Difference between revisions of "Abi"

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This page presents some idea for an embedded middleware named ABI based on publish/subscribe like Ivy.
#REDIRECT [[ABI]]
 
= 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. 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.
 
// Message IDs
#define DATA_ID 0
// Callbacks
typedef void (*abi_callback)(void); // Generic callback definition
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>
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
EXTERN abi_event abi_queues[ABI_MESSAGE_NB]; // Magic trick to avoid generating .c file
#define ABI_FOREACH(head,el) for(el=head; el; el=el->next)
#define ABI_PREPEND(head,new) { (add)->next = head; head = add; }
// 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 !

Latest revision as of 07:11, 26 November 2014

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