fd(4)fd(4)NAMEfd - file descriptor files
DESCRIPTION
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 descrip‐
tors 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 direc‐
tory, 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.
RESTRICTIONS
The /dev/fd file descriptors should not be exported.
EXAMPLES
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 operation.
ERRORS
The following error condition exists: The file descriptor is not valid.
RELATED INFORMATION
Commands: chmod(1), mkdir(1), mount(8).
Functions: creat(2), dup(2), open(2). delim off
fd(4)
