Character Device Drivers:-
Character special files or character devices relate to devices through which the system transmits data one character at a time by, for example, getchar. These device nodes often serve for stream communication with devices such as mice, keyboards, virtual terminals, and serial modems, and usually do not support random access to data.
In most implementations, character devices use unbuffered input and output routines. The system reads each character from the device immediately or writes each character to the device immediately.
There are two major ways for a kernel module to talk to processes. One is through device files (like the files in the /dev directory), the other is to use the proc file system. Since one of the major reasons to write something in the kernel is to support some kind of hardware device, we’ll begin with device files.
The original purpose of device files is to allow processes to communicate with device drivers in the kernel, and through them with physical devices (modems, terminals, etc.). The way this is implemented is the following.
Each device driver, which is responsible for some type of hardware, is assigned its own major number. The list of drivers and their major numbers is available in /proc/devices. Each physical device managed by a device driver is assigned a minor number. The /dev directory is supposed to include a special file, called a device file, for each of those devices, whether or not it’s really installed on the system.
Kernel Symbol Table:-
In programming language, a symbol is either a variable or a function. Or more generally, we can say, a symbol is a name representing an space in the memory, which stores data (variable, for reading and writing) or instructions (function, for executing). To make life easier for cooperation among various kernel function unit, there are thousands of global symbols in Linux kernel.
In general,when we have to use some variable or function outside the scope we use EXTERN or Declare globally similarly in kernel space we use symbol table .Once we EXPORT a module to symbol table it become a part of kernel .On that kernel any user can import that module .The table contains the addresses of global kernel items—functions and variables—that are needed to implement modularized drivers. When a module is loaded, any symbol exported by the module becomes part of the kernel symbol table.you can stack new modules on top of other modules. Module stacking is implemented in the mainstream kernel sources as well.
