mirror of https://github.com/mkerrisk/man-pages
320 lines
9.0 KiB
Groff
320 lines
9.0 KiB
Groff
.\" Copyright 2002 Urs Thuermann (urs@isnogud.escape.de)
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.\" and Copyright 2015 Michael Kerrisk <mtk.manpages@gmail.com>
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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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.\" the License, or (at your option) any later version.
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.\"
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.\" The GNU General Public License's references to "object code"
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.\" and "executables" are to be interpreted as the output of any
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.\" document formatting or typesetting system, including
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.\" intermediate and printed output.
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.\"
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.\" This manual is distributed in the hope that it will be useful,
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.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
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.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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.\" GNU General Public License for more details.
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.\"
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.\" You should have received a copy of the GNU General Public
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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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.\" %%%LICENSE_END
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.\"
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.TH LOOP 4 2020-06-09 "Linux" "Linux Programmer's Manual"
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.SH NAME
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loop, loop-control \- loop devices
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.SH SYNOPSIS
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#include <linux/loop.h>
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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 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.
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You could do
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.PP
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.in +4n
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.EX
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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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.EE
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.in
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.PP
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See
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.BR losetup (8)
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for another example.
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.PP
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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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.PP
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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 (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 using the (third)
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.BR ioctl (2)
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argument.
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This argument is a pointer to
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.I loop_info
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structure, defined in
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.I <linux/loop.h>
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as:
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.IP
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.in +4n
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.EX
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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/w (r/o before
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Linux 2.6.25) */
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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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.EE
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.in
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.IP
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The encryption type
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.RI ( lo_encrypt_type )
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should be one of
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.BR LO_CRYPT_NONE ,
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.BR LO_CRYPT_XOR ,
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.BR LO_CRYPT_DES ,
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.BR LO_CRYPT_FISH2 ,
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.BR LO_CRYPT_BLOW ,
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.BR LO_CRYPT_CAST128 ,
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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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or (since Linux 2.6.0)
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.BR LO_CRYPT_CRYPTOAPI .
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.IP
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The
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.I lo_flags
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field is a bit mask that can include zero or more of the following:
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.RS
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.TP
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.BR LO_FLAGS_READ_ONLY
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The loopback device is read-only.
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.TP
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.BR LO_FLAGS_AUTOCLEAR " (since Linux 2.6.25)"
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.\" commit 96c5865559cee0f9cbc5173f3c949f6ce3525581
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The loopback device will autodestruct on last close.
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.TP
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.BR LO_FLAGS_PARTSCAN " (since Linux 3.2)"
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.\" commit e03c8dd14915fabc101aa495828d58598dc5af98
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Allow automatic partition scanning.
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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.
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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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.TP
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.BR LOOP_CHANGE_FD " (since Linux 2.6.5)"
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Switch the backing store of the loop device to the new file identified
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file descriptor specified in the (third)
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.BR ioctl (2)
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argument, which is an integer.
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This operation is possible only if the loop device is read-only and
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the new backing store is the same size and type as the old backing store.
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.TP
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.BR LOOP_SET_CAPACITY " (since Linux 2.6.30)"
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.\" commit 53d6660836f233df66490707365ab177e5fb2bb4
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Resize a live loop device.
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One can change the size of the underlying backing store and then use this
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operation so that the loop driver learns about the new size.
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This operation takes no argument.
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.TP
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.BR LOOP_SET_DIRECT_IO " (since Linux 4.10)"
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.\" commit ab1cb278bc7027663adbfb0b81404f8398437e11
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Set DIRECT I/O mode on the loop device, so that
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it can be used to open backing file.
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The (third)
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.BR ioctl (2)
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argument is an unsigned long value.
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A non-zero represents direct I/O mode.
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.TP
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.BR LOOP_SET_BLOCK_SIZE " (since Linux 4.14)"
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.\" commit 89e4fdecb51cf5535867026274bc97de9480ade5
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Set the block size of the loop device.
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The (third)
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.BR ioctl (2)
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argument is an unsigned long value.
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This value must be a power of two in the range
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[512,pagesize];
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otherwise, an
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.B EINVAL
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error results.
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.PP
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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
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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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.IP
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.in +4n
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.EX
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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; i /* ioctl r/w (r/o before
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Linux 2.6.25) */
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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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.EE
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.in
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.SS /dev/loop-control
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Since Linux 3.1,
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.\" commit 770fe30a46a12b6fb6b63fbe1737654d28e84844
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the kernel provides the
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.I /dev/loop-control
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device, which permits an application to dynamically find a free device,
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and to add and remove loop devices from the system.
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To perform these operations, one first opens
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.IR /dev/loop-control
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and then employs one of the following
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.BR ioctl (2)
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operations:
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.TP
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.B LOOP_CTL_GET_FREE
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Allocate or find a free loop device for use.
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On success, the device number is returned as the result of the call.
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This operation takes no argument.
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.TP
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.B LOOP_CTL_ADD
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Add the new loop device whose device number is specified
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as a long integer in the third
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.BR ioctl (2)
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argument.
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On success, the device index is returned as the result of the call.
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If the device is already allocated, the call fails with the error
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.BR EEXIST .
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.TP
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.B LOOP_CTL_REMOVE
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Remove the loop device whose device number is specified
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as a long integer in the third
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.BR ioctl (2)
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argument.
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On success, the device number is returned as the result of the call.
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If the device is in use, the call fails with the error
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.BR EBUSY .
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.SH FILES
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.TP
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.IR /dev/loop*
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The loop block special device files.
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.SH EXAMPLES
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The program below uses the
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.I /dev/loop-control
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device to find a free loop device, opens the loop device,
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opens a file to be used as the underlying storage for the device,
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and then associates the loop device with the backing store.
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The following shell session demonstrates the use of the program:
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.PP
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.in +4n
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.EX
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$ \fBdd if=/dev/zero of=file.img bs=1MiB count=10\fP
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10+0 records in
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10+0 records out
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10485760 bytes (10 MB) copied, 0.00609385 s, 1.7 GB/s
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$ \fBsudo ./mnt_loop file.img\fP
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loopname = /dev/loop5
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.EE
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.in
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.SS Program source
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\&
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.EX
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#include <fcntl.h>
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#include <linux/loop.h>
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#include <sys/ioctl.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <unistd.h>
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#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \e
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} while (0)
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int
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main(int argc, char *argv[])
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{
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int loopctlfd, loopfd, backingfile;
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long devnr;
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char loopname[4096];
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if (argc != 2) {
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fprintf(stderr, "Usage: %s backing\-file\en", argv[0]);
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exit(EXIT_FAILURE);
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}
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loopctlfd = open("/dev/loop\-control", O_RDWR);
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if (loopctlfd == \-1)
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errExit("open: /dev/loop\-control");
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devnr = ioctl(loopctlfd, LOOP_CTL_GET_FREE);
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if (devnr == \-1)
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errExit("ioctl\-LOOP_CTL_GET_FREE");
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sprintf(loopname, "/dev/loop%ld", devnr);
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printf("loopname = %s\en", loopname);
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loopfd = open(loopname, O_RDWR);
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if (loopfd == \-1)
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errExit("open: loopname");
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backingfile = open(argv[1], O_RDWR);
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if (backingfile == \-1)
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errExit("open: backing\-file");
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if (ioctl(loopfd, LOOP_SET_FD, backingfile) == \-1)
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errExit("ioctl\-LOOP_SET_FD");
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exit(EXIT_SUCCESS);
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}
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.EE
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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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