File descriptor

In Unix and Unix-like computer operating systems, a file descriptor (FD, less frequently fildes) is a process-unique identifier (handle) for a file or other input/output resource, such as a pipe or network socket.

File descriptors typically have non-negative integer values, with negative values being reserved to indicate "no value" or error conditions.

File descriptors are a part of the POSIX API. Each Unix process (except perhaps daemons) should have three standard POSIX file descriptors, corresponding to the three standard streams:

Integer value Name <unistd.h> symbolic constant[1] <stdio.h> file stream[2]
0 Standard input STDIN_FILENO stdin
1 Standard output STDOUT_FILENO stdout
2 Standard error STDERR_FILENO stderr
  1. ^ The Open Group. "The Open Group Base Specifications Issue 7, IEEE Std 1003.1-2008, 2016 Edition". Retrieved 2017-09-21.
  2. ^ The Open Group. "The Open Group Base Specifications Issue 7, IEEE Std 1003.1-2008, 2016 Edition". <stdio.h>. Retrieved 2017-09-21.