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Version: 0.8.0

Reaction Declarations

This article has examples in the following target languages:

Sometimes, it is inconvenient to mix Lingua Franca code with target code. Rather than defining reactions (i.e., complete with inlined target code), it is also possible to just declare them and provide implementations in a separate file. The syntax of reaction declarations is the same as for reaction definitions, except they have no implementation. Reaction declarations can be thought of as function prototypes.


Consider the following program that has a single reaction named hello and is triggered at startup. It has no implementation.

target C { cmake-include: ["hello.cmake"], files: ["hello.c"] } main reactor HelloDecl { reaction hello(startup) }

The cmake-include target property is used to make the build system aware of an externally supplied implementation. The contents of hello.cmake is as follows:

target_sources(${LF_MAIN_TARGET} PRIVATE hello.c)

The files target property is used to make the file that has the implementation in hello.c accessible, which could look something like this:

#include <stdio.h>
#include "../include/HelloDecl/HelloDecl.h"

void hello(hellodecl_self_t* self) {
printf("Hello declaration!\n");

File Structure

In the above example, the C file used #include to import a file called HelloDecl.h. This file was generated from the Lingua Franca source file when the LF program was compiled. The file HelloDecl.h is named after the main reactor, which is called HelloDecl, and its parent directory, include/HelloDecl, is named after the file, HelloDecl.lf.

In general, compiling a Lingua Franca program that uses reaction declarations will always generate a directory in the include directory for each file in the program. This directory will contain a header file for each reactor that is defined in the file.

As another example, if an LF program consists of files F1 and F2, where F1 defines reactors A and B and F2 defines the reactor C and the main reactor F2, then the directory structure will look something like this:

├ F1/
│ ├ A.h
│ └ B.h
└ F2/
├ C.h
└ F2.h
├ F1.lf // defines A and B
└ F2.lf // defines C and F2

There is no particular location where you are required to place your C files or your CMake files. For example, you may choose to place them in a directory called c that is a sibling of the src directory.

The Generated Header Files

The generated header files are necessary in order to separate your C code from your LF code because the describe the signatures of the reaction functions that you must implement.

In addition, they define structs that will be referenced by the reaction bodies. This includes the self struct of the reactor to which the header file corresponds, as well as structs for its ports, its actions, and the ports of its child reactors.

As with preambles, programmer discipline is required to avoid breaking the deterministic semantics of Lingua Franca. In particular, although the information exposed in these header files allows regular C code to operate on ports and self structs, such information must not be saved in global or static variables.

Linking Your C Code

As with any Lingua Franca project that uses external C files, projects involving external reactions must use the cmake-include target property to link those files into the main target.

This is done using the syntax

target_sources(${LF_MAIN_TARGET} PRIVATE <files>)

where <files> is a list of the C files you need to link, with paths given relative to the project root (the parent of the src directory).