loop(4) Device Drivers Manual loop(4) loop, loop-control - #include -- , . , , , , , mount(8). : $ 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 losetup(8). . ioctl(2): LOOP_SET_FD , ioctl(2). LOOP_CLR_FD . LOOP_SET_STATUS ( ioctl(2)) . loop_info, : 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]; }; (lo_encrypt_type) : 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 LO_CRYPT_CRYPTOAPI ( Linux 2.6.0). lo_flags , : LO_FLAGS_READ_ONLY . LO_FLAGS_AUTOCLEAR ( Linux 2.6.25) . LO_FLAGS_PARTSCAN ( Linux 3.2) . LO_FLAGS_DIRECT_IO ( 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 . ioctl(2) struct loop_info. LOOP_CHANGE_FD ( Linux 2.6.5) (backing store) , , ioctl(2), . , , . LOOP_SET_CAPACITY ( Linux 2.6.30) (live) . , , . . LOOP_SET_DIRECT_IO ( 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 ( 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 ( 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 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: o set the correct block size immediately by setting loop_config.block_size; o explicitly request direct I/O mode by setting LO_FLAGS_DIRECT_IO in loop_config.info.lo_flags; and o explicitly request read-only mode by setting LO_FLAGS_READ_ONLY in loop_config.info.lo_flags. Linux 2.6, ioctl(2): LOOP_SET_STATUS64 LOOP_GET_STATUS64 LOOP_SET_STATUS LOOP_GET_STATUS, loop_info64, , : 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 Linux 3.1, /dev/loop-control, , . /dev/loop-control, ioctl(2): LOOP_CTL_GET_FREE . . . LOOP_CTL_ADD ; ioctl(2). . , EEXIST. LOOP_CTL_REMOVE ; ioctl(2). . , EBUSY. /dev/loop* . , , /dev/loop-control , , , , . : $ 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 #include #include #include #include #include #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) Artyom Kunyov , Azamat Hackimov , Dmitry Bolkhovskikh , Katrin Kutepova , Konstantin Shvaykovskiy , Yuri Kozlov ; GNU 3 , . . , , . Linux man-pages 6.06 31 2023 . loop(4)