When you use ALTLIB when ISPF is active, you can define the libraries (user, application, and system) that are active for each application. Libraries that you define while running an application are in effect only while that application has control. When the application completes, the previous libraries (user, application, and system) are automatically reactivated.

If you are in split-screen mode in ISPF and you issue the ALTLIB command from a one-screen session, the changes affect only that screen session. The ALTLIB search order is not valid across split screens.

The libraries that are originally used when an application gets control are determined through the NEWAPPL and PASSLIB parameters on the ISPF SELECT service. For more information about the SELECT service, see z/OS ISPF Services Guide.

When NEWAPPL is specified and PASSLIB is not specified (that is, you want to isolate the selected function from the application currently in control, but you do not want to pass library definitions specified with the ALTLIB command and ISPF LIBDEF service on to the new application), the current set of libraries, if any exist, are not used by the application being selected. The deactivation of these libraries takes place BEFORE the application is selected. The current library definitions are automatically reactivated when the application being selected terminates.

When both NEWAPPL and PASSLIB are specified (that is, you want to isolate the selected function from the application currently in control and you want to pass library definitions specified with the ALTLIB command and ISPF LIBDEF service on to the new application), the current set of libraries, if any exist, are made available to the selected application. Any changes you make to this set of libraries while this application is running are in effect only while this application has control. After the selected application terminates, the original set of libraries is reactivated.

When NEWAPPL and PASSLIB are not specified (that is, you do not want to isolate the selected function), the current set of libraries remains in effect because the selected function does not represent a new application. If the selected function changes any of the library definitions, the changes apply through all select levels of the application of which the selected function is a part.

ALTLIB within line mode TSO/E works just like an ISPF application. However, if you use ALTLIB from within line mode TSO/E and start ISPF, the libraries you specified in line mode TSO/E will not be available until ISPF is terminated.