Dynamic Allocation

Introduction Differences from TSO/E Dynamic Allocation
Service Descriptions

Introduction

Dynamic allocation is the system facility that permits data sets to be allocated for use by your program as they are required. When they are no longer needed, your program can request that dynamic allocation routines "unallocate" or free the data sets. In contrast, data sets allocated using JCL DD statements are allocated statically. That is, they are allocated for your program's use possibly well before your program requires them; and they may be held by your program long after the allocations are necessary, resulting in loss of concurrent use.

To eliminate this problem, Shadow REXXTOOLS provides implementations of the ALLOCATE and FREE commands. These commands are implemented under the REXXTOOL host command environment (address REXXTOOL). Like their TSO counterparts, the Shadow REXXTOOLS ALLOCATE and FREE commands are invoked by coding host command expressions in REXX programs. Unlike the TSO commands, the Shadow REXXTOOLS commands are strictly for use under Shadow REXX programs.

Using the Shadow REXXTOOLS ALLOCATE command you can:

Create new data sets. The data sets can be assigned attributes (e.g., DCB) directly on the command, or indirectly by referring to model data sets or SMS classes.
Associate a ddname with a data set. The ddname can be referenced by the EXECIO command and by the Shadow REXXTOOLS VSAM functions.
Concatenate several data sets. The data sets can be grouped together under one ddname.
Allocate SYSOUT data sets. The output can be routed to writers, printers, and other systems.

Using the Shadow REXXTOOLS FREE command you can:

Disassociate a data set with your program. The data set then becomes available for other users in the system.
Override the disposition and destination of data sets. The new values override the values that were specified at data set allocation.

In addition, the Shadow REXXTOOLS ALLOCATE and FREE commands return extensive diagnostic information to your REXX program, including the text of allocation messages. You may use this information to drive conditional logic in your programs.

Differences from TSO/E Dynamic Allocation

The Shadow REXXTOOLS ALLOCATE command differs from the TSO/E ALLOCATE command in the following respects:

Generation data sets are supported by the REXXTOOLS ALLOCATE command. They are not supported by TSO ALLOCATE.
The ALTFILE and USING keywords are not supported by the REXXTOOLS ALLOCATE command.
The abbreviations for keywords are not always the same. In the service descriptions that follow, the minimum acceptable abbreviation for a keyword is shown in bold and underlined.
The return codes are different and there are more of them. Refer to the section "Returned Information" under the description for the ALLOCATE command.
The handling of messages is different. Messages are returned in special variables, and message printing is suppressed by the OPTIONS NOMSGS command.
System prompting for additional information is not supported.
The REUSE keyword always frees and reallocates the ddname.
In some cases the TSO ALLOCATE command will assume keyword values. The Shadow REXXTOOLS implementation assumes only the following:
- If you specify a data set status of NEW, a disposition of CATALOG will be provided automatically.
- If you do not specify a status (OLD, NEW, SHR, MOD), but you do specify LIKE, or any of the DCB or space-related keywords, a status of NEW is provided automatically.

The Shadow REXXTOOLS FREE command differs from the TSO/E FREE command in the following respects:

The ATTRLIST and OUTDES keywords are not supported.
The abbreviations for keywords are not always the same. In the service descriptions that follow, the minimum acceptable abbreviation for a keyword is shown in bold and underlined.
The return codes are different and there are more of them. Refer to the section "Returned Information" under the description for the FREE command.
The handling of messages is different. Messages are returned in special variables, and message printing is suppressed by the OPTIONS NOMSGS command.
System prompting for additional information is not supported.

Service Descriptions

The sections that follow describe the syntax and operation of the Dynamic Allocation commands.

ALLOCATE

FREE