loop(4) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | FILES | EXAMPLES | SEE ALSO | COLOPHON

loop(4)                 Kernel Interfaces Manual                 loop(4)

NAME         top

       loop, loop-control - loop devices

SYNOPSIS         top

       #include <linux/loop.h>

DESCRIPTION         top

       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:

       LOOP_SET_FD
              Associate the loop device with the open file whose file
              descriptor 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 a
              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/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];
                  };

              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.

              LO_FLAGS_DIRECT_IO (since Linux 4.10)
                     Use direct I/O mode to access the backing file.

              The only lo_flags that can be modified by LOOP_SET_STATUS
              are LO_FLAGS_AUTOCLEAR and LO_FLAGS_PARTSCAN.

       LOOP_GET_STATUS
              Get the status of the loop device.  The (third) ioctl(2)
              argument 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.

       LOOP_SET_DIRECT_IO (since Linux 4.10)
              Set DIRECT I/O mode on the loop device, so that it can be
              used to open backing file.  The (third) ioctl(2) argument
              is an unsigned long value.  A nonzero represents direct
              I/O mode.

       LOOP_SET_BLOCK_SIZE (since Linux 4.14)
              Set the block size of the loop device.  The (third)
              ioctl(2) argument is an unsigned long value.  This value
              must be a power of two in the range [512,pagesize];
              otherwise, an EINVAL error results.

       LOOP_CONFIGURE (since Linux 5.8)
              Setup and configure all loop device parameters in a single
              step using the (third) ioctl(2) argument.  This argument
              is a pointer to a loop_config structure, defined in
              <linux/loop.h> as:

                  struct loop_config {
                      __u32               fd;
                      __u32               block_size;
                      struct loop_info64  info;
                      __u64               __reserved[8];
                  };

              In addition to doing what LOOP_SET_STATUS can do,
              LOOP_CONFIGURE can also be used to do the following:

              •  set the correct block size immediately by setting
                 loop_config.block_size;

              •  explicitly request direct I/O mode by setting
                 LO_FLAGS_DIRECT_IO in loop_config.info.lo_flags; and

              •  explicitly request read-only mode by setting
                 LO_FLAGS_READ_ONLY in loop_config.info.lo_flags.

       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; 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];
                  };

   /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 operations, 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 operation 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 EEXIST.

       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         top

       /dev/loop*
              The loop block special device files.

EXAMPLES         top

       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

   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         top

       losetup(8), mount(8)

COLOPHON         top

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Linux man-pages 6.9.1          2024-06-15                        loop(4)