I am trying to understand character special files. From wikipedia, I understand that these files "provide an interface" for devices that transmit data one character at a time. My understanding is that the system somehow calls the character device instead of calling the device driver directly. But how does the file provide this interface? Is it an executable that translates the system call? Can someone explain what's up.
They are actually just that - interfaces. Encoded by a "major" and "minor" number they provide a hook to the kernel.
They come in two flavors (well, three, but named pipes are out of the scope of this epxlanation for now): Character Devices and Block Devices.
Block Devices tend to be storage devices, capable of buffering output and storing data for later retrieval
Character Devices are things like audio or graphics cards, or input devices like keyboard and mouse.
In each case, when the kernel loads the correct driver (either at boot time, or via programs like udev) it scans the various buses to see if any devices handled by that driver are actually present on the system. If so, it sets up a device that 'listens' on the appropriate major/minor number.
(For instance, the Digital Signal Processor of the first audio card found by your system gets the major/minor number pair of 14/3; the second gets 14,35, etc.)
It's up to udev to create an entry in
(In significantly older or minimum-footprint versions of linux,
Then, when a userspace program tries to access a file that's marked as a 'character special file' with the appropriate major/minor number (for instance, your audio player trying to send digital audio to
Character devices can be created by kernel modules (or the kernel itself). When a device is created, the creator provides pointers to functions that implement handle standard calls like open, read, etc. The Linux kernel then associates those functions with the character device, so for example when a user-mode application calls the read() function on a character device file, it will result in a syscall and then the kernel will route this call to a read function specified when creating the driver. There's a step-by-step tutorial on creating a character device here, you can create a sample project and the step through it using a debugger to understand how the device object is created and when the handlers are invoked.
Every file, device or otherwise, supports 6 basic operations within the VFS:
Additionally, device files support I/O Control, which allows other miscellaneous operations not covered by the first 6.
In the case of a character special, seek and tell are not implemented since they support a streaming interface. That is, reading or writing directly such as is done with redirection in the shell: