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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)
     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]);
         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)
         exit(EXIT_SUCCESS); }


     losetup(8), mount(8)


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     description  of  the project, information about reporting bugs, and the
     latest    version    of    this    page,    can     be     found     at

Linux 2017-09-15 LOOP(4)

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