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)