Systems Network Architecture Considerations (SNA) › Allocating a Session › Issuing the #TREQ ALLOC Statement

Issuing the #TREQ ALLOC Statement

After you have set the session and conversation attributes in the UIOCB, you must issue a #TREQ ALLOC statement to allocate the session.

Coding Considerations

You should consider the following parameters when coding your #TREQ ALLOC statement:

• The OPTNS=ANY/CONN/IMM parameter of the #TREQ ALLOC statement establishes criteria for choosing a session. The session you need can be in one of three states:
  • Immediately available—The session has already been established with the requested LU and is not currently in use.

    Note: (LU6.2 only); the session must be a contention winner to be considered immediately available.

    For more information about contention winners, see the System Generation Guide.

  • Disconnected—The session has not yet been established.
  • Busy—The session has been established, but is currently allocated to another logical unit. The session will become immediately available when that logical unit ends its conversation.

  The options on the #TREQ ALLOC statement are as follows:

  • ANY (default) specifies that CA IDMS/DC tries to allocate a session in the following order:

    A session that is immediately available and currently unused.

    A session that is disconnected.

    A session that is busy; CA IDMS/DC will wait for a busy session and return control to your program once the session is allocated.

  • CONN requests CA IDMS/DC not to wait for a busy session. CA IDMS/DC will first attempt to allocate an immediately available session, then a disconnected session.
  • IMM specifies that only immediately available sessions are acceptable for the allocation request.
• You can specify whether your #TREQ ALLOC request is made synchronous (default) by specifying OPTNS=WAIT or asynchronous by specifying OPTNS=NOWAIT.

  Note: If you specify OPTNS=ANY, do not request asynchronous processing with OPTNS=NOWAIT. OPTNS=ANY implies that the request may wait for a busy session.

• The UIOCB parameter of the #TREQ ALLOC statement establishes a UIOCB for the conversation.

Example of LU-LU Session Allocation

The following example illustrates how you would allocate an LU-LU session, establishing the UIOCB, and setting session and conversation attributes:

• The first statement obtains storage for the UIOCB.
• The next statement establishes the remote logical unit.
• The next four statements establish LU6.2 conversation attributes.
• The #TREQ ALLOC statement allocates the session, initiates the conversation, and names the UIOCB.

  UIOSTG   #GETSTG TYPE=(USER,LONG),PLIST=*,LEN=UIOLEN,INIT=X'00',       *
                 STGID=UIOCBD,ADDR=(R1)
  *                                         SESSION ATTRIBUTES
  ATTR     MVC   UIORLU,=C'VTMFO178'     REMOTE LU
  *                                         CONVERSATION ATTRIBUTES
           MVC   UIOUSER,=C'BRANCH01'    USER ID: DENVER BRANCH
           MVC   UIOPASS,=C'DENPR  '     USER PASSWORD: DENVER
           MVI   UIOCONV,UIOCONVM        MAPPED MODE
           MVI   UIOSYNC,UIOSYNCC        MAXIMUM SYNC-LEVEL
           #TREQ ALLOC,UIOCBA=UIOCB,COND=ALL
  

After Issuing #TREQ ALLOC

After you have issued your #TREQ ALLOC request, you need to perform the following:

• Check the value in register 15:
  • If register 15 contains a nonzero value, the allocation request failed. The UIOUCM2 field in the UIOCB indicates whether the problem is permanent or temporary:
    • If CA IDMS/DC returns UIOALFR to the UIOUCM2 field, the allocate request was denied due to a temporary problem; for example, CA IDMS/DC was unable to wait for a busy session. In this case, you should issue the #TREQ ALLOC request again.
    • If CA IDMS/DC returns UIOALFN to the UIOUCM2 field, a permanent error was encountered.
    • If CA IDMS/DC returns UIOALFS to the UIOUCM2 field, the specified sync level for the conversation is not supported. This is a permanent error.
  • If register 15 contains 0, the session has been successfully established. Register 1 contains the logical terminal address (LTEADDR) of the remote LU. The logical terminal address (also stored in UIOLTEA) must be specified on all subsequent #TREQ requests in that session because a single task can have conversations with many logical units.
• If the #TREQ request was asynchronous (OPTNS=NOWAIT), you must issue a #TREQ CHECK statement before you make any further I/O requests. Your program must specify the LTE address of the remote LU (UIOLTEA) to identify the conversation.