FD(4)                  FreeBSD 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
     existing 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
     ignored.

IMPLEMENTATION NOTES
     By default, /dev/fd is provided by devfs(5), 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(5) on /dev/fd.

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

SEE ALSO
     tty(4), devfs(5), fdescfs(5)

FreeBSD 13.1-RELEASE-p6          June 9, 1993          FreeBSD 13.1-RELEASE-p6