When you implicitly invoke a CLIST or REXX exec, the EXEC command processor, by default, searches system-level libraries allocated to a system file (SYSPROC or SYSEXEC). You can use the ALTLIB command to specify alternative CLIST or REXX exec libraries for implicit execution. This command performs the same function for CLISTs and REXX execs as the LIBDEF command in ISPF performs for panels, messages, tables, skeletons, and load libraries. With ALTLIB, a user or ISPF application can easily activate and deactivate CLIST and REXX exec libraries as the need arises. This flexibility provides a potential performance benefit by reducing the time it takes to search for these libraries.

The ALTLIB command lets you specify alternative libraries on the user, application, and system-level. The user-level includes libraries previously allocated to the SYSUEXEC or SYSUPROC file. The application-level includes libraries specified on the ALTLIB command using the data set or file name. The system-level includes libraries previously allocated to the SYSEXEC or SYSPROC file.

During implicit execution, the EXEC command processor searches the libraries in the following order: user-level, application-level, and system-level. At the application and system-levels, the ALTLIB command uses the virtual lookaside facility (VLF) to provide potential increases in library search speed.