nonnegative. 
Josip Globevnik