For software maintenance and evolution, a common problem is to understand how each requirement is implemented in the source code. The basic solution of this problem is to find the fragment of source code that is corresponding to the implementation of each requirement. This can be viewed as a requirement-slicing problem -- slicing the source code according to each individual requirement. In this paper, we present an approach to find the set of functions that is corresponding to each requirement. The main idea of our method is to combine the information retrieval technology with the static analysis of source code structures. First, we retrieve the initial function sets through some information retrieval model using functional requirements as the queries and identifier information (such as function names, parameter names, variable names etc.) of functions in the source code as target documents. Then we complement each retrieved initial function set by analyzing the call graph extracted from the source code. A premise of our approach is that programmers should use meaningful names as identifiers. Furthermore, we perform an experimental study based on a GNU system. We use two basic metrics: precision and recall (which are the common practice in the information retrieval field), to evaluate our approach. We also compare the results directly acquired from information retrieval with those that are complemented through static source code structure analysis.