The drive toward many-core architectures has been tremendous during the last decade. Along with this trend, the community has been searching, investigating and looking for programming models that provide both control on the data locality and flexibility of the data handling. In this paper, the authors present the design and implementation of POSH, an Open-Source implementation of the OpenSHMEM standard. They present a model for its communications, and prove some properties on the memory model defined in the OpenSHMEM specification. They present some performance measurements of the communication library featured by POSH and compare them with an existing one-sided communication library.