2015-05-01 11:47:02 +00:00
|
|
|
.\" Copyright 2002 Urs Thuermann (urs@isnogud.escape.de)
|
2015-05-01 13:03:50 +00:00
|
|
|
.\" and Copyright 2015 Michael Kerrisk <mtk.manpages@gmail.com>
|
2015-05-01 11:47:02 +00:00
|
|
|
.\"
|
|
|
|
.\" %%%LICENSE_START(GPLv2+_DOC_FULL)
|
|
|
|
.\" This is free documentation; you can redistribute it and/or
|
|
|
|
.\" modify it under the terms of the GNU General Public License as
|
|
|
|
.\" published by the Free Software Foundation; either version 2 of
|
|
|
|
.\" the License, or (at your option) any later version.
|
|
|
|
.\"
|
|
|
|
.\" The GNU General Public License's references to "object code"
|
|
|
|
.\" and "executables" are to be interpreted as the output of any
|
|
|
|
.\" document formatting or typesetting system, including
|
|
|
|
.\" intermediate and printed output.
|
|
|
|
.\"
|
|
|
|
.\" This manual is distributed in the hope that it will be useful,
|
|
|
|
.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
|
|
|
|
.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
|
|
|
|
.\" GNU General Public License for more details.
|
|
|
|
.\"
|
|
|
|
.\" You should have received a copy of the GNU General Public
|
|
|
|
.\" License along with this manual; if not, write to the Free
|
|
|
|
.\" Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111,
|
|
|
|
.\" USA.
|
|
|
|
.\" %%%LICENSE_END
|
|
|
|
.\"
|
2015-05-07 06:29:01 +00:00
|
|
|
.TH LOOP 4 2005-12-05 "Linux" "Linux Programmer's Manual"
|
2015-05-01 11:47:02 +00:00
|
|
|
.SH NAME
|
2015-05-01 13:03:50 +00:00
|
|
|
loop, loop-control \- loop devices
|
2015-05-01 11:47:02 +00:00
|
|
|
.SH SYNOPSIS
|
|
|
|
#include <linux/loop.h>
|
|
|
|
.SH DESCRIPTION
|
|
|
|
The loop device is a block device that maps its data blocks not to a
|
|
|
|
physical device such as a hard disk or optical disk drive,
|
|
|
|
but to the blocks of
|
|
|
|
a regular file in a filesystem or to another block device.
|
|
|
|
This can be useful for example to provide a block device for a filesystem
|
|
|
|
image stored in a file, so that it can be mounted with the
|
|
|
|
.BR mount (8)
|
|
|
|
command.
|
|
|
|
You could do
|
|
|
|
.nf
|
|
|
|
.IP
|
|
|
|
$ \fBdd if=/dev/zero of=file.img bs=1MiB count=10\fP
|
|
|
|
$ \fBsudo losetup /dev/loop4 file.img \fP
|
|
|
|
$ \fBsudo mkfs -t ext4 /dev/loop4\fP
|
|
|
|
$ \fBsudo mkdir /myloopdev\fP
|
|
|
|
$ \fBsudo mount /dev/loop4 /myloopdev\fP
|
|
|
|
.fi
|
|
|
|
.LP
|
|
|
|
See
|
|
|
|
.BR losetup (8)
|
|
|
|
for another example.
|
|
|
|
|
|
|
|
A transfer function can be specified for each loop device for
|
|
|
|
encryption and decryption purposes.
|
|
|
|
|
|
|
|
The following
|
|
|
|
.BR ioctl (2)
|
|
|
|
operations are provided by the loop block device:
|
|
|
|
.TP
|
|
|
|
.B LOOP_SET_FD
|
|
|
|
Associate the loop device with the open file whose file descriptor is
|
|
|
|
passed as the (third)
|
|
|
|
.BR ioctl (2)
|
|
|
|
argument.
|
|
|
|
.TP
|
|
|
|
.B LOOP_CLR_FD
|
|
|
|
Disassociate the loop device from any file descriptor.
|
|
|
|
.TP
|
|
|
|
.B LOOP_SET_STATUS
|
|
|
|
Set the status of the loop device using the (third)
|
|
|
|
.BR ioctl (2)
|
|
|
|
argument.
|
|
|
|
This argument is a pointer to
|
|
|
|
.I loop_info
|
|
|
|
structure, defined in
|
|
|
|
.I <linux/loop.h>
|
|
|
|
as:
|
|
|
|
|
|
|
|
.nf
|
|
|
|
.in +4n
|
|
|
|
struct loop_info {
|
|
|
|
int lo_number; /* ioctl r/o */
|
|
|
|
dev_t lo_device; /* ioctl r/o */
|
|
|
|
unsigned long lo_inode; /* ioctl r/o */
|
|
|
|
dev_t lo_rdevice; /* ioctl r/o */
|
|
|
|
int lo_offset;
|
|
|
|
int lo_encrypt_type;
|
|
|
|
int lo_encrypt_key_size; /* ioctl w/o */
|
|
|
|
int lo_flags; /* ioctl r/o */
|
|
|
|
char lo_name[LO_NAME_SIZE];
|
|
|
|
unsigned char lo_encrypt_key[LO_KEY_SIZE];
|
|
|
|
/* ioctl w/o */
|
|
|
|
unsigned long lo_init[2];
|
|
|
|
char reserved[4];
|
|
|
|
};
|
|
|
|
.in
|
|
|
|
.fi
|
|
|
|
|
2015-05-01 11:51:39 +00:00
|
|
|
The encryption type
|
|
|
|
.RI ( lo_encrypt_type )
|
|
|
|
should be one of
|
2015-05-01 11:47:02 +00:00
|
|
|
.BR LO_CRYPT_NONE ,
|
|
|
|
.BR LO_CRYPT_XOR ,
|
|
|
|
.BR LO_CRYPT_DES ,
|
|
|
|
.BR LO_CRYPT_FISH2 ,
|
|
|
|
.BR LO_CRYPT_BLOW ,
|
|
|
|
.BR LO_CRYPT_CAST128 ,
|
|
|
|
.BR LO_CRYPT_IDEA ,
|
|
|
|
.BR LO_CRYPT_DUMMY ,
|
|
|
|
.BR LO_CRYPT_SKIPJACK ,
|
|
|
|
or (since Linux 2.6.0)
|
|
|
|
.BR LO_CRYPT_CRYPTOAPI .
|
2015-05-01 11:59:59 +00:00
|
|
|
|
|
|
|
The
|
|
|
|
.I lo_flags
|
|
|
|
field is a bit mask that can include zero or more of the following:
|
|
|
|
.RS
|
|
|
|
.TP
|
|
|
|
.BR LO_FLAGS_READ_ONLY
|
|
|
|
The loopback device is read-only.
|
|
|
|
.TP
|
|
|
|
.BR LO_FLAGS_AUTOCLEAR " (since Linux 2.6.25)"
|
|
|
|
.\" commit 96c5865559cee0f9cbc5173f3c949f6ce3525581
|
|
|
|
The loopback device will autodestruct on last close.
|
|
|
|
.TP
|
|
|
|
.BR LO_FLAGS_PARTSCAN " (since Linux 3.2)"
|
|
|
|
.\" commit e03c8dd14915fabc101aa495828d58598dc5af98
|
|
|
|
Allow automatic partition scanning.
|
|
|
|
.RE
|
2015-05-01 11:47:02 +00:00
|
|
|
.TP
|
|
|
|
.B LOOP_GET_STATUS
|
|
|
|
Get the status of the loop device.
|
|
|
|
The (third)
|
|
|
|
.BR ioctl (2)
|
|
|
|
argument must be a pointer to a
|
|
|
|
.IR "struct loop_info" .
|
2015-05-01 21:03:31 +00:00
|
|
|
.TP
|
2015-05-01 21:11:14 +00:00
|
|
|
.BR LOOP_CHANGE_FD " (since Linux 2.6.5)"
|
|
|
|
Switch the backing store of the loop device to the new file identified
|
|
|
|
file descriptor specified in the (third)
|
|
|
|
.BR ioctl (2)
|
|
|
|
argument, which is an integer.
|
|
|
|
This operation is possible only if the loop device is read-only and
|
|
|
|
the new backing store is the same size and type as the old backing store.
|
|
|
|
.TP
|
2015-05-01 21:03:31 +00:00
|
|
|
.BR LOOP_SET_CAPACITY " (since Linux 2.6.30)"
|
|
|
|
.\" commit 53d6660836f233df66490707365ab177e5fb2bb4
|
|
|
|
Resize a live loop device.
|
|
|
|
One can change the size of the underlying backing store and then use this
|
|
|
|
operation so that the loop driver learns about the new size.
|
|
|
|
This operation takes no argument.
|
2015-05-01 11:47:02 +00:00
|
|
|
.PP
|
|
|
|
Since Linux 2.6, there are two new
|
|
|
|
.BR ioctl (2)
|
|
|
|
operations:
|
|
|
|
.TP
|
|
|
|
.BR LOOP_SET_STATUS64 ", " LOOP_GET_STATUS64
|
|
|
|
These are similar to
|
|
|
|
.BR LOOP_SET_STATUS " and " LOOP_GET_STATUS
|
|
|
|
described above but use the
|
|
|
|
.I loop_info64
|
|
|
|
structure,
|
|
|
|
which has some additional fields and a larger range for some other fields:
|
|
|
|
|
|
|
|
.nf
|
|
|
|
.in +4n
|
|
|
|
struct loop_info64 {
|
|
|
|
uint64_t lo_device; /* ioctl r/o */
|
|
|
|
uint64_t lo_inode; /* ioctl r/o */
|
|
|
|
uint64_t lo_rdevice; /* ioctl r/o */
|
|
|
|
uint64_t lo_offset;
|
|
|
|
uint64_t lo_sizelimit;/* bytes, 0 == max available */
|
|
|
|
uint32_t lo_number; /* ioctl r/o */
|
|
|
|
uint32_t lo_encrypt_type;
|
|
|
|
uint32_t lo_encrypt_key_size; /* ioctl w/o */
|
|
|
|
uint32_t lo_flags; /* ioctl r/o */
|
|
|
|
uint8_t lo_file_name[LO_NAME_SIZE];
|
|
|
|
uint8_t lo_crypt_name[LO_NAME_SIZE];
|
|
|
|
uint8_t lo_encrypt_key[LO_KEY_SIZE]; /* ioctl w/o */
|
|
|
|
uint64_t lo_init[2];
|
|
|
|
};
|
|
|
|
.in
|
|
|
|
.fi
|
2015-05-01 13:03:50 +00:00
|
|
|
.SS /dev/loop-control
|
|
|
|
Since Linux 3.1,
|
|
|
|
.\" commit 770fe30a46a12b6fb6b63fbe1737654d28e84844
|
|
|
|
the kernel provides the
|
|
|
|
.I /dev/loop-control
|
|
|
|
device, which permits an application to dynamically find a free device,
|
|
|
|
and to add and remove loop devices from the system.
|
|
|
|
To perform these operations, one first opens
|
|
|
|
.IR /dev/loop-control
|
|
|
|
and then employs one of the following
|
|
|
|
.BR ioctl (2)
|
|
|
|
operations:
|
|
|
|
.TP
|
|
|
|
.B LOOP_CTL_GET_FREE
|
|
|
|
Allocate or find a free loop device for use.
|
|
|
|
On success, the device number is returned as the result of the call.
|
|
|
|
This operation takes no argument.
|
|
|
|
.TP
|
|
|
|
.B LOOP_CTL_ADD
|
|
|
|
Add the new loop device whose device number is specified
|
|
|
|
as a long integer in the third
|
|
|
|
.BR ioctl (2)
|
|
|
|
argument.
|
|
|
|
On success, the device index is returned as the result of the call.
|
|
|
|
If the device is already allocated, the call fails with the error
|
|
|
|
.BR EEXIST .
|
|
|
|
.TP
|
|
|
|
.B LOOP_CTL_REMOVE
|
|
|
|
Remove the loop device whose device number is specified
|
|
|
|
as a long integer in the third
|
|
|
|
.BR ioctl (2)
|
|
|
|
argument.
|
|
|
|
On success, the device number is returned as the result of the call.
|
|
|
|
If the device is in use, the call fails with the error
|
|
|
|
.BR EBUSY .
|
2015-05-01 13:11:33 +00:00
|
|
|
.SH EXAMPLE
|
|
|
|
The program below uses the
|
|
|
|
.I /dev/loop-control
|
|
|
|
device to find a free loop device, opens the loop device,
|
|
|
|
opens a file to be used as the underlying storage for the device,
|
|
|
|
and then associates the loop device with the backing store.
|
|
|
|
The following shell session demonstrates the use of the program:
|
|
|
|
|
|
|
|
.nf
|
|
|
|
.in +4n
|
|
|
|
$ \fBdd if=/dev/zero of=file.img bs=1MiB count=10\fP
|
|
|
|
10+0 records in
|
|
|
|
10+0 records out
|
|
|
|
10485760 bytes (10 MB) copied, 0.00609385 s, 1.7 GB/s
|
|
|
|
$ \fBsudo ./mnt_loop file.img\fP
|
|
|
|
loopname = /dev/loop5
|
|
|
|
.in
|
|
|
|
.fi
|
|
|
|
.SS Program source
|
|
|
|
\&
|
|
|
|
.nf
|
|
|
|
|
|
|
|
#include <fcntl.h>
|
|
|
|
#include <linux/loop.h>
|
|
|
|
#include <sys/ioctl.h>
|
|
|
|
#include <stdio.h>
|
|
|
|
#include <stdlib.h>
|
|
|
|
#include <unistd.h>
|
|
|
|
|
|
|
|
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \\
|
|
|
|
} while (0)
|
|
|
|
|
|
|
|
int
|
|
|
|
main(int argc, char *argv[])
|
|
|
|
{
|
|
|
|
int loopctlfd, loopfd, backingfile;
|
|
|
|
long devnr;
|
|
|
|
char loopname[4096];
|
|
|
|
|
|
|
|
if (argc != 2) {
|
|
|
|
fprintf(stderr, "Usage: %s backing\-file\\n", argv[0]);
|
|
|
|
exit(EXIT_FAILURE);
|
|
|
|
}
|
|
|
|
|
|
|
|
loopctlfd = open("/dev/loop\-control", O_RDWR);
|
|
|
|
if (loopctlfd == \-1)
|
|
|
|
errExit("open: /dev/loop\-control");
|
|
|
|
|
|
|
|
devnr = ioctl(loopctlfd, LOOP_CTL_GET_FREE);
|
|
|
|
if (devnr == \-1)
|
|
|
|
errExit("ioctl\-LOOP_CTL_GET_FREE");
|
|
|
|
|
|
|
|
sprintf(loopname, "/dev/loop%ld", devnr);
|
|
|
|
printf("loopname = %s\\n", loopname);
|
|
|
|
|
|
|
|
loopfd = open(loopname, O_RDWR);
|
|
|
|
if (loopfd == \-1)
|
|
|
|
errExit("open: loopname");
|
|
|
|
|
|
|
|
backingfile = open(argv[1], O_RDWR);
|
|
|
|
if (backingfile == \-1)
|
|
|
|
errExit("open: backing\-file");
|
|
|
|
|
|
|
|
if (ioctl(loopfd, LOOP_SET_FD, backingfile) == \-1)
|
|
|
|
errExit("ioctl\-LOOP_SET_FD");
|
|
|
|
|
|
|
|
exit(EXIT_SUCCESS);
|
|
|
|
}
|
|
|
|
.fi
|
2015-05-01 11:47:02 +00:00
|
|
|
.SH FILES
|
2015-05-01 13:03:50 +00:00
|
|
|
.IR /dev/loop* :
|
|
|
|
the loop block special device files
|
2015-05-01 11:47:02 +00:00
|
|
|
.SH "SEE ALSO"
|
|
|
|
.BR losetup (8),
|
|
|
|
.BR mount (8)
|