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NAME

       ioctl_fat - manipulating the FAT filesystem

SYNOPSIS

       #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

       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

       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

       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

       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

       This API is Linux-specific.

EXAMPLE

   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

       ioctl(2)

COLOPHON

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