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
^The Open Group. "The Open Group Base Specifications Issue 7, IEEE Std 1003.1-2008, 2016 Edition". Retrieved 2017-09-21.
^The Open Group. "The Open Group Base Specifications Issue 7, IEEE Std 1003.1-2008, 2016 Edition". <stdio.h>. Retrieved 2017-09-21.
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