14.1.3.2. Named Queues

Your program creates (and deletes) named queues. You can name the queue yourself or leave the naming to the language processor. Your program must know the name of the queue to use a named queue. To obtain the name of the queue, use the RXQUEUE function:

previous_queue=rxqueue("set",newqueuename)

This sets the new queue name and returns the name of the previous queue.

The following Rexx instructions manipulate the queue:

PULL or PARSE PULL—reads a string from the head of the queue. If the queue is empty, these instructions take input from .INPUT.
PUSH—stacks a line on top of the queue (LIFO).
QUEUE—adds a string to the tail of the queue (FIFO).

Rexx functions that manipulate QUEUE: as a device name are:

LINEIN("QUEUE:")--reads a string from the head of the queue. If the queue is empty the program waits for an entry to be placed on the queue.
LINEOUT("QUEUE:","string")--adds a string to the tail of the queue (FIFO).
QUEUED—returns the number of items remaining in the queue.

Here is an example of using a queue:

Example 14.3. Sample Rexx Procedure Using a Queue

/*                                                                            */
/* push/pull WITHOUT multiprogramming support                                 */
/*                                                                            */
push date() time()                                /* push date and time       */
do 1000                                           /* let's pass some time     */
  nop                                             /* doing nothing            */
end                                               /* end of loop              */
pull a b                                          /* pull them                */
say "Pushed at " a b ", Pulled at " date() time() /* say now and then         */

/*                                                                            */
/*              push/pull WITH multiprogramming support                       */
/*        (no error recovery, or unsupported environment tests)               */
/*                                                                            */
newq = RXQUEUE("Create")                          /* create a unique queue    */
oq = RXQUEUE("Set",newq)                          /* establish new queue      */
push date() time()                                /* push date and time       */
do 1000                                           /* let's spend some time    */
  nop                                             /* doing nothing            */
end                                               /* end of loop              */
pull a b                                          /* get pushed information   */
say "Pushed at " a b ", Pulled at " date() time() /* tell user                */
call RXQUEUE "Delete",newq                   /* destroy unique queue created  */
call RXQUEUE "Set",oq               /* reset to default queue (not required)  */

Special considerations:

External programs that must communicate with a Rexx procedure through defined data queues can use the Rexx-provided queue or the queue that QUEUE: references (if the external program runs in a child process), or they can receive the data queue name through some interprocess communication technique, including argument passing, placement on a prearranged logical queue, or the use of usual interprocess communication mechanisms (for example, pipes, shared memory, or IPC queues).
Named queues are available across the entire system. Therefore, the names of queues must be unique within the system. If a queue named anyque exists, using the following function:
```
newqueue = RXQUEUE("Create", "ANYQUE")
```
results in an error.