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The term "file descriptor" is generally used in POSIX Operating System s. In Microsoft Windows terminology and in the context of the C Standard I/O Library , "file handle" is preferred, though the standard I/O library file handle is technically a different object (see below). In POSIX, a file descriptor is an Integer , specifically of the C type int. There are 3 standard POSIX file descriptors which presumably every process (save perhaps a Daemon ) should expect to have: Generally, a file descriptor is an Index for an entry in a Kernel -resident data structure containing the details of all open files. In POSIX this data structure is called a file descriptor table, and each process has its own file descriptor table. The user application passes the abstract key to the kernel through a System Call , and the kernel will access the file on behalf of the application, based on the key. The application itself cannot read or write the file descriptor table directly. In Unix-like systems, file descriptors can refer to files, Directories , Block or Character Device s (also called "special files"), Socket s, FIFO s (also called Named Pipe s), or unnamed Pipe s.
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