LOOP(4) Linux Programmer's Manual LOOP(4)
NAME
loop, loop-control - loop devices
SYNOPSIS
#include <linux/loop.h>
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 filesys-
tem image stored in a file, so that it can be mounted with the mount(8)
command. You could do
$ dd if=/dev/zero of=file.img bs=1MiB count=10
$ sudo losetup /dev/loop4 file.img
$ sudo mkfs -t ext4 /dev/loop4
$ sudo mkdir /myloopdev
$ sudo mount /dev/loop4 /myloopdev
See losetup(8) for another example.
A transfer function can be specified for each loop device for encryp-
tion and decryption purposes.
The following ioctl(2) operations are provided by the loop block
device:
LOOP_SET_FD
Associate the loop device with the open file whose file descrip-
tor is passed as the (third) ioctl(2) argument.
LOOP_CLR_FD
Disassociate the loop device from any file descriptor.
LOOP_SET_STATUS
Set the status of the loop device using the (third) ioctl(2)
argument. This argument is a pointer to loop_info structure,
defined in <linux/loop.h> as:
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];
};
The encryption type (lo_encrypt_type) should be one of
LO_CRYPT_NONE, LO_CRYPT_XOR, LO_CRYPT_DES, LO_CRYPT_FISH2,
LO_CRYPT_BLOW, LO_CRYPT_CAST128, LO_CRYPT_IDEA, LO_CRYPT_DUMMY,
LO_CRYPT_SKIPJACK, or (since Linux 2.6.0) LO_CRYPT_CRYPTOAPI.
The lo_flags field is a bit mask that can include zero or more
of the following:
LO_FLAGS_READ_ONLY
The loopback device is read-only.
LO_FLAGS_AUTOCLEAR (since Linux 2.6.25)
The loopback device will autodestruct on last close.
LO_FLAGS_PARTSCAN (since Linux 3.2)
Allow automatic partition scanning.
LOOP_GET_STATUS
Get the status of the loop device. The (third) ioctl(2) argu-
ment must be a pointer to a struct loop_info.
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) 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.
LOOP_SET_CAPACITY (since Linux 2.6.30)
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.
Since Linux 2.6, there are two new ioctl(2) operations:
LOOP_SET_STATUS64, LOOP_GET_STATUS64
These are similar to LOOP_SET_STATUS and LOOP_GET_STATUS
described above but use the loop_info64 structure, which has
some additional fields and a larger range for some other fields:
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];
};
/dev/loop-control
Since Linux 3.1, the kernel provides the /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 opera-
tions, one first opens /dev/loop-control and then employs one of the
following ioctl(2) operations:
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 oper-
ation takes no argument.
LOOP_CTL_ADD
Add the new loop device whose device number is specified as a
long integer in the third 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 EEX-
IST.
LOOP_CTL_REMOVE
Remove the loop device whose device number is specified as a
long integer in the third 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 EBUSY.
FILES
/dev/loop*
The loop block special device files.
EXAMPLE
The program below uses the /dev/loop-control device to find a free loop
device, opens the loop device, opens a file to be used as the underly-
ing storage for the device, and then associates the loop device with
the backing store. The following shell session demonstrates the use of
the program:
$ dd if=/dev/zero of=file.img bs=1MiB count=10
10+0 records in
10+0 records out
10485760 bytes (10 MB) copied, 0.00609385 s, 1.7 GB/s
$ sudo ./mnt_loop file.img
loopname = /dev/loop5
Program source
#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);
}
SEE ALSO
losetup(8), mount(8)
COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 LOOP(4)
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