ioctl_fat.2: Tweaks to Heinrich's new page

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.\" Copyright (C) 2014, Heinrich Schuchardt <xypron.glpk@gmx.de>
.\"
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.TH IOCTl-FAT 2 2015-01-23 "Linux" "Linux Programmer's Manual"
.SH "NAME"
ioctl_fat \- manipulating the FAT filesystem
.SH SYNOPSIS
.nf
.B #include <linux/msdos_fs.h>
.br
.B #include <sys/ioctl.h>
.sp
.BI "int ioctl(int " fd ", FAT_IOCTL_GET_ATTRIBUTES, uint32_t * " attr );
.BI "int ioctl(int " fd ", FAT_IOCTL_SET_ATTRIBUTES, uint32_t * " attr );
.BI "int ioctl(int " fd ", FAT_IOCTL_GET_VOLUME_ID, uint32_t * " id );
.BI "int ioctl(int " fd ", VFAT_IOCTL_READDIR_BOTH,
.BI "          struct __fat_dirent[2] " entry );
.BI "int ioctl(int " fd ", VFAT_IOCTL_READDIR_SHORT,
.BI "          struct __fat_dirent[2] " entry );
.SH DESCRIPTION
The
.BR ioctl (2)
system call can be used to read and write metadata of FAT filesystems that
are not accessible using other system calls.
.SS Reading and setting file attributes
Files and directories in the FAT filesystem possess an attribute bit mask that
can be read with
.B FAT_IOCTL_GET_ATTRIBUTES
and written with
.BR FAT_IOCTL_SET_ATTRIBUTES .
.PP
The
.I fd
argument contains a file descriptor for a file or directory.
It is sufficient to create the file descriptor by calling
.BR open (2)
with the
.B O_RDONLY
flag.
.PP
The
.I attr
argument contains a pointer to a bit mask.
The bits of the bit mask are:
.TP
.B ATTR_RO
This bit specifies that the file or directory is read-only.
.TP
.B ATTR_HIDDEN
This bit specifies that the file or directory is hidden.
.TP
.B ATTR_SYS
This bit specifies that the file is a system file.
.TP
.B ATTR_VOLUME
This bit specifies that the file is a volume label.
This attribute is read-only.
.TP
.B ATTR_DIR
This bit specifies that this is a directory.
This attribute is read-only.
.TP
.B 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.
.PP
The zero value
.B ATTR_NONE
can be used to indicate that no attribute bit is set.
.SS Reading the volume label
FAT filesystems are identified by a volume label.
The volume label can be read with
.BR FAT_IOCTL_GET_VOLUME_ID .
.PP
The
.I 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
.BR open (2)
with the
.B O_RDONLY
flag.
.PP
The
.I id
argument is a pointer to the field that will be filled with the volume ID.
Typically the volume label is displayed to the user as a group of two
16-bit fields:
.PP
.in +4n
.nf
printf("Volume ID %4x-%4x\\n", id >> 16, id & 0xFFFF);
.fi
.in
.SS 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.
.PP
The short filenames in a directory can be read with
.BR VFAT_IOCTL_READDIR_SHORT .
.B VFAT_IOCTL_READDIR_BOTH
reads both the short and the long filenames.
.PP
The
.I fd
argument must be a file descriptor for a directory.
It is sufficient to create the file descriptor by calling
.BR open (2)
with the
.B O_RDONLY
flag.
The file descriptor can be used only once to iterate over the directory
entries by calling
.BR ioctl (2)
repeatedly.
.PP
The
.I entry
argument is a two-element array of the following structures:

.in +4n
.nf
struct __fat_dirent {
    long            d_ino;
    __kernel_off_t  d_off;
    uint32_t short  d_reclen;
    char            d_name[256];
};
.fi
.in
.PP
The first entry in the array is for the short filename.
The second entry is for the long filename.
.PP
The field
.I d_reclen
contains the length of the filename in the field
.IR d_name .
A length of 0 for the short filename signals that the end of the directory
has been reached.
.\" FIXME Missing here are explanations of the fileds ;d_ino' and 'd_off'
.SH RETURN VALUE
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B ENOTDIR
This error is returned by
.B VFAT_IOCTL_READDIR_SHORT
and
.B VFAT_IOCTL_READDIR_SHORT
if
.I fd
does not refer to a directory.
.TP
.B ENOTTY
The file descriptor
.I fd
does not refer to an object in a FAT filesystem.
.PP
For further error values, see
.BR ioctl (2).
.SH VERSIONS
.B FAT_IOCTL_GET_VOLUME_ID
was introduced in version 3.11
.\" commit 6e5b93ee55d401f1619092fb675b57c28c9ed7ec
of the Linux kernel.
.PP
.BR FAT_IOCTL_GET_ATTRIBUTES ,
.BR FAT_IOCTL_SET_ATTRIBUTES ,
.BR VFAT_IOCTL_READDIR_BOTH ,
and
.B VFAT_IOCTL_READDIR_SHORT
were introduced before version 2.6.28 of the Linux kernel.
.SH "CONFORMING TO"
This API is Linux-specific.
.SH EXAMPLE
.SS Toggling the archive flag
The following program demonstrates the usage of
.BR ioctl ()
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.
.PP
The following was recorded when applying the program for the file
.IR /mnt/user/foo :

.in +4n
.nf
# ./toggle_archive_flag /mnt/user/foo
Archive flag is set
Toggling archive flag
Archive flag is not set
.fi
.in
.SS Program source
.in +4n
.nf
#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);

    return EXIT_SUCCESS;
}
.fi
.in
.SS Reading the volume label
The following program demonstrates the use of
.BR ioctl (2)
to display the volume label of a FAT filesystem.
.PP
The following output was recorded when applying the program for
directory
.IR /mnt/user :

.in +4n
.nf
$ ./display_volume_id /mnt/user
Volume ID 6443-6241
.fi
.in
.SS Program source
.in +4n
.nf
#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 %4x\-%4x\\n", id >> 16, id & 0xFFFF);

    close(fd);

    return EXIT_SUCCESS;
}
.fi
.in
.SS Listing a directory
The following program demonstrates the use of
.BR ioctl (2)
to list a directory.
.PP
The following was recorded when applying the program to the directory
.IR /mnt/user :

.in +4n
.nf
$ ./fat_dir /mnt/user
\[char46] -> ''
\[char46]. -> ''
ALONGF~1.TXT -> 'a long filename.txt'
UPPER.TXT -> ''
LOWER.TXT -> 'lower.txt'
.fi
.in
.SS Program source
.in +4n
.nf
#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 d_reclen is zero, the end of the directory
         * list has been reached.
         */
        if (ret == \-1 || entry[0].d_reclen == 0)
            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);

    return EXIT_SUCCESS;
}
.fi
.in
.SH SEE ALSO
.BR ioctl (2)