An integer that identifies an open file within a process. This number is obtained as a result of opening a file. Operations which read, write, or close a file would take the file descriptor as an input parameter.

In many operating system implementations, file descriptors are small integers which index a table of open files. In Unix, file descriptors 0, 1 and 2 correspond to the standard input, standard output and standard error files respectively.

See file descriptor leak.