NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | EXAMPLE | SEE ALSO | COLOPHON

IOCTL-FAT(2)              Linux Programmer's Manual             IOCTL-FAT(2)

NAME         top

       ioctl_fat - manipulating the FAT filesystem

SYNOPSIS         top

       #include <linux/msdos_fs.h>
       #include <sys/ioctl.h>

       int ioctl(int fd, FAT_IOCTL_GET_ATTRIBUTES, uint32_t *attr);
       int ioctl(int fd, FAT_IOCTL_SET_ATTRIBUTES, uint32_t *attr);
       int ioctl(int fd, FAT_IOCTL_GET_VOLUME_ID, uint32_t *id);
       int ioctl(int fd, VFAT_IOCTL_READDIR_BOTH,
                 struct __fat_dirent[2] entry);
       int ioctl(int fd, VFAT_IOCTL_READDIR_SHORT,
                 struct __fat_dirent[2] entry);

DESCRIPTION         top

       The ioctl(2) system call can be used to read and write metadata of
       FAT filesystems that are not accessible using other system calls.

   Reading and setting file attributes
       Files and directories in the FAT filesystem possess an attribute bit
       mask that can be read with FAT_IOCTL_GET_ATTRIBUTES and written with
       FAT_IOCTL_SET_ATTRIBUTES.

       The fd argument contains a file descriptor for a file or directory.
       It is sufficient to create the file descriptor by calling open(2)
       with the O_RDONLY flag.

       The attr argument contains a pointer to a bit mask.  The bits of the
       bit mask are:

       ATTR_RO
              This bit specifies that the file or directory is read-only.

       ATTR_HIDDEN
              This bit specifies that the file or directory is hidden.

       ATTR_SYS
              This bit specifies that the file is a system file.

       ATTR_VOLUME
              This bit specifies that the file is a volume label.  This
              attribute is read-only.

       ATTR_DIR
              This bit specifies that this is a directory.  This attribute
              is read-only.

       ATTR_ARCH
              This bit indicates that this file or directory should be
              archived.  It is set when a file is created or modified.  It
              is reset by an archiving system.

       The zero value ATTR_NONE can be used to indicate that no attribute
       bit is set.

   Reading the volume ID
       FAT filesystems are identified by a volume ID.  The volume ID can be
       read with FAT_IOCTL_GET_VOLUME_ID.

       The fd argument can be a file descriptor for any file or directory of
       the filesystem.  It is sufficient to create the file descriptor by
       calling open(2) with the O_RDONLY flag.

       The id argument is a pointer to the field that will be filled with
       the volume ID.  Typically the volume ID is displayed to the user as a
       group of two 16-bit fields:

           printf("Volume ID %04x-%04x\n", id >> 16, id & 0xFFFF);

   Reading short file names of a directory
       A file or directory on a FAT filesystem always has a short filename
       consisting of up to 8 capital letters, optionally followed by a
       period and up to 3 capital letters for the file extension.  If the
       actual filename does not fit into this scheme, it is stored as a long
       filename of up to 255 UTF-16 characters.

       The short filenames in a directory can be read with
       VFAT_IOCTL_READDIR_SHORT.  VFAT_IOCTL_READDIR_BOTH reads both the
       short and the long filenames.

       The fd argument must be a file descriptor for a directory.  It is
       sufficient to create the file descriptor by calling open(2) with the
       O_RDONLY flag.  The file descriptor can be used only once to iterate
       over the directory entries by calling ioctl(2) repeatedly.

       The entry argument is a two-element array of the following
       structures:

           struct __fat_dirent {
               long            d_ino;
               __kernel_off_t  d_off;
               uint32_t short  d_reclen;
               char            d_name[256];
           };

       The first entry in the array is for the short filename.  The second
       entry is for the long filename.

       The d_ino and d_off fields are filled only for long filenames.  The
       d_ino field holds the inode number of the directory.  The d_off field
       holds the offset of the file entry in the directory.  As these values
       are not available for short filenames, the user code should simply
       ignore them.

       The field d_reclen contains the length of the filename in the field
       d_name.  To keep backward compatibility, a length of 0 for the short
       filename signals that the end of the directory has been reached.
       However, the preferred method for detecting the end of the directory
       is to test the ioctl(2) return value.  If no long filename exists,
       field d_reclen is set to 0 and d_name is a character string of length
       0 for the long filename.

RETURN VALUE         top

       On error, -1 is returned, and errno is set to indicate the error.

       For VFAT_IOCTL_READDIR_BOTH and VFAT_IOCTL_READDIR_SHORT a return
       value of 1 signals that a new directory entry has been read and a
       return value of 0 signals that the end of the directory has been
       reached.

ERRORS         top

       ENOENT This error is returned by VFAT_IOCTL_READDIR_BOTH and
              VFAT_IOCTL_READDIR_SHORT if the file descriptor fd refers to a
              removed, but still open directory.

       ENOTDIR
              This error is returned by VFAT_IOCTL_READDIR_BOTH and
              VFAT_IOCTL_READDIR_SHORT if the file descriptor fd does not
              refer to a directory.

       ENOTTY The file descriptor fd does not refer to an object in a FAT
              filesystem.

       For further error values, see ioctl(2).

VERSIONS         top

       VFAT_IOCTL_READDIR_BOTH and VFAT_IOCTL_READDIR_SHORT first appeared
       in Linux 2.0.

       FAT_IOCTL_GET_ATTRIBUTES and FAT_IOCTL_SET_ATTRIBUTES first appeared
       in Linux 2.6.12.

       FAT_IOCTL_GET_VOLUME_ID was introduced in version 3.11 of the Linux
       kernel.

CONFORMING TO         top

       This API is Linux-specific.

EXAMPLE         top

   Toggling the archive flag
       The following program demonstrates the usage of ioctl(2) to
       manipulate file attributes.  The program reads and displays the
       archive attribute of a file.  After inverting the value of the
       attribute, the program reads and displays the attribute again.

       The following was recorded when applying the program for the file
       /mnt/user/foo:

           # ./toggle_fat_archive_flag /mnt/user/foo
           Archive flag is set
           Toggling archive flag
           Archive flag is not set

   Program source (toggle_fat_archive_flag.c)
           #include <fcntl.h>
           #include <linux/msdos_fs.h>
           #include <stdint.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <sys/ioctl.h>
           #include <unistd.h>

           /*
            * Read file attributes of a file on a FAT filesystem.
            * Output the state of the archive flag.
            */
           static uint32_t
           readattr(int fd)
           {
               uint32_t attr;
               int ret;

               ret = ioctl(fd, FAT_IOCTL_GET_ATTRIBUTES, &attr);
               if (ret == -1) {
                   perror("ioctl");
                   exit(EXIT_FAILURE);
               }

               if (attr & ATTR_ARCH)
                   printf("Archive flag is set\n");
               else
                   printf("Archive flag is not set\n");

               return attr;
           }

           int
           main(int argc, char *argv[])
           {
               uint32_t attr;
               int fd;
               int ret;

               if (argc != 2) {
                   printf("Usage: %s FILENAME\n", argv[0]);
                   exit(EXIT_FAILURE);
               }

               fd = open(argv[1], O_RDONLY);
               if (fd == -1) {
                   perror("open");
                   exit(EXIT_FAILURE);
               }

               /*
                * Read and display the FAT file attributes.
                */
               attr = readattr(fd);

               /*
                * Invert archive attribute.
                */
               printf("Toggling archive flag\n");
               attr ^= ATTR_ARCH;

               /*
                * Write the changed FAT file attributes.
                */
               ret = ioctl(fd, FAT_IOCTL_SET_ATTRIBUTES, &attr);
               if (ret == -1) {
                   perror("ioctl");
                   exit(EXIT_FAILURE);
               }

               /*
                * Read and display the FAT file attributes.
                */
               readattr(fd);

               close(fd);

               exit(EXIT_SUCCESS);
           }

   Reading the volume ID
       The following program demonstrates the use of ioctl(2) to display the
       volume ID of a FAT filesystem.

       The following output was recorded when applying the program for
       directory /mnt/user:

           $ ./display_fat_volume_id /mnt/user
           Volume ID 6443-6241

   Program source (display_fat_volume_id.c)
           #include <fcntl.h>
           #include <linux/msdos_fs.h>
           #include <stdint.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <sys/ioctl.h>
           #include <unistd.h>

           int
           main(int argc, char *argv[])
           {
               uint32_t id;
               int fd;
               int ret;

               if (argc != 2) {
                   printf("Usage: %s FILENAME\n", argv[0]);
                   exit(EXIT_FAILURE);
               }

               fd = open(argv[1], O_RDONLY);
               if (fd == -1) {
                   perror("open");
                   exit(EXIT_FAILURE);
               }

               /*
                * Read volume ID.
                */
               ret = ioctl(fd, FAT_IOCTL_GET_VOLUME_ID, &id);
               if (ret == -1) {
                   perror("ioctl");
                   exit(EXIT_FAILURE);
               }

               /*
                * Format the output as two groups of 16 bits each.
                */
               printf("Volume ID %04x-%04x\n", id >> 16, id & 0xFFFF);

               close(fd);

               exit(EXIT_SUCCESS);
           }

   Listing a directory
       The following program demonstrates the use of ioctl(2) to list a
       directory.

       The following was recorded when applying the program to the directory
       /mnt/user:

           $ ./fat_dir /mnt/user
           . -> ''
           .. -> ''
           ALONGF~1.TXT -> 'a long filename.txt'
           UPPER.TXT -> ''
           LOWER.TXT -> 'lower.txt'

   Program source
           #include <fcntl.h>
           #include <linux/msdos_fs.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <sys/ioctl.h>
           #include <unistd.h>

           int
           main(int argc, char *argv[])
           {
               struct __fat_dirent entry[2];
               int fd;
               int ret;

               if (argc != 2) {
                   printf("Usage: %s DIRECTORY\n", argv[0]);
                   exit(EXIT_FAILURE);
               }

               /*
                * Open file descriptor for the directory.
                */
               fd = open(argv[1], O_RDONLY | O_DIRECTORY);
               if (fd == -1) {
                   perror("open");
                   exit(EXIT_FAILURE);
               }

               for (;;) {

                   /*
                    * Read next directory entry.
                    */
                   ret = ioctl( fd, VFAT_IOCTL_READDIR_BOTH, entry);

                   /*
                    * If an error occurs, the return value is -1.
                    * If the end of the directory list has been reached,
                    * the return value is 0.
                    * For backward compatibility the end of the directory
                    * list is also signaled by d_reclen == 0.
                    */
                   if (ret < 1)
                       break;

                   /*
                    * Write both the short name and the long name.
                    */
                   printf("%s -> '%s'\n", entry[0].d_name, entry[1].d_name);
               }

               if (ret == -1) {
                   perror("VFAT_IOCTL_READDIR_BOTH");
                   exit(EXIT_FAILURE);
               }

               /*
                * Close the file descriptor.
                */
               close(fd);

               exit(EXIT_SUCCESS);
           }

SEE ALSO         top

       ioctl(2)

COLOPHON         top

       This page is part of release 4.10 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                            2017-03-13                     IOCTL-FAT(2)