Waiting for messages
If you want a program to wait until a message arrives on a queue, specify the MQGMO_WAIT option in the Options field of the MQGMO structure. Use the WaitInterval field of the MQGMO structure to specify the maximum time (in milliseconds) that you want an MQGET call to wait for a message to arrive on a queue.
If the message does not arrive within this time, the MQGET call completes with the MQRC_NO_MSG_AVAILABLE reason code.
We can specify an unlimited wait interval using the constant MQWI_UNLIMITED in the WaitInterval field. However, events outside your control could cause your program to wait for a long time, so use this constant with caution. IMS applications must not specify an unlimited wait interval because this would prevent the IMS system terminating. (When IMS terminates, it requires all dependent regions to end.) Instead, IMS applications can specify a finite wait interval; then, if the call completes without retrieving a message after that interval, issue another MQGET call with the wait option.
If more than one program is waiting on the same shared queue to remove a message, only one program is activated by a message arriving. However, if more than one program is waiting to browse a message, all the programs can be activated. For more information, see the description of the Options field of the MQGMO structure in the Application Programming Reference.
If the state of the queue or the queue manager changes before the wait interval expires, the following actions occur:
- If the queue manager enters the quiescing state, and you used the MQGMO_FAIL_IF_QUIESCING option, the wait is canceled and the MQGET call completes with the MQRC_Q_MGR_QUIESCING reason code. Without this option, the call remains waiting.
- On z/OS, if the connection (for a CICS or IMS™ application) enters the quiescing state, and you used the MQGMO_FAIL_IF_QUIESCING option, the wait is canceled and the MQGET call completes with the MQRC_CONN_QUIESCING reason code. Without this option, the call remains waiting.
- If the queue manager is forced to stop, or is canceled, the MQGET call completes with either the MQRC_Q_MGR_STOPPING or the MQRC_CONNECTION_BROKEN reason code.
- If the attributes of the queue (or a queue to which the queue name resolves) are changed so that get requests are now inhibited, the wait is canceled and the MQGET call completes with the MQRC_GET_INHIBITED reason code.
- If the attributes of the queue (or a queue to which the queue name resolves) are changed in such a way that the FORCE option is required, the wait is canceled and the MQGET call completes with the MQRC_OBJECT_CHANGED reason code.
If you want your application to wait on more than one queue, use the signal facility of WebSphere MQ for z/OS (see Signaling). For more information about the circumstances in which these actions occur, see the Application Programming Reference.
Parent topic:
Writing a WebSphere MQ application
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