Microsoft KB Archive/246723

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How to programmatically timeout an (MC_)ALLOCATE verb

Article ID: 246723

Article Last Modified on 12/4/2007


  • Microsoft Host Integration Server 2004 Standard Edition
  • Microsoft Host Integration Server 2000 Standard Edition
  • Microsoft SNA Server 3.0 Service Pack 4
  • Microsoft SNA Server 4.0
  • Microsoft SNA Server 3.0 Service Pack 2
  • Microsoft SNA Server 3.0 Service Pack 3
  • Microsoft SNA Server 3.0 Service Pack 4
  • Microsoft SNA Server 4.0
  • Microsoft SNA Server 4.0 Service Pack 1
  • Microsoft SNA Server 4.0 Service Pack 2
  • Microsoft SNA Server 4.0 Service Pack 3

This article was previously published under Q246723


This article demonstrates how a developer can use the APPC WinAsyncAPPCEx() function in combination with the Win32 WaitForSingleObjectEx() to expire an (MC_)ALLOCATE verb when a predefined time-out interval has elapsed.

Microsoft does not recommend canceling an outstanding [MC_]ALLOCATE unless the request has been outstanding for a reasonable period of time (at least 20 seconds). If [MC_]ALLOCATE is issued with rtn_ctl = AP_WHEN_SESSION_ALLOCATED, the request does not complete until an available bound LU6.2 session is allocated from an SNA Server computer that supports the Local LU/ Remote LU/Mode being requested. A couple possible causes for slow [MC_]ALLOCATE response time include the following:

  • If all available LU6.2 sessions are already in use, then the [MC_]ALLOCATE will block until a current conversation ends, creating an available LU6.2 session. If this is occurring frequently, consider increasing the parallel session limit in the SNA Server Mode and the remote application subsystem.
  • If the APPC LU session is supported through an on-demand connection, and the connection is slow to activate (such as over SDLC)

Also, in order for the SNA Server computer(s) to properly handle cancelled [MC_]ALLOCATE requests, the computer(s) should be running at least SNA Server 4.0 SP2 or greater. For more information, click the following article number to view the article in the Microsoft Knowledge Base:

195996 SNA Server incorrectly handles cancelled allocate requests


When you use the following sample code, an APPC Allocate request will timeout if it has not completed in 25 seconds. This code is used for demonstration purposes only.

The following code snippet begins with construction of the verb control block (vcb) used for the Allocate request. When the vcb is complete, a pointer to the vcb is passed to the WinAsyncAPPCEx() function along with a Win32 handle used for event notification. The handle that is passed to the WinAsyncAPPEx() function is also registered with the WaitForSingleObjectEx(). The second argument specified in the WaitFOrSingleObjectEx is the timeout value specified in milliseconds. If the function succeeds, the return value indicates the event that caused the function to return. If the return value is WAIT_TIMEOUT, then the function completed as a result of the expired timer.

For more details concerning APPC Programming and Win32 APIs, please see the Microsoft Platform SDK.


  // Construct verb control block (vcb)

  vcb->allocate.opcode = AP_B_ALLOCATE;
  vcb->allocate.opext = AP_BASIC_CONVERSATION;
  vcb->allocate.conv_type = AP_BASIC_CONVERSATION;
  memcpy( vcb->allocate.tp_id,TPid, 8);
  vcb->allocate.sync_level = AP_CONFIRM_SYNC_LEVEL;
  vcb->allocate.rtn_ctl = AP_WHEN_SESSION_ALLOCATED;
  CopyMemory(vcb->allocate.plu_alias,RLUName,strlen(RLUName) );<BR/>
  CopyMemory(vcb->allocate.mode_name,ModeName, strlen(ModeName) );
  ConvertAToE(vcb->allocate.mode_name, 8);
  FillMemory(vcb->allocate.tp_name, sizeof(vcb->allocate.tp_name), 0x20);
  CopyMemory(vcb->allocate.tp_name, "GREGORY", 7);
  vcb-> = AP_NONE;

  // Create an unnamed event object
  hHandle = CreateEvent(NULL, FALSE, FALSE, NULL);

  // - Use async entry point for the verb.
  // - Pass the event handle (created above) along with pointer to vcb

  verbid = WinAsyncAPPCEx(hHandle,(long)vcb);

if (verbid == 0)
        return false;

    // Sleep for 25 seconds before timing out 

    wait_code = WaitForSingleObjectEx(hHandle,25000,true);
    ASSERT(wait_code != WAIT_ABANDONED); 
    if (wait_code == WAIT_TIMEOUT)

        return false;
    // Allocate successful. Return and do more useful things
    return true;


Additional query words: HIS 2000 HIS2004

Keywords: kbinfo KB246723