Dynamically allocating the internal reader

You can allocate SYSOUT data sets to the special external writer, INTRDR, just as you would any other external writer. For example, your program can issue an SVC 99 (for details on SVC 99, see z/OS MVS Programming: Assembler Services Guide) and write JCL-images directly to the internal reader.

The following text units are required on the dynamic allocation request to allocate an internal reader:

DALSYSOU - to indicate that this is a SYSOUT data set and the default MSGCLASS for jobs that are submitted thought this internal reader. If '*' is specified, the MSGCLASS is the same as the MSGCLASS, job or TSO/E logon that allocated the internal reader.
DALSPGNM - you must specify "INTRDR" to indicate that an internal reader is being allocated.
DALDDNAM or DALRTDDN - specifies the DD name to associate with the internal reader or to request that the system assign a DD name.
DALSSREQ or DALUASSR - optionally specify the name of the subsystem that the internal reader should be associated with. The name must be that of an active JES2 subsystem on this member. To use DALSSREQ, the caller must be APF authorized.

Note: If DALSSREQ or DALUASSR is specified, the address space that allocates the internal reader does not have to be associated with the JES2 that is specified. The allocating address space can be associated with the master address space (such as a started task running SUB=MSTR) or running under another JES subsystem (such as a job associated with the primary subsystem allocating an internal reader on a secondary subsystem). However, having an internal reader allocated will prevent the owning JES2 from shutting down. IBM recommends that applications using DALSSREQ or DALUASSR not keep the internal reader allocated for an extended period of time or have a mechanism to request that the internal reader be unallocated. This prevents the internal reader allocation from impacting the starting and stopping of JES2 subsystems.