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FD(4)			     Kernel Interfaces Manual			   FD(4)

NAME
     fd, stdin, stdout, stderr -- file descriptor files

DESCRIPTION
     The  files  /dev/fd/0 through /dev/fd/# refer to file descriptors which can
     be accessed through the file system.  If the file descriptor  is  open  and
     the  mode	the file is being opened with is a subset of the mode of the ex-
     isting descriptor, the call:

	   fd = open("/dev/fd/0", mode);

     and the call:

	   fd = fcntl(0, F_DUPFD, 0);

     are equivalent.

     Opening the files /dev/stdin, /dev/stdout and /dev/stderr is equivalent  to
     the following calls:

	   fd = fcntl(STDIN_FILENO,  F_DUPFD, 0);
	   fd = fcntl(STDOUT_FILENO, F_DUPFD, 0);
	   fd = fcntl(STDERR_FILENO, F_DUPFD, 0);

     Flags  to the open(2) call other than O_RDONLY, O_WRONLY and O_RDWR are ig-
     nored.

IMPLEMENTATION NOTES
     By default, /dev/fd is provided by devfs(4), which provides nodes	for  the
     first  three file descriptors.  Some sites may require nodes for additional
     file descriptors; these can be made available  by	mounting  fdescfs(4)  on
     /dev/fd.

FILES
     /dev/fd/#
     /dev/stdin
     /dev/stdout
     /dev/stderr

SEE ALSO
     devfs(4), fdescfs(4), tty(4)

FreeBSD ports 15.1		  June 9, 1993				   FD(4)

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<https://man.freebsd.org/cgi/man.cgi?query=stderr&sektion=4&manpath=FreeBSD+15.1-RELEASE+and+Ports>

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