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"Inter-process" communication

Diagram showing communication pathways three process loops on the RT target

RT inter-process communication
RT guide

Exchange data between process loops running in parallel on the RT target.

LabVIEW RT block diagram snippet: Process Loop #1 writes an indicator, Process Loop #2 reads the indicator value with a local variable

Local variable (RT)
RT code

Use a local variable (front-panel indicator) to communicate between two parallel process loops contained within the same VI, and use a local variable to stop parallel loops with one "stop" button.

LabVIEW RT block diagrams: VI #1 writes a global variable, VI #2 reads the same global variable

Global variable (RT)
RT code

Use a global variable to communicate between two parallel process loops contained within different VIs under the same target, and use a global variable to stop parallel loops with one "stop" button.

LabVIEW RT block diagram snippet: Read a single-process shared variable in one process loop that was written by another process loop

Single-process shared variable (SPSV)
RT code

A single-process shared variable (SPSV) behaves like a global variable that links deterministic and non-deterministic process loops, effectively shielding the deterministic loop from elements that contribute jitter.

LabVIEW RT block diagram snippet: Functional global variable subVI with unitialized while-loop shift register and case structure

Functional global variable (FGV)
RT code

Use a functional global variable (FGV) to communicate between two parallel process loops contained within different VIs under the same target, and use a FGV to stop parallel loops with one "stop" button. The "functional" nature of the FGV means that you can create additional functionality beyond that of a basic global variable, e.g., counting and calculations that operate on the stored value.

LabVIEW RT block diagram: for-loop structure enqueues 10 integers, then another for-loop structure dequeues the same elements

Queue
RT code

Use a queue to send messages and data between two or more parallel process loops contained within a VI or other VIs. Queues also serve as the foundation for the "Queued State Machine" design pattern.

LabVIEW RT block diagram snippet: Process Loop #1 writes a channel wire, Process Loop #2 reads the channel wire

Channel wire
RT code

Use a channel wire to communicate between two (or more) parallel process loops contained within the same VI, and use a channel wire to stop parallel loops with one "stop" button. Requires LabVIEW 2016 or later version.

Diagram showing communication pathways three process loops on the FPGA target

FPGA inter-process communication
RT guide

Exchange data between process loops running in parallel on the RT target.

LabVIEW FPGA block diagram snippet: Process Loop #1 writes an indicator, Process Loop #2 reads the indicator value with a local variable

Local variable (FPGA)
FPGA code

Use a local variable (front-panel indicator) to communicate between two parallel process loops contained within the same VI.

LabVIEW FPGA block diagrams: VI #1 writes a global variable, VI #2 reads the same global variable

Global variable (FPGA)
FPGA code

Use a global variable to communicate between two parallel process loops contained within different VIs under the same target.

LabVIEW RT block diagram snippet: While-loop structure containing guard clause, "Command Parser" kernel subVI, and queue

Queued state machine
RT code

State machines perform system control, data processing, and any task that involves executing a sequence of activities in response to inputs from the surrounding physical system, the user interface, and other processes within the system. The queued state machine is a particular implementation style that is flexible and versatile, easy to maintain, and computationally efficient.

LabVIEW RT block diagram snippet: System Manager and Code Validator message-handling loops

Queued message handler with multiple process loops
RT code

The queued message handler contains multiple process loops operating independently and in parallel that communicate with each other by sending messages through queues. Each process is a well-defined task implemented by the "Queued State Machine" design pattern. Breaking up the system into self-contained tasks greatly simplifies the design of complex systems.

LabVIEW front panels of the "Home Security System" application: RT target with virtual user interface and PC host remote interface

System controller application example: Home Security System
RT code PC code

Example of a complete RT system controller based on the Queued Message Handler (QMH) design pattern with multiple parallel task loops implementing behaviors with queued state machines (QSMs), various inter-process communication techniques (queues and local variables), and inter-target communication techniques (network-published shared variables (NPSVs) and network streams). The PC host human-machine interface (HMI) can remotely connect to the system through the network, monitor the status of the security system, and control it remotely.

LabVIEW PC block diagram: producer-loop contains event structure and guard clause; consumer-loop contains guard clause and case structure; both loops share the same queue reference

Event-driven producer-consumer state machine
PC code

Create a responsive user interface based on two loops operating in parallel: the "producer" loop event structure responds immediately to user interactions such as button clicks and mouse movements that send commands through a queue to the "consumer" loop which performs the required tasks. Separating the state machine into two loops allows the user interface to remain responsive should a consumer task require an unusual amount of time or must wait for a shared resource to become available.