Uploaded by Diego Alvarez

Re-entrant VIs

advertisement
Home (http://www.ni.com/) ❯ Support (http://www.ni.com/support/) ❯ Manuals (http://www.ni.com/manuals) ❯ LabVIEW 2015 Help (/reference/en-XX/help/371361M-01)
Reentrancy: Allowing Simultaneous Calls to the Same SubVI
By default, LabVIEW configures VIs for non-reentrant execution. In non-reentrant execution, when a VI is called as a subVI
simultaneously by other VIs, only one of those calls can enter and execute the subVI at a time. LabVIEW ensures that
callers of the non-reentrant subVI take turns using the subVI. A non-reentrant VI has only a single data space to store the
parameters of its callers, any temporary values computed during execution, and any state that the non-reentrant VI
maintains between calls. However, if you configure a VI for reentrant execution, LabVIEW can enter and execute the
reentrant VI simultaneously from all callers. In reentrant execution, LabVIEW allocates multiple instances, or clones, of the
data space so that each call site, or usage of the subVI on the block diagram of the caller, can execute simultaneously, in
parallel, such that each call site uses a separate clone.
Note (FPGA Module) FPGA VIs are reentrant by default (/reference/en-XX/help/371599L01/lvfpgaconcepts/fpga_determine_re_nre_subvi/).
Types of Reentrant Execution
LabVIEW provides the following types of reentrant VIs:
• Non-reentrant execution—LabVIEW allocates a single data space for use by all instances of the subVI.
• Shared clone reentrant execution—LabVIEW allocates a pool of clones, initially populated with one clone, to
share amongst callers. When a caller calls a shared clone VI, it pulls a clone from the pool and returns the clone
when the call finishes executing. If the pool is empty when a caller calls a shared clone VI, LabVIEW creates a new
clone for that call and places the clone in the pool when execution finishes, thereby expanding the clone pool.
• Preallocated clone reentrant execution—LabVIEW allocates a separate, non-shareable, clone for each use of the
subVI.
To configure the reentrancy settings for a VI, refer to the Execution (../../lvdialog/execution/) page of the VI Properties
(../../lvdialog/vi_properties_dialog_box/) dialog box.
Choosing Which Type of Reentrant Execution to Use
Usage
Considerations
Non-reentrant
Ability to maintain
state, such as in
uninitialized shift
registers
Maintains a single instance of the Does not maintain state—Each call
data space and uses its state
site pulls the data space of a clone
across all call sites.
randomly from the pool of clones.
Call sites may end up sharing state.
Maintains state for each call
site—Each call site has its
own separate and specific
clone.
Determinism for
multiple,
simultaneous
calling VIs
Call sites might wait in line
behind other simultaneous
callers—The time required for an
instance of a subVI to begin
executing varies with how many
other instances are scheduled to
execute before it.
Possible waiting—Call sites may
need to wait for LabVIEW to create
a new clone if the pool of clones is
empty. When the pool is not empty,
pulling a clone and returning the
clone to the pool can introduce nondeterministic delays.
No waiting—LabVIEW
allocates a clone for each
call site prior to execution.
This allows deterministic
memory usage and
execution speed.
Call overhead
when no
simultaneous
calling VIs exist
Medium—LabVIEW must check
for simultaneous callers.
Highest—LabVIEW must atomically Lowest—LabVIEW already
pull clones from and return clones preallocates a clone to each
to the pool of clones. Calls may
call site.
need to wait for LabVIEW to add a
clone to the pool.
Memory usage
Lowest—LabVIEW allocates only Medium—LabVIEW must allocate
a single data space for the subVI. clones only for the maximum
number of instances running
simultaneously.
Shared Clone Reentrancy
Preallocated Clone
Reentrancy
Highest—LabVIEW must
allocate a clone for each
instance of the subVI.
Note (FPGA Module) Shared clone reentrant VIs and preallocated clone reentrant VIs behave the same when
used in FPGA VIs.
Asynchronously Calling Multiple Instances of a Reentrant VI
Refer to the Asynchronously Calling Multiple Instances of a VI for Parallel Execution (../../lvhowto/acbr_call_clones/) topic
for information about asynchronously calling reentrant VIs.
Viewing the Front Panel of the Clone of a Reentrant VI
Each clone has an individual front panel. To view the front panel while the clone is not running, double-click the instance of
This site uses cookies to offer you a better browsing experience. Learn more about our privacy policy.
the subVI whose panel you want to view. You can also set the reentrant VI to open the front panel during execution
(http://www.ni.com/legal/privacy/unitedstates/us/)
(../../lvhowto/opening_subvi_front_panels/). The title bar of the subVI contains (clone) to indicate that it is a clone rather
Table of Contents (/reference/en
XX/help/371361M-01)
LabVIEW 2015 Help
Edition Date: June 2015
Part Number: 371361M-01
»View Product Info
(http://sine.ni.com/nips/cds/view/p/lang/en/
Download Help (Windows Only)
(http://www.ni.com/pdf/manuals/371361m.zip
than the original VI. To access the original VI from the clone, select View»Browse Relationships»Reentrant Items. You
can also use the Front Panel:Open (../../lvprop/vi_open_front_panel/) method within the subVI to programmatically open
the front panel of each clone when it runs. The title bar of the subVI contains (clone) to indicate that it is a clone of the
source VI.
You can interact with the front panel of a reentrant VI similarly to the front panel of a non-reentrant VI, particularly in the
following ways:
• Configuring an Event (../../glang/event_structure/) structure to handle events for front panel objects
• Using the front panel as a subpanel (../../lvhowto/loading_panel_in_subpanel/)
• Using properties and methods to configure controls and indicators (../../lvhowto/prog_controlling_fp_objs/)
Examples of Reentrant Execution
The following two sections describe examples of reentrant VIs that wait and do not share data.
Ensuring Simultaneous Execution of SubVI Instances
The following block diagram alerts the user when both his Lunch Time and his Break Time arrive. However, the
reentrancy setting of the Alarm Timer VI is critical for producing accurate results.
The user specifies a Lunch Time and a Break Time. For the purposes of this example, assume Lunch Time is
12:00pm and Break Time is 10:00am.
The Alarm Timer VI waits until the specified input time. Because the instances of the Alarm Timer VI appear in parallel
parts of the diagram, it is impossible to predict which instance executes first. For the purposes of this example, assume
the Lunch Time timer executes first.
If the Alarm Timer VI is configured for non-reentrant execution, the Break Time timer cannot begin running until 12:00pm
because it must wait for the Lunch Time timer to finish executing. This delay prevents the Break Time timer from
triggering an alarm at 10:00am. Therefore, to ensure that both instances of the subVI can execute simultaneously and
trigger their alarms at the appropriate time, you must configure the Alarm Timer VI to use one of the types of reentrant
execution.
Storing Data between Multiple Calls to Each Instance of a SubVI
To maintain state between calls to a subVI from a specific call site, you must use preallocated clone reentrant execution.
For example, you create a VI, ExpAvg, that calculates a running exponential average of the last four data points that have
been passed to the VI. LabVIEW can maintain the most recent four data points as state in an uninitialized shift register in a
single iteration While Loop.
Another VI uses the ExpAvg subVI to calculate the running average of two data acquisition channels. The VI monitors the
voltages at two points in a process and displays the exponential running average on a strip chart. The block diagram of the
VI contains two instances of the ExpAvg subVI. The calls alternate — one for Channel 0, and one for Channel 1. Assume
Channel 0 runs first. If the ExpAvg subVI is not reentrant, the call for Channel 1 uses the average computed by the call for
Channel 0, and the call for Channel 0 uses the average computed by the call for Channel 1. By making the ExpAvg subVI
reentrant, each call can run independently without sharing the data.
Overriding Reentrancy Settings for SubVIs in Timed Structures
(Real-Time, Windows) You can configure LabVIEW to either preallocate or share clones of a shared-clone reentrant VI
within a Timed Loop or Timed Sequence (../con_timed_structures/) structure. For example, if you place a shared-clone
reentrant VI inside of a Timed Loop or Timed Sequence structure, you can set the structure to preallocate clones for
instances of the reentrant VI you call within the structure. Instances of the reentrant VI that you call outside the structure
continue to share clones. To set the allocation of a VI within a Timed Loop or a Timed Sequence structure, right-click the
structure, select Shared Clone Allocation, and select one of the following:
• Automatic—LabVIEW decides whether to set the structure as Preallocate or Allocate As Needed based on the
context of the structure. LabVIEW will append the shortcut menu item with text that indicates the setting LabVIEW
chooses. For example, if LabVIEW chooses Preallocate, the menu item becomes Automatic (Preallocate).
• Preallocate—LabVIEW creates a clone VI for each call to the reentrant VI within the structure. The Timed Loop or
Timed Sequence structure displays an
icon to indicate that LabVIEW is preallocating clones for any reentrant VI
in the structure.
• Allocate As Needed—LabVIEW does not create a clone VI until you make a call to the reentrant VI within the
structure. The Timed Loop or Timed Sequence structure displays an
icon to indicate that LabVIEW is sharing
This site uses cookies to offer you a better browsing experience. Learn more about our privacy policy.
clones for any reentrant VI in the structure.
(http://www.ni.com/legal/privacy/unitedstates/us/)
Debugging Reentrant VIs
To allow debugging on a reentrant VI, select File»VI Properties to display the VI Properties
(../../lvdialog/vi_properties_dialog_box/) dialog box, select Execution from the pull-down menu, and place a checkmark in
the Allow debugging checkbox.
To debug a specific instance of a VI with preallocated clones, open that instance of the subVI. Although you cannot edit
the clone VI, you can use the block diagram of the clone for debugging purposes. For example, you can set breakpoints
(../../lvhowto/breakpoints/), use probes (../../lvhowto/using_the_probe_tool/), enable execution highlighting
(../../lvhowto/execution_highlighting/), and single-step (../../lvhowto/single_step_mode/) through execution.
To debug an instance of a VI that shares clones, apply breakpoints, probes, and execution highlighting within the source
VI, not within the clone VI. Clone VIs do not maintain these settings across calls.
Note When you debug applications and shared libraries, you cannot debug reentrant panels that an Open VI
Reference (../../glang/open_vi_reference/) function creates. You also cannot debug reentrant panels that are entry
points to LabVIEW-built shared libraries.
WAS THIS ARTICLE HELPFUL?
Helpful
Not Helpful


PRODUCT
SUPPORT
COMPANY
MISSION
Order status and history
Submit a service request
About National Instruments
NI equips engineers and scientists with
(http://www.ni.com/status/)
(https://sine.ni.com/srm/app/myServiceRequests) (http://www.ni.com/company/)
Order by part number
Manuals (http://www.ni.com/manuals/)
(http://sine.ni.com/apps/utf8/nios.store?
action=purchase_form)
Activate a product
(http://sine.ni.com/myproducts/app/main.xhtml?
lang=en)
Drivers
systems that accelerate productivity,
innovation, and discovery.
Events (http://www.ni.com/enus/events.html)
(http://www.ni.com/downloads/drivers/)
Careers (http://www.ni.com/careers/)
Alliance Partners
Contact Us (http://www.ni.com/contact-us/)
(http://www.ni.com/alliance/)
(http://twitter.com/niglobal)
(http://www.facebook.com/NationalInstruments)
(http://linkedin.com/company/national-
Order and payment information
(http://www.ni.com/how-to-buy/)
instruments)
(http://www.ni.com/rss/)
(http://www.youtube.com/nationalinstruments)
(http://privacy-policy.truste.com/privacy-seal/seal?rid=342a514a-c792-40e3-ae36-17c7491387a7) Legal |
(http://www.ni.com/legal/)Privacy (http://www.ni.com/legal/privacy/unitedstates/us/) | © 2018 National Instruments. All rights
reserved.
This site uses cookies to offer you a better browsing experience. Learn more about our privacy policy.
(http://www.ni.com/legal/privacy/unitedstates/us/)
United States
Download