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 kernal space we use symbol table .Once we EXPORT a module to symbol table it become a part of kerner .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.
