Writing exits for the ALTLIB command

TSO/E users issue the ALTLIB command to define alternative user-level or application-level libraries containing REXX execs and CLISTs. The defined libraries are searched before, or instead of, the system-level libraries. Therefore, the user does not need:

To allocate application-level REXX execs or CLISTs to the appropriate ddname before implicitly executing them.
To include all application-level REXX execs or CLISTs in the user's logon procedure.

Table 1 lists the search order of the user-, application-, and system-level libraries. Also shown are the ddnames associated with each library level. These ddnames can be allocated either dynamically by the ALLOCATE command or included as part of a logon procedure.

Table 1. Library search order
Search order	Library level		Associated ddname
1.	User	REXX exec	SYSEXEC
2.	User	CLIST	SYSUPROC
3.	Application	REXX exec	Define with FILE or DATASET operand
4.	Application	CLIST	Define with FILE or DATASET operand
5.	System	REXX exec	SYSEXEC (installation can define this ddname)
6.	System	CLIST	SYSPROC

With the defaults that TSO/E provides, and before an ALTLIB command is invoked, TSO/E searches the system EXEC library (default ddname SYSEXEC) first, followed by the system CLIST library (ddname SYSPROC). Note that your system programmer can change this by

Defining an alternate ddname of SYSEXEC
Indicating that TSO/E is not to search the system-level exec ddname of SYSEXEC. Then only the system-level CLIST (SYSPROC) is searched.

You can alter the default library search order by using either the ALTLIB command or the EXECUTIL command.

Use EXECUTIL to indicate that the system-level exec ddname is to be searched for the duration of the current REXX language processor environment.
Use ALTLIB to indicate that the system-level exec ddname is to be searched for the duration of the current application. ALTLIB always overrides EXECUTIL within an application.

Use ALTLIB DISPLAY to see which libraries are being searched for.

TSO/E provides initialization and termination exits that you can use to customize the ALTLIB command. These exits receive control during ALTLIB command processing, as follows:

The initialization exit receives control before the ALTLIB command processor invokes the parse service routine to syntax check the input parameters.
The termination exit receives control just before the ALTLIB command terminates processing.