When the DYNAM(DLL) option is used to build a DLL module, a side file might be generated along with it. The side file is saved in the data set represented by the SYSDEFSD ddname. The side file contains the symbols from which other DLLs can import; that is, which symbols the DLL "exports". Consequently, a side file contains a collection of IMPORT control statements that can be used by other DLLs in order to resolve their own external references during dynamic linking.

SYSDEFSD can be a sequential data set, a z/OS UNIX file, a PDS, or a PDSE. If your job binds multiple DLLs and SYSDEFSD represents a sequential data set or a z/OS UNIX file, the side file records of a given DLL can overwrite or append to the records of a previously saved side file, depending on the DISP or PATHOPTS parameter of your side file ddname.

The SYSDEFSD DD statement is optional. However, when it is absent, the binder issues a warning message if at bind time a module (DLL) generates export records and the DYNAM(DLL) binder option has been specified. Note that the side file can be referred to as the definition side deck by other products.

Table 1 shows the data set requirements for SYSDEFSD.