fd - file descriptor files
The /dev/fd file system is a pseudo-file system layered
beneath the Virtual File System (VFS). The file descriptor
files (fd*) are those files that are accessible
through file descriptors. The file descriptors use the
naming convention /dev/fd/0, /dev/fd/1, /dev/fd/2 and so
on up to any number.
To make the /dev/fd file system known to the operating
system, you must create the directory with the correct
privileges, then you must mount the file system. The following
steps describe how to create the directory, mount
the file system both manually and automatically, and how
to dismount the file system: Create the directory using
the mkdir and chmod commands: mkdir /dev/fd; chmod 777
/dev/fd Mount the file system manually using the mount
command: mount -t fdfs /dev/fd /dev/fd Mount the file system
automatically by editing either the /etc/fstab file or
the /sbin/bcheckrc file.
Add the following entry to the /etc/fstab file:
/dev/fd /dev/fd fdfs rw 0 0 This entry mounts the
pseudodevice /dev/fd on the /dev/fd directory with
read/write privileges. The file system type is
fdfs and the zeros (0) in the remaining fields
specify that the file system is not to be backed up
nor can file system checks be performed by the fsck
command as this is a virtual file system.
Add the following entry to the /sbin/bcheckrc file:
# # mount fdfs # echo 'Mounting /dev/fd filesystem'
/sbin/mount -a -v -t fdfs Again, the /dev/fd file
system should not be mounted in this manner if an
entire system is to be backed up starting from the
root directory. Dismount the file system using the
umount command: umount /dev/fd For correct truncate()
behavior on fd files, you must load your
program using the -lsys5 flag.
The /dev/fd file descriptors should not be exported.
The following example show how the open and dup functions
have the same effect if file descriptor n is opened:
fd = open("/dev/fd/n", mode);
fd = dup(n);
In the above example, the open function is equal to the
creat function and mode is ignored. Using the dup function,
subsequent reads or writes on the fd file descriptor
files fail unless the original file descriptor enables the
The following error condition exists: The file descriptor
is not valid.
Commands: chmod(1), mkdir(1), mount(8).
Functions: creat(2), dup(2), open(2). delim off
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