The call flow

Before we proceed further, let us have a look at the call flow. Asterisk calls the zaptel using system calls on the devices created by zaptel. A large chunk of such calls are ioctl calls.

To give an example, when a channel has to be configured, asterisk, through chan_zap.so, makes a ZT_CHANCONFIG ioctl. The user data is put into the form of zt_chanconfig structure and passed to the ioctl function.

zt_ctl_ioctl function is invoked as a result of this. This in turn does some generic operations such as error checking etc and then assigns the values from the user space structure to the kernel space structure.

When, there are device specific job to be done, zt_ctl_ioctl calls the device specific function that is already populated in the span structure that we used while registering our device with zaptel. In this particular case, they don't handle the clear function. For clear, they call the

res = chans[ch.chan]->span->chanconfig(chans[ch.chan], ch.sigtype);

In short, in this example we have seen
Asterisk <--> chan_zap.so <-> zaptel.ko (kernel) <-> device driver <-> Zaptel device <-> Phone/switch/PSTN