The Linux Telephony Kernel API

Herlein explains the integration of the telephony device driver into the Linux kernel.
Controlling Phone Devices—the ioctl System Call

Commands for the phone device to perform certain actions are not written to the phone device using a write call—only audio data is written to the device. To control the phone device, a set of ioctl functions are defined to handle the basic phone activities. This basic set of ioctl functions are defined in /usr/include/linux/telephony.h. Vendors who wish to extend that basic set of capabilities may do so, but those functions are limited to their own device driver and are outside the scope of the common Linux telephony API.

An example will best illustrate this potential. Using the telephony API with a Quicknet Internet PhoneJACK card and a phone plugged into the card (called an FXS port or POTS port by telephony folks), Listing 2 shows a brief program to ring the phone.

Listing 2. Ringing the Phone

You'll notice that the device is opened, defaulting to /dev/phone0 if the user does not provide a specific device name on the command line. The maximum number of rings is set with an ioctl call using the PHONE_MAXRINGS constant. The phone is then instructed to ring using the PHONE_RING ioctl. This example program is a simplified version of the LGPL module ring.c found in Quicknet's Software Developer Kit (SDK). It's too simple to do anything more than illustrate the technique and demonstrate the use of ioctls to control a phone device, but real-world programs need not be much more complicated, the API is fairly simple. All the Linux telephony API ioctl constants are defined in the header file /etc/include/linux/telephony.h.

It's this basic fact that allows software written for sound cards to be adapted easily for use with phone devices; like sound cards, the only data written using the read and write system calls is audio data. In addition, the low-level constants used in the defined telephony ioctl calls were designed to avoid conflict with existing sound card ioctls. Porting an application that expects a sound card to use a phone card instead will primarily involve handling the errors returned from the sound card ioctl calls your code makes. It should be possible (though perhaps not easy) to write a wrapper that opens the telephony device and spawns a child process (inheriting the open file descriptor to the phone device) that runs software expecting a sound card interface. While it's not totally transparent, it's possible and should not actually be that difficult.

Asynchronous Event Notification

Events that occur on the telephony side of the device need to be communicated to the user-space software running the phone. Old and crude methods for this required the software to poll the device continuously for status and changes. The Linux telephony API avoids that, of course, and provides two different techniques, both generically called “asynchronous event notification”. The first method uses signals for indication, and the second uses the “exception bit” in the file descriptor set for exceptions. I'll cover both techniques in order.

The use of signals for event notification requires three steps: first, prepare and declare a signal handler function for the SIGIO signal; second, set the process ID (PID) for the running process to receive the signal; third, enable the generation of the signal on the open files descriptor using the fcntl system call. An excellent description of these steps can be found in Chapter 12 of Advanced Programming in the UNIX Environment by W. Richard Stevens (a worthwhile book to have in any case). Again, a short example will probably clear this up. Assuming you have an open file descriptor ixj1 that is an open phone device, you can enable asynchronous event notification with signals with the following code snip:

signal(SIGIO, &getdata);
fcntl(ixj1, F_SETOWN, getpid());
oflags1 = fcntl(ixj1, F_GETFL);
fcntl(ixj1, F_SETFL, oflags1 | FASYNC);

and a related signal handling function (getdata in the code above) to process the data. When you get the signal, you still do not know what kind of event occurred—only that an event did occur. Your program would then have to make an ioctl call to the phone device to ask what kind of event was detected (more on this below). In addition, if your program has more than one open phone device file descriptor, you will not know which one generated the signal. And, signals can be complicated to deal with and can be unreliable in a multi-threaded program and, so, are avoided by some developers. These factors limit the effectiveness of this method, leading to the more useful case of using the “exception bit” in the file descriptor set for exceptions.

It's common for programs to set read and write exception sets and then use the select( ) system call to wait for a file descriptor to be readable or writable. A less well-known aspect of the select( ) call is the “exception set”. The Linux telephony API uses this exception set to signal a process that a telephony event has occurred. Listing 3 presents a simple example using the file descriptor ixj1.

Listing 3. Using an Exception Set

This extremely simple (and not really useful) example is purely to illustrate the technique of using the exception set and select( ) to detect events. The phone device driver will set the appropriate bit in the exception set if an event has occurred, causing select( ) to return. The user can screen the read, write and exception descriptor sets to determine if its own file descriptor was marked by the device driver as ready for that kind of operation. If data is ready to be read, the statement FD_ISSET(ixj1,&rfds) will return TRUE; FD_ISSET(ixj1,&wfds) statement returns TRUE if the device is ready to be written to. And, if a telephony event has occurred, the FD_ISSET(ixj1,&efds) will return TRUE. So, how do you detect what specific event occurred?

The API provides a special PHONE_EXCEPTION ioctl call and an associated telephony_exception structure to decode the return value. This call will set bits in the structure that indicate which of the telephony events occurred (there may have been several). In the above example “hookstate” bit is examined in the statement if(ixje.bits.hookstate), and if that bit is set it indicates a change of status. An ioctl call is then made to determine if the phone is on or off hook. Real-world code would have used a large select or an extended if-else-if ladder to examine the contents of ixje.bits after the PHONE_EXCEPTION ioctl call. A detailed explanation of how to use this technique is beyond the scope of this article, but please refer to the /usr/include/linux/telephony.h file for details of what kinds of events can be detected.

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