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Obtain Queue Function

LabVIEW 2013 Help

Edition Date: June 2013

Part Number: 371361K-01

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Owning Palette: Queue Operations Functions

Requires: Base Development System

Returns a reference to a queue.

Use this reference when calling other Queue Operations functions.

Details  Example

max queue size is the maximum number of elements you want the queue to hold. The default is –1, specifying that the queue can hold an unlimited number of elements.

If a queue reaches max queue size, the Enqueue Element or Enqueue Element at Opposite End functions wait until the Dequeue Element or Flush Queue functions remove elements from the queue.

If a queue with the same name exists, LabVIEW ignores this input and uses the size from the existing queue.
Note  (Real-Time Module) max queue size preallocates the specified number of elements in the queue when running on an RT target.


Note  When not running on an RT target, max queue size only limits the number of elements in the queue and does not preallocate the queue. To preallocate memory for a queue, enqueue that number of elements and then flush the queue. The space remains allocated for further use of the queue. Refer to the Details section for more information on memory allocation for queues.
name contains the name of the queue that you want to obtain or create. The default is an empty string to create an unnamed queue.
element data type is the type of data that you want the queue to contain. You can wire any data type to this input.
create if not found? specifies whether you want to create a new queue if one with the same name as name does not exist. If TRUE (default), the function creates a queue if one with the same name does not exist.
error in describes error conditions that occur before this node runs. With the following exception, this input provides standard error in functionality.

This node runs normally even if an error occurred before this node runs.
queue out is a reference to the existing queue or the new queue created by this function.
created new? is TRUE if the function created a new queue.
error out contains error information. This output provides standard error out functionality.

Obtain Queue Details

When you enqueue and dequeue resizable data types, such as paths, strings and arrays, you do not affect the memory used by queues. Queues transfer data, but do not generate copies of the data. Use the Lossy Enqueue Element function if you are concerned that elements waiting to enter an already-full queue might cause jitter in a deterministic loop.

Use named queues to pass data between two sections of a block diagram or between two VIs in the same application instance. If you do not wire name, the function creates a new, unnamed queue reference. If you wire name, the function searches for an existing queue with the same name and returns a new reference to the existing queue. If a queue with the same name does not already exist and create if not found? is TRUE, the function creates a new, named queue reference.

If you use the Obtain Queue function to return a reference to a named queue inside a loop, LabVIEW creates a new reference to the named queue each time the loop iterates. If you use Obtain Queue in a tight loop, LabVIEW slowly increases how much memory it uses because each new reference uses an additional four bytes. These bytes are released automatically when the VI stops running. However, in a long-running application it may appear as if LabVIEW is leaking memory since the memory usage keeps increasing. To prevent this unintended memory allocation, use the Release Queue function in the loop to release the queue reference for each iteration.

This function might return error codes 1, 2, 1094, 1100, 1491, or 1548.

Note  You cannot use queues or named queues to communicate across application instances. If you attempt to communicate with a named queue in another application instance, LabVIEW does not return an error but you might not receive the expected results.

If you obtain a queue reference in one application instance, you cannot use that queue reference in another application instance. If you attempt to use a queue reference in another application instance, LabVIEW returns error 1491.

Example

Refer to the Queue Overflow and Underflow VI in the labview\examples\Synchronization\Queue directory for an example of using the Obtain Queue function.


 

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