                 LinPac - Packet Radio Terminal for Linux
               --------------------------------------------
                               Version 0.06

                  (c) 1998 - 1999 by Radek Burget OK2JBG


                  Extrenal Application Programming Guide




Contents

1  Introduction

2  What is an extrenal program

3  How do applications communicate with LinPac

4  Using the application library
   4.1 The simplest application

5  Application programming
   5.1 Events
   5.2 Sending and receiving events
   5.3 Synchronization
   5.4 Shared memory
   5.5 Connection status

6  The application library interface
   6.1 Constants
   6.2 Data structures
   6.3 Global variables
   6.4 Functions
       6.4.1 Uninterruptable versions of some system calls
       6.4.2 Basic communication functions
       6.4.3 Automatic event handling functions
       6.4.4 Environment functions
       6.4.5 User functions
       6.4.6 Tool functions


1  Introduction

  This guide is written for programmers who want to add some new functions
to LinPac. In following text the basic knowledge about Linux programming is
assumed. It's also recommended to read the user manual first.
  

2  What is an external program

  An external program is a standard Linux application which uses LinPac to
communicate with remote user. There are basicaly two types of LinPac
external programs:

  - Normal programs that can be used without LinPac too. LinPac allows to
    redirect the input stream (stdin) of the application and any of the
    output streams (stdout, stderr) or both of them. LinPac can also provide
    the CR/LF conversions in this streams. This parametres are set when
    adding the external program to LinPac - see the section 7 "Creating new
    commands" in the user manual.

  - Application written using the application interface of LinPac. This
    applications can share some information with LinPac and they can control
    almost all functions of LinPac.

There are no specialities when creating an application of the first type.
Just write the program to work on the Linux console and add it to LinPac.
Following sections of this guide are dedicated to the second type of
applications.


3  How do applications communicate with LinPac

  There are two types of communication between LinPac and the application:

  - shared memory: LinPac holds some information like connected station
    callsigns, connection status and internal variables in the shared memory
    block. Each application can attach this data and read or modify them.

  - named pipes: There are two named pipes, one for reading data from
    application and other for sending data to application.

  Shared memory and the pipes are maintained by the application interface
  library and shouldn't be contrlled directly.


4  Using the application library

  During LinPac installation the application library 'liblinpac' is created
and installed by default to /usr/local/lib. The interface to this library is
contained in the file 'lpapp.h' and it's installed by default to
/usr/local/include/linpac. Next chapter shows how to use the library with
the user program.

4.1  The simplest application

  Following application example just tries to contact LinPac and prints the
result.

---------------------------- File test1.cc ---------------------------------
#include <stdio.h>
#include <unistd.h>
#include <linpac/lpapp.h>

int main()
{
  if (start_appl(LP_PIPE_PATH))
  {
    printf("Application started\n");
    sleep(1);
    printf("Application finished\n");
    end_appl();
  }
  else
  {
    printf("LinPac is not running\n");
    return 1;
  }
  return 0;
}
----------------------------------------------------------------------------

The function 'start_appl()' tries to contact LinPac and returns 1 in case of
succes or 0 when LinPac cannot be connected (probably it's not running).
This function should precede the usage of any other application library
function. The LP_PIPE_PATH constant contains the path to LinPac named pipe.

The function 'end_appl()' closes the connection to LinPac.

How to compile this example:

          gcc -o test1 test1.cc -llinpac

  This example just detects if LinPac is running and it can be executed
directly from the shell. When running from the shell, no streams are
redirected to LinPac and the application seems to run on channel 0 of
LinPac. It's useful for some applications that are used to control linpac
from outside. However it's not a typical case.

  For most of the applications it's better to copy the executable
to the $LINPACDIR/bin directory and add it to the file
$LINPACDIR/bin/commands as described in the user manual. After this the
application can be executed as the LinPac command. In this case the streams
are properly redirected and the application output is visible in LinPac
window. It's also possible to select the channel for running the
application.


5  Application programming

5.1 Events

  LinPac is completely driven by events. Each part of LinPac including the
application can generate the event to inform other parts (internal modules
or applications) that something has happend. Each event is sent to all
LinPac components and application. For example when some station connects to
some LinPac channel, the internal AX.25 interface generates the event
reporting that the station has connected and includes its callsign.
All components and applications now know who has connected and they can do
some actions (the output window prints the information about the connect,
the macro processor executes the cinit.mac macro ...). Each application can
handle all the events too and it can generate events which are handled by
other components.

  The event is represented by the following structure:

struct Event
{
   int type;
   int chn;
   int x,y;
   char ch;
   void *data;
} Event;

  The meaning of each field is following:

  type - Determines the type of the event. Actually it says what happend.
         There is a symbolic constant defined for each known event.
  chn  - It says the channel for which the event applies (for example if the
         type of the event reports some data received, the chn field
         contains the number of the channel which has received data). There
         are many events that apply for all the channels. For this events
         this field is not signigicant.
  x, y - The meaning of field depends on the event type. The y field is
         usually not used (it's used by some internale events only).
  ch   - This field is used by some internal events only.
  data - Depends on the type of the event too. It usually points to some
         string data or a char buffer.

All the event types are described in the file events.txt


5.2 Sending and receiving events

  For sending events the function

  int emit_event(int chn, int type, int x, void *data);

  is used. This generates new event using specified values. Each argument
corresponds with one of the fields in the Event structure.

  There are two modes of handling the incomming events:

a) Reading each event on demand
  This mode is started by the event_handling_off() call. In this mode events
are read using the function
  
  int get_event(Event *ev);

This function returns 0 when no event is available. When there is some event
available, it returns 1 and fills the Event structure with the received
event data. 
WARNING1: The 'data' field in your Event structure must point to some
          dynamicaly allocated buffer. The size of the buffer is reallocated
          automaticaly after receiving an event. When the 'data' field is
          set to NULL, new buffer is allocated. This field _must_not_ be
          uninitialized.
WARNING2: The application _must_ read all events in this mode. It's not a
          good idea to stop reading the events because the event queue can
          overflow and cause serious problems.

b) Automatical event processing
  This mode is started by the event_handling_on() call. All the events are
read automaticaly. The user can define his own function that is called
automaticaly when an event occurs. When there's no such function defined,
all events are discarted.
  
  The event handling function must have following prototype:

  void some_function(Event *ev);

(the function name can be different). After initializing the application the
event handling function must be registered using the function
set_event_handler() from the apllication library.

Following example is an application that prints the types of all events
received and stops when an event EV_ABORT is received. This event can be
generated using the ':ABort' command in LinPac.

----------------------------------------------------------------------------
#include <stdio.h>
#include <linpac/lpapp.h>

int aborted = 0;

//User event handling function. This function is called each time
//an event occurs
void my_event_handler(Event *ev)
{
  printf("The event of type %i has been received\n", ev->type);
  if (ev->type == EV_ABORT) aborted = 1;
}

int main()
{
  if (start_appl(LP_PIPE_PATH))
  {
    event_handling_on();  //turn on automatical event handling
    set_event_handler(my_event_handler); //define own event handler
    
    printf("Application started\n");
    printf("Stop with the ':Abort' command\n");

    do ; while(!aborted); //wait until application is aborted

    printf("Application finished\n");
    
    end_appl();
  }
  else
  {
    printf("LinPac is not running\n");
    return 1;
  }
  return 0;
}
----------------------------------------------------------------------------

WARNING: Note that some system calls can be interrupted when the event is
         received. Interrupted system call returns the error result and
         sets errno to EAGAIN (for example the read() call returns -1) and
         it must be called again. To avoid this use the interrupt-safe
         versions of the system calls contained in the application library
         (see chapter 6.3.1)


5.3 Synchronization

  The event generated by an applicatoin is sent to all the modules and
applications including the application that has generated the event. When
there's the need to wait until the event is accepted by LinPac, the simplest
way is to wait until the event we have sent is received back.

  For testing that all the events were processed there is an event EV_VOID.
It's not handled by any module. After sending all events just generate the
EV_VOID event and wait until it returns. After that it's sure that all
previous events have been processed.


5.4 Shared memory

  All the shared data are represented by folowing structure:

typedef struct
{
  char call[MAX_CHN+1][10];       /* callsign for each channel */
  char cwit[MAX_CHN+1][10];       /* callsign of connected station */
  char cphy[MAX_CHN+1][10];       /* physical connection to */
  int port[MAX_CHN+1];            /* connected on which port (0..n) */
  char descript[MAX_CHN+1][128];  /* reserved (currently not used) */
  int state[MAX_CHN+1];           /* connection status */
  char env[MAX_CHN+1][ENV_SIZE];  /* channel environment */
} shared_data;

The constants MAX_CHN and ENV_SIZE are defined in the library and shouldn't
be changed. The fields have following meaning:

  call - callsign of the channel that was set using the :mycall command
  cwit - callsign of station connected to the channel
  cphy - callsign of the station we are physicaly connected to. In case of
         direct connection cphy is equal to cwit.
  port - port used for the connection. 0 means the first port in axports, 
         1 is the second one...
  state - connection status. Following states can occur:
                 ST_DISC - disconnected
                 ST_DISP - disconnecting
                 ST_TIME - disconnecting for timeout
                 ST_CONN - connected
                 ST_CONP - connecting in progress
  env - the variable environment for the channel. This contains the list of
        strings NAME=VALUE separated by '\0'. End of the environment is
        makred with two characters '\0'. The application library provides
        some functions for working with this environment.

When the application is initialized, the start_appl() function creates
following pointer:

shared_data *shd

that points to the shared structure.
The contents of the structure is managed by LinPac and it's not recommended
to modify the fields within an application (except the environment and
some special cases). Beter way to change this fields is to generate
appropriate event (e.g. EV_CALL_CHANGE).


5.5 Connection status

There are two special events reserved for obtaining the AX.25 connection 
status. When the application wants to get the status of the connection on
certain LinPac channel, it generates the EV_STAT_REQ event on this channel.
As the answer LinPac generates the EV_STATUS event. The 'data' field of
this event points to the ax25_status structure (see chapter 6.2). When
there is no active connection on the channel, no EV_STATUS event is
generated.


6  The application library interface

6.1 Constants

LPAPP_VERSION - version of LinPac that the library came with
MAX_CHN       - number of regular LinPac's channels
ENV_SIZE      - environment size for channel
AXPORTS       - path to axports file

ST_xxxx       - connection status constants (see chapter 5.4)

6.2 Data structures

struct ax25_status - contains the AX.25 connection status:

typedef struct
{
  char devname[8];
  int state;
  int vs, vr, va;
  int t1, t2, t3, t1max, t2max, t3max;
  int idle, idlemax;
  int n2, n2max;
  int rtt;
  int window;
  int paclen;
  bool dama;
  int sendq, recvq;
} ax25_status;


6.3 Global variables

shared data *shd - pointer to shared structure (see chapter 5.4)
int app_chn      - channel number this application is running on
int app_pid      - the PID of this application

6.4 Functions

6.4.1 Uninterruptable versions of some system calls

Following functions work the same way as the original system calls, but they
are interrupt-safe (do not fail with errno == EAGAIN).

size_t safe_read(int fd, void *buf, size_t count);
size_t safe_write(int fd, const void *buf, size_t count);
char *safe_fgets(char *s, int size, FILE *stream);
int safe_fgetc(FILE *stream);

6.4.2 Basic communication functions

int start_appl(char *pipename)
  Starts the communication with LinPac. The pipename parameter contains the
name of the named pipe used for communication (use LP_PIPE_PATH here).
Non-zero return value means success, zero value means that LinPac cannot be
contacted (probably it's not running).

int get_event(Event *ev)
  Read the event from the queue. Non-zero return value means succesful read,
zero value means that the event queue is empty. The 'data' field of the
event structure _must_ be initialized before using this function (to NULL or
to some buffer). This function shouldn't be used when automatic event
processing is used.

int emit_event(int chn, int type, int x, void *data)
  Generate new event. The arguments correspond with the fields in the event
structure. Return value is always 0.

void wait_event(int chn, int type)
  Wait until the event with the same 'chn' and 'type' values is received.

void discard_event(Event *ev)
  Free the memory used by the data field of Event structure received 
using get_event().

void clear_pipe()
  Removes all events from the event queue. This has no use when automatic
event processing is on.

void end_appl()
  Closes the connection to LinPac.


6.4.3 Automatic event handling functions

void event_handling_on()
  Switches the automatic event handling on. From this point each event is
automaticaly read from the queue, treated with an event handler function
(if defined) and discarted.

void event_handling_off()
  Switches the automatic event handling off. Events must be read from the
queue using the get_event() function.

void set_event_handler(handler_type handler)
  Defines the event handler function - a function like
   void my_handler(Event *ev)

  The event handler is called automaticaly each time some event is received
and the automatic event handling is on.


6.4.4 Environment functions

void set_var(int chn, char *name, char *contents)
  Change the value of the variable. 'name' is the name of the variable,
'contents' is the new value. 'chn' is the channel number (0..MAXCHN)
When the variable doesn't exist, it's created.

void del_var(int chn, char *var)
  Delete the variable. 'var' is the pointer to the begining of the variable
in the environment (pointer to the statement NAME=VALUE)

char *find_var(int chn, char *name)
  Returns the poiner to the begining of the variable in channel environment.

char *get_var(int chn, char *name)
  Returns the pointer to the value of the variable. 'name' is the name of
the variable.

char *env_end(int chn)
  Returns the pointer to end of the environment (behind the last variable).

char *clear_var_names(int chn, char *name)
  Delete all variables for which the contents of 'name' is the left
substring of their name. ($name*)


6.4.5 User functions

void appl_result(const char *fmt, ...)
  Set the result of the application. This function generates the
EV_APP_RESULT event with the message string. The argument format is the same
as for printf()

void statline(const char *fmt, ...)
  Displays or changes the additional status line. Using this function can be
displayed one status line only. This function generates the EV_CHANGE_STLINE
event with the x field (line ID) containing the PID of the application.
For displaying more than one status line for the application other
EV_CHANGE_STLINE events must be generated manualy.

void remove_statline()
  Removes the status line.

void disable_screen()
  Disables displaying the data in the QSO window on application's channel.
The EV_DISABLE_SCREEN event is used.

void enable_screen()
  Enables displaying the data in the QSO window. The EV_ENABLE_SCREEN event
is generated.


6.4.6 Tool functions

char *time_stamp(int utc)
  Returns the pointer to a c-string that contains actual time. If utc=0 then
local time is used else the UTC time is used.

char *date_stamp(int utc)
  Returns the date-string.

void replace_macros(int chn, char *s)
  Replaces the variables in the string (%xxx) with their values. The
%(command) macro is not replaced.

void get_port_name(int n)
  Returns the name of the n-th port in axports (starting with 0).
