A processing apparatus 2 has a secure domain 90 and a less secure domain 80. Security protection hardware 40 performs security checking operations when the processing circuitry 2 calls between domains. A data store 6 stores several software libraries 100 and library management software 110. The library management software 110 selects at least one of the libraries 100 as an active library which is executable by the processing circuitry 4 and at least one other library 100 as inactive libraries which are not executable. In response to an access to an inactive library, the library management software 110 switches which library is active.