The APPCCMD CONTROL=SENDRCV macroinstruction allows both a send and a receive on a single API request. Use of CONTROL=SENDRCV reduces the path length on half-duplex conversations that experience frequent changes in the direction of data flow. This function is supported for applications using both HPDT and non-HPDT interfaces. This section describes considerations for using CONTROL=SENDRCV for HPDT requests.

The HPDT interface can be used only for the sending function of the APPCCMD CONTROL=SENDRCV macroinstruction. The application can send data directly from CSM storage using the XBUFLST option. However, data received is always copied into application storage.

When the application specifies OPTCD=XBUFLST on the CONTROL=SENDRCV request, all entries in the buffer list except the last specify the address and length of data to be sent. The data to be sent resides in CSM buffers. VTAM® does not track logical records supplied by the application. Like OPTCD=BUFFLST, the last entry specifies the address and length of an area in which data is to be received. When this macroinstruction completes, another field in this last entry contains the number of bytes placed in this receive buffer by VTAM. This receive buffer is not a CSM buffer.

Figure 1 shows the difference between the normal buffer list and the extended buffer list used on an APPCCMD CONTROL=SENDRCV macroinstruction.

Note that the receive portion of the extended buffer list is the same as entries in the buffer list built by the BUFFLST parameter. There can be multiple send XBUFLST entries. There is one receive BUFFLST entry.

The send buffers are mapped by the ISTBLXEN DSECT and the receive buffer is mapped by the ISTBLENT DSECT. The layout of these DSECTs is shown in the z/OS Communications Server: SNA Programmer's LU 6.2 Reference.

For more information about this macroinstruction, refer to z/OS Communications Server: SNA Programmer's LU 6.2 Reference.