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 described in conf/abi.xml analoguous to 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/modules/core/abi_common.h (sw/airborne/modules/core/abi.h is a convenience header that only includes 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.
 // 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 {
 uint8_t id;
 abi_callback cb;
 struct _abi_event_struct * next;
 };
 typedef struct _abi_event_struct abi_event;
 
 #ifdef ABI_C
 #define ABI_EXTERN
 #else
 #define ABI_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 "modules/core/abi_common.h"
 // Message IDs
 #define DATA_ID 0
 
 // Callbacks
 typedef void (*abi_callbackDATA)(uint8_t sender_id, 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(uint8_t sender_id, abi_event * ev, abi_callbackDATA cb) {
 ev->id = sender_id; // Store sender ID
 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(uint8_t sender_id, const float * a, const struct bla * b) {
 // Call all callback functions
 abi_event* e;
 ABI_FOREACH(abi_queues[DATA_ID],e) {
 if (e->id == ABI_BROADCAST || e->id == sender_id) { // call function only if selected source or broadcast address
 abi_callbackDATA cb = (abi_callbackDATA)(e->cb); // C black magic
 cb(sender_id, 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 "modules/core/abi.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 "modules/core/abi.h"
 
 abi_event ev;
 
 void data_cb(uint8_t sender_id, 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. ABI_BROADCAST is a macro equals to 0 and is used as a broadcast address, which means that input values will be received from all sources for this message.
 AbiBindMsgDATA(ABI_BROADCAST, &ev, data_cb);
== Airborne code for publisher ==
Include header of course.
 #include "modules/core/abi.h"
In any function, call the send function. Sender id 0 is reserved (broadcast address).
 #define SENDER_ID 1
 float var = 2.;
 struct bla s;
 AbiSendMsgDATA(SENDER_ID, &var, &s);
That's it !
Category:Developer_Documentation