| loop(4) | Device Drivers Manual | loop(4) |
loop, loop-control - loop devices
#include <linux/loop.h>
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 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 encryption and decryption purposes.
The following ioctl(2) operations are provided by the loop block device:
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/w (r/o before
Linux 2.6.25) */
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];
};
struct loop_config {
__u32 fd;
__u32 block_size;
struct loop_info64 info;
__u64 __reserved[8];
};
Since Linux 2.6, there are two new ioctl(2) operations:
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; i /* ioctl r/w (r/o before
Linux 2.6.25) */
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];
};
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 operations, one first opens /dev/loop-control and then employs one of the following ioctl(2) operations:
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 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:
$ 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
#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);
}
losetup(8), mount(8)
| 2023-10-31 | Linux man-pages 6.7 |