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File Descriptors are non-negative integers that act as an abstract handle to “Files” or I/O resources (like pipes, sockets, or data streams). These descriptors help us interact with these I/O resources and make working with them very easy. The I/O system is visible to a user process as a stream of bytes (I/O stream). A Unix process uses descriptors (small unsigned integers) to refer to I/O streams. The system calls related to the I/O operations take a descriptor as as argument.
Valid file descriptor ranges from 0 to a max descriptor number that is configurable (ulimit, /proc/sys/fs/file-max). Kernel assigns desc. for std input(0), std output(1) and std error(2) of the FD table. If a file open is not successful, fd return -1. Image may be NSFW.
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When a process makes a successful request to open a file, the kernel returns a file descriptor which points to an entry in the kernel's global file table. The file table entry contains information such as the inode of the file, byte offset, and the access restrictions for that data stream (read-only, write-only, etc.).
