Abi

This page presents some idea for an embedded middleware named ABI based on publish/subscribe like Ivy.

General idea

• Give an easy way to allow software components to exchange data with minimum delay nor execution overhead.
• The components don't need to know each other, they only need the message format.
• Each subscriber set a callback function that will be called when new data are sent.

Implementation

Message definition

The messages are describe in conf/messages.xml like any other messages in paparazzi. Name are unique, IDs starts from 0. "type" field should be a real C type. Field name might be useless.

<class name="airborne">
 <message name="DATA" id="0">
  <field name="a" type="float"/>
  <field name="b" type="struct bla *"/>
 </message>
 ...
</class>

Code generation

The generated code will be in var/include/abi_messages.h and include some structure definition from sw/airborne/subsystems/abi_common.h. Bind and Send functions are generated, as well as callback type definition. A linked list is used to store the binded callbacks for each message. The head of the list is in an array to allow a fast access.

// Code in abi_common.h
// Here, include header custom structures you may want to use in ABI messages
// Default is to include pprz algebra libraries

typedef void (*abi_callback)(void); // Generic callback definition

struct _abi_event_struct {
 abi_callback cb;
 struct _abi_event_struct * next;
};
typedef struct _abi_event_struct abi_event;

#ifdef ABI_C
#define EXTERN
#else
#define EXTERN extern
#endif

#define ABI_FOREACH(head,el) for(el=head; el; el=el->next)
#define ABI_PREPEND(head,new) { (add)->next = head; head = add; }

// Code generated in var/include/abi_messages.h
#include "subsystems/abi_common.h
// Message IDs
#define DATA_ID 0

// Callbacks
typedef void (*abi_callbackDATA)(const float a, const struct bla * b); // Specific callback for DATA message (arguments are const to prevent modifying them)

// Array and linked list
#define ABI_MESSAGE_NB <highest id of the messages in airborne class + 1>

EXTERN abi_event abi_queues[ABI_MESSAGE_NB]; // Magic trick to avoid generating .c file

// Bind and Send for each messages
static inline void AbiBindMsgDATA(abi_event * ev, abi_callbackDATA cb) {
 ev->cb = (abi_callback)cb; // Store callback
 ABI_PREPEND(abi_queues[DATA_ID],ev); // add to the head of the list (because I'm lazy)
}

static inline void AbiSendMsgDATA(float a, struct bla * b) {
 // Call all callback functions
 abi_event* e;
 ABI_FOREACH(abi_queues[DATA_ID],e) {
  abi_callbackDATA cb = (abi_callbackDATA)(e->cb); // C black magic
  cb(a,b);
 }
}

Airborne code for main AP

This part of the code should be written and called only once in a .c file (like main.c, but it can be somewhere else)

#define ABI_C 1

#include "abi_messages.h"

Airborne code for subscriber

Include header and declare an abi_event as a global variable. Write the callback function with the proper prototype.

#include "abi_messages.h"

abi_event ev;

void data_cb(const float a, const struct bla * b) {
 // do something here
}

In the initialization function (or later) call the binding functions for the message you want to receive.

AbiBindMsgDATA(&ev, data_cb);

Airborne code for publisher

Include header of course.

#include "abi_messages.h"

In any function, call the send function.

float var = 2.;
struct bla s;
AbiSendMsgDATA(var,&s);

That's it !

A dev branch of ABI with a test program is in the github repository git://github.com/gautierhattenberger/paparazzi.git, branch abi