fd(4)							     Kernel Interfaces Manual							     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: 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)