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

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

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

#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);
}

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

#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);
}

$ ./fat_dir /mnt/user
[char46] -> ''
[char46]. -> ''
ALONGF~1.TXT -> 'a long filename.txt'
UPPER.TXT -> ''
LOWER.TXT -> 'lower.txt'

#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);
}