ioctl_fat

NAME

ioctl_fat - manipulating the FAT filesystem

LIBRARY

Standard C library (libc, -lc)

SYNOPSIS

#include <linux/msdos_fs.h>     /* Definition of [V]FAT_* and
                                   ATTR_* constants*/"
#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 entry[2]);
int ioctl(int fd, VFAT_IOCTL_READDIR_SHORT,
          struct __fat_dirent entry[2]);

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 filenames 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.

STANDARDS

This API is Linux-specific.

EXAMPLES

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)
{
    int       ret;
    uint32_t  attr;

    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[])
{
    int       fd;
    int       ret;
    uint32_t  attr;

    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[])
{
    int       fd;
    int       ret;
    uint32_t  id;

    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[])
{
    int                  fd;
    int                  ret;
    struct __fat_dirent  entry[2];

    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)