Shady Asterisk

Hmm... you must be wondering why I call him so. Because he is of dubious character. He appears nice outside and what does he do within?

I got a feel of it when I was writing the channel. I wrote the skeleton channel and made an entry for that in extensions.conf.

exten => 200,1,Dial(Sarin/200)
exten => 200,n,Hangup()

From the console do "console dial 200". Then I got a message that "Couldn't call 200". I checked again and again to find what has gone wrong. Nothing as far as I could see.

Then I enabled debug mode. To enable the debug mode in asterisk, I had to do two things.

1.1 Edit the /etc/asterisk/logger.conf.
1.2 Look for a line that looks like "console => notice,warning,error"
1.3 Add debug to that list. "console => notice,warning,error,debug"
2. Now start asterisk with debug option. "asterisk -vvvdc"

(This for enabling debug prints on console)

Then, when I tried to call that number, I found that response from "ast_call" indicates a failure. And, what was causing it?

ast_call checks ast_check_hangup. Inside that function, there is this block of code:

if (!chan->tech_pvt) /* yes if no technology private data */
return 1;
Shame on you asterisk. You were checking the private elements of a channel!. You had no business poking around that pointer. It is private to the channel and it is upto the channel to decide to have it or not to have it (have it as NULL). If you wanted to do anything with it, you should not have called it pvt!

Anyway, I assigned a int pointer to it and then things went fine.

The channel woes

Today I was trying to register an asterisk channel. However my module registration failed with the error that module does not provide any description. I had half a mind to edit the line from Loader.c and load my module. However, I also noticed that I was compiling the module for one version (SVN trunk) and using it on some other version (1.4.15).

Then I downloaded asterisk from svn and compiled and installed it on some other machine. Then I just put in my simple module code, compiled, copied to /usr/lib/asterisk/modules/ and on the command line told "module load chan_thin" The stuff that was not happening for last two days just happened!

The sad part of my story is this. Till now the asterisk was giving me this message: "Module 'chan_thin.so' does not provide a description". I had tried various things including injecting the code of AST_MODULE_INFO macro into my module and adding checks on the functions used by that macro. I kept getting that vague error only because of some version change. (OK, it is still my mistake. Lot of things could have happened in a version change)

Anyway, now I have a module that goes in and does nothing. I have also checked oss channel driver in asterisk. I feel that is the most simple channel driver which I can use as a datum. More on it later. Before I end, let me put down a partial function call tree.

1. AST_MODULE_INFO -> Defines the module info and the register function. This registers ast_module_info
2. load_module -> This is made available to asterisk using ast_module_info structure. It is called when we load the module. For channel registration, ast_channel_register is called during load module. It passes a structure ast_channel_tech
3. ast_channel_tech -> This has all the callback functions. For OSS channel, the structure is as follows.

static struct ast_channel_tech oss_tech = {
        .type = "Console",
        .description = tdesc,
        .capabilities = AST_FORMAT_SLINEAR, /* overwritten later */
        .requester = oss_request,
        .send_digit_begin = oss_digit_begin,
        .send_digit_end = oss_digit_end,
        .send_text = oss_text,
        .hangup = oss_hangup,
        .answer = oss_answer,
        .read = oss_read,
        .call = oss_call,
        .write = oss_write,
        .write_video = console_write_video,
        .indicate = oss_indicate,
        .fixup = oss_fixup,
};

4. oss_request -> This is the function that is called by asterisk when a call comes to your channel. The number is passed to you as a parameter to the function.

Got to do few more things. Rest later.

Making of a channel

I need to answer 5 technical questions before I start making the channel

1. What does asterisk need from the channel? (What are the bare minimum features/functionalities of a channel?)
2. What all can asterisk take from the channel? (What is the largest possible set of features/functionalities that a channel can support out of the box?)
3. What are the features of the channel needed by the customer?
4. Is there any feature that a normal asterisk channel cannot support or it is very difficult to support?
5. How to add support to a new feature in channel?

I am searching the answer for the first question now.

Asterisk Channel

We have decided to make a new channel in Asterisk and almost completely move away from the zaptel channel. We had a discussion where we discussed how to proceed with the creation of the channel. In the discussion I was told that there is only a standard load_module function that is provided by Asterisk to define the channel. However, I was sure from my previous encounters with the channel that there needs to be something more than that and I even vaguely remembered that it is a structure.
Today I went back and had a look at the channel code and I found that channels infact use the structure ast_channel. I have not seen the code in detail. Will come back and post more once I see the channels in detail.