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loop.4: Edits to Urs Thuermann's page
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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Michael Kerrisk
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man4/loop.4
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man4/loop.4
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.\" loop.4
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.\" Copyright 2002 Urs Thuermann (urs@isnogud.escape.de)
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.\"
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.\" %%%LICENSE_START(GPLv2+_DOC_FULL)
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.\" This is free documentation; you can redistribute it and/or
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.\" modify it under the terms of the GNU General Public License as
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.\" published by the Free Software Foundation; either version 2 of
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@ -20,28 +20,30 @@
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.\" License along with this manual; if not, write to the Free
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.\" Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111,
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.\" USA.
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.\"
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.\" $Id: loop.4,v 1.2 2005/12/05 19:03:01 urs Exp urs $
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.\" %%%LICENSE_END
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.\"
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.TH LOOP 4 "2005-12-05" "Linux" "Linux Programmer's Manual"
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.SH NAME
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loop \- Driver for loop devices
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loop \- driver for loop devices
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.SH SYNOPSIS
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#include <linux/loop.h>
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...
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.SH DESCRIPTION
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The loop device is a block device that maps its data blocks not to a
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physical device like a hard disk or CDROM drive, but to the blocks of
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a regular file in a file system or to another block device. This can
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be useful for example to provide a block device for a file system
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physical device such as a hard disk or optical disk drive,
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but to the blocks of
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a regular file in a filesystem or to another block device.
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This can be useful for example to provide a block device for a filesystem
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image stored in a file, so that it can be mounted with the
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.BR mount (8)
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command. You could do
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command.
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You could do
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.nf
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.IP
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# dd if=/dev/fd0 of=floppy.img
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# losetup /dev/loop0 floppy/img
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# mount /dev/loop0 /mnt
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$ \fBdd if=/dev/zero of=file.img bs=1MiB count=10\fP
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$ \fBsudo losetup /dev/loop4 file.img \fP
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$ \fBsudo mkfs -t ext4 /dev/loop4\fP
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$ \fBsudo mkdir /myloopdev\fP
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$ \fBsudo mount /dev/loop4 /myloopdev\fP
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.fi
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.LP
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See
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@ -51,38 +53,49 @@ for another example.
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A transfer function can be specified for each loop device for
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encryption and decryption purposes.
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The following ioctl's are provided by the loop block device:
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The following
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.BR ioctl (2)
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operations are provided by the loop block device:
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.TP
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.B LOOP_SET_FD
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Associate the loop device with the open file whose file descriptor is
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passed as the ioctl command argument.
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passed as the (third)
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.BR ioctl (2)
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argument.
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.TP
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.B LOOP_CLR_FD
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Disassociate the loop device from any file descriptor.
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.TP
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.B LOOP_SET_STATUS
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Set the status of the loop device as defined by the struct loop_info
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passed as command argument. The loop_info structure is defined in
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<linux/loop.h> as
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.PP
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.RS
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Set the status of the loop device as defined by the
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.IR "struct loop_info"
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passed as the (third)
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.BR ioctl (2)
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argument.
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The
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.I loop_info
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structure is defined in
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.I <linux/loop.h>
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as:
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.nf
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.ft B
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.in +4n
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struct loop_info {
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int lo_number; /* ioctl r/o */
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dev_t lo_device; /* ioctl r/o */
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unsigned long lo_inode; /* ioctl r/o */
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dev_t lo_rdevice; /* ioctl r/o */
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int lo_offset;
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int lo_encrypt_type;
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int lo_encrypt_key_size; /* ioctl w/o */
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int lo_flags; /* ioctl r/o */
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char lo_name[LO_NAME_SIZE];
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unsigned char lo_encrypt_key[LO_KEY_SIZE]; /* ioctl w/o */
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unsigned long lo_init[2];
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char reserved[4];
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int lo_number; /* ioctl r/o */
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dev_t lo_device; /* ioctl r/o */
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unsigned long lo_inode; /* ioctl r/o */
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dev_t lo_rdevice; /* ioctl r/o */
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int lo_offset;
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int lo_encrypt_type;
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int lo_encrypt_key_size; /* ioctl w/o */
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int lo_flags; /* ioctl r/o */
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char lo_name[LO_NAME_SIZE];
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unsigned char lo_encrypt_key[LO_KEY_SIZE];
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/* ioctl w/o */
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unsigned long lo_init[2];
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char reserved[4];
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};
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.ft R
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.in
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.fi
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The encryption type should be one of
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@ -95,46 +108,47 @@ The encryption type should be one of
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.BR LO_CRYPT_IDEA ,
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.BR LO_CRYPT_DUMMY ,
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.BR LO_CRYPT_SKIPJACK .
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.RE
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.TP
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.B LOOP_GET_STATUS
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Get the status of the loop device. The ioctl command argument must be
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a pointer to a struct loop_info.
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Get the status of the loop device.
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The (third)
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.BR ioctl (2)
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argument must be a pointer to a
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.IR "struct loop_info" .
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.PP
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Since Linux kernel 2.6 there are two new ioctl's:
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Since Linux 2.6, there are two new
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.BR ioctl (2)
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operations:
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.TP
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.BR LOOP_SET_STATUS64 ", " LOOP_GET_STATUS64
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These are similar to
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.BR LOOP_SET_STATUS " and " LOOP_GET_STATUS
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described above but use the loop_info64 structure, that has some
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additional fields and a larger range for some other fields:
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.PP
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.RS
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.nf
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.ft B
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struct loop_info64 {
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__u64 lo_device; /* ioctl r/o */
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__u64 lo_inode; /* ioctl r/o */
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__u64 lo_rdevice; /* ioctl r/o */
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__u64 lo_offset;
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__u64 lo_sizelimit;/* bytes, 0 == max available */
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__u32 lo_number; /* ioctl r/o */
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__u32 lo_encrypt_type;
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__u32 lo_encrypt_key_size; /* ioctl w/o */
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__u32 lo_flags; /* ioctl r/o */
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__u8 lo_file_name[LO_NAME_SIZE];
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__u8 lo_crypt_name[LO_NAME_SIZE];
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__u8 lo_encrypt_key[LO_KEY_SIZE]; /* ioctl w/o */
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__u64 lo_init[2];
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};
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.fi
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.RE
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described above but use the
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.I loop_info64
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structure,
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which has some additional fields and a larger range for some other fields:
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.nf
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.in +4n
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struct loop_info64 {
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uint64_t lo_device; /* ioctl r/o */
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uint64_t lo_inode; /* ioctl r/o */
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uint64_t lo_rdevice; /* ioctl r/o */
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uint64_t lo_offset;
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uint64_t lo_sizelimit;/* bytes, 0 == max available */
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uint32_t lo_number; /* ioctl r/o */
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uint32_t lo_encrypt_type;
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uint32_t lo_encrypt_key_size; /* ioctl w/o */
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uint32_t lo_flags; /* ioctl r/o */
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uint8_t lo_file_name[LO_NAME_SIZE];
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uint8_t lo_crypt_name[LO_NAME_SIZE];
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uint8_t lo_encrypt_key[LO_KEY_SIZE]; /* ioctl w/o */
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uint64_t lo_init[2];
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};
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.in
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.fi
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.SH FILES
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/dev/loop[0-7]: The loop block special device files
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.SH "SEE ALSO"
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.BR losetup (8),
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.BR mount (8)
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