Issuing the CONTROL=ALLOC macroinstruction

After the FMH-5 is built, the application program issues the APPCCMD CONTROL=ALLOC macroinstruction to place the FMH-5 in the SEND buffer destined for the partner LU. For conversations allocated on full-duplex-capable sessions, the FMH-5 is not buffered, but sent immediately. For conversations established on half-duplex sessions, the FMH-5 is not sent through the network until the application program issues an APPCCMD macroinstruction that flushes the SEND buffer, or until the application program sends enough additional data to cause VTAM® to flush the buffer. The application program uses the AREA parameter on the macroinstruction to point to the FMH-5. The RECLEN parameter specifies the length of the FMH-5.

As an alternative, application programs can issue the APPCCMD CONTROL=PREALLOC macroinstruction, which reserves a session for a conversation without sending the FMH-5. For more information about preallocating a conversation, see Reserving a session for a conversation.

Application programs have several choices of what to specify on the APPCCMD CONTROL=ALLOC macroinstruction. These choices are shown in Table 1.

Table 1. Types of conversation allocation
Allocation Type	Description
QUALIFY=ALLOCD	Attempts to start a session for the conversation if one is not available. If a session cannot be established, the session request is queued until a session is available.
QUALIFY=CONVGRP	Can complete successfully if the session specified by the conversation group identifier is available. If the session is not available, the request is queued until it is available.
QUALIFY=CONWIN	Can complete successfully only after a contention-winner session has been allocated for use by the newly created conversation. If no contention-winner session is currently available, the APPCCMD is queued for later completion.
QUALIFY=IMMED	Can complete successfully only if a contention-winner session is available immediately for use.
QUALIFY=WHENFREE	Can complete successfully only if a session is available or pending or one can be activated. This request is never queued waiting for a conversation to complete and free a session.

For an explanation of the types of conversation allocation, see Types of conversation allocation. For details on the syntax and operands for the different forms of the CONTROL=ALLOC macroinstruction, refer to the descriptions in the z/OS Communications Server: SNA Programmer's LU 6.2 Reference.

For optimal performance in allocating a session for a conversation, design the application program so that a contention-winner session is always available.