Index for Section 4

Alphabetical listing for F

Bottom of page

fd(4)
NAME
  fd - 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 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:

   1.  Create the directory using the mkdir and chmod commands:
	    mkdir /dev/fd; chmod 777 /dev/fd

   2.  Mount the file system manually using the mount command:
	    mount -t fdfs /dev/fd /dev/fd

   3.  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.

   4.  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:

  [EBADF]   The file descriptor is not valid.

RELATED INFORMATION
  Commands: chmod(1), mkdir(1), mount(8).

  Functions: creat(2), dup(2), open(2).

Index for Section 4

Alphabetical listing for F

Top of page