LOOP(4)                    Linux Programmer's 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 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

              Associate the loop device with the open file whose file descrip-
              tor is passed as the (third) ioctl(2) argument.

              Disassociate the loop device from any file descriptor.

              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_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:

                     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.

              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

       Since Linux 2.6, there are two new ioctl(2) operations:

              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];

       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:

              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.

              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-

              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.

              The loop block special device files.

       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)

       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]);

           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)

           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)


       losetup(8), mount(8)

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Linux                             2017-09-15                           LOOP(4)
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