File Descriptor

In Unix like system File descriptor is abstract indicator for accessing file and are represented in integer value.  FD is a computer program term and highly used while programming in Unix like system using language like C, This is similar to file handle in windows like system.

By default operating system 3 file descriptor namely from 0-2. Where 0 represent standard input (stdin), 1 for standard output (stdout) and 2 for standard errors (stderr).

Operating system keeps index entry of opened files somewhere in Kernal’s address space which can be generally referred as file descriptor table. When a user process use open() or socket() (system calls to interface to the kernel) you are returned a file descriptor, which is an integer (These entries are represented by integers like ...100, 101, 102....)  And are maintains its file descriptor table in its address space. When a process want to access the file it passes the file descriptor to kernel with system call which in turns reads the file on behalf and returns the handle or result.