Message-Id: <5984550D-1518-490C-9A3F-3F9B1FA2EA00@dilger.ca>
Date:	Tue, 3 Feb 2015 01:49:44 -0700
From:	Andreas Dilger <adilger@...ger.ca>
To:	Heinrich Schuchardt <xypron.glpk@....de>
Cc:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	Ogawa Hirofumi <hirofumi@...l.parknet.co.jp>,
	Linux Filesystem Development List 
	<linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-man <linux-man@...r.kernel.org>
Subject: Re: [PATCH v3 1/1] ioctl-fat.2: new manpage for the ioctl fat API

On Jan 23, 2015, at 12:54 PM, Heinrich Schuchardt <xypron.glpk@....de> wrote:
> 
> The ioctl(2) system call may be used to retrieve information about
> the fat file system and to set file attributes.
> 
> This new manpage describes the details.
> 
> Michael Kerrisk suggested to CC linux-fsdevel@...r.kernel.org and
> linux-kernel@...r.kernel.org for review.
> 
> version 3: correct typos
> version 2: consider comments by Michael Kerrisk
> verison 1: original patch
> 
> Signed-by: Heinrich Schuchardt <xypron.glpk@....de>
> ---
> man2/ioctl-fat.2 | 442 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 442 insertions(+)
> create mode 100644 man2/ioctl-fat.2
> 
> diff --git a/man2/ioctl-fat.2 b/man2/ioctl-fat.2
> new file mode 100644
> index 0000000..80891d8
> --- /dev/null
> +++ b/man2/ioctl-fat.2
> @@ -0,0 +1,442 @@
> +.\" Copyright (C) 2014, Heinrich Schuchardt <xypron.glpk@....de>
> +.\"
> +.\" %%%LICENSE_START(VERBATIM)
> +.\" Permission is granted to make and distribute verbatim copies of this
> +.\" manual provided the copyright notice and this permission notice are
> +.\" preserved on all copies.
> +.\"
> +.\" Permission is granted to copy and distribute modified versions of
> +.\" this manual under the conditions for verbatim copying, provided that
> +.\" the entire resulting derived work is distributed under the terms of
> +.\" a permission notice identical to this one.
> +.\"
> +.\" Since the Linux kernel and libraries are constantly changing, this
> +.\" manual page may be incorrect or out-of-date.  The author(s) assume.
> +.\" no responsibility for errors or omissions, or for damages resulting.
> +.\" from the use of the information contained herein.  The author(s) may.
> +.\" not have taken the same level of care in the production of this.
> +.\" manual, which is licensed free of charge, as they might when working.

There are extraneous periods at the end of each of these lines.

> +.\" professionally.
> +.\"
> +.\" Formatted or processed versions of this manual, if unaccompanied by
> +.\" the source, must acknowledge the copyright and authors of this work.
> +.\" %%%LICENSE_END
> +.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)
> +function can be used to read and write metadata of the 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 the 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 the bit mask.
> +The bits of the bit mask are
> +.TP
> +.B ATTR_RO
> +This bit specifies that the file or directory is read-only.

It's too bad that these constants have such generic names. It would
be better to use MSDOS_ATTR_* or FAT_ATTR_*, since these are also
exposed to userspace, but are only used by FAT.

> +.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.

It might be useful if the FS_IOC_GETFLAGS ioctl was implemented for
FAT, and this was mapped to FS_IMMUTABLE_FL as it is in the kernel,
so that the more common lsattr and chattr commands worked on it, as
they do with ext*, btrfs, xfs, reiserfs, etc.

I guess these aren't problems with the man page per-se, but something
for the FAT developers to think about.

Cheers, Andreas

> +.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 only used 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
> +Field
> +.I d_reclen
> +specifies the length of the filename in field
> +.IR d_name .
> +A length of 0 for the short filename signals that the end of the directory
> +has been reached.
> +.SH RETURN VALUE
> +On error, -1 is returned, and 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 the file descriptor does not point to a directory.
> +.TP
> +.B ENOTTY
> +This error signals that the file descriptor is not for 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 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 the ioctl API to manipulate
> +file attributes.
> +It reads and displays the archive attribute of a file.
> +After inverting the value of the attribute, it reads and displays it again.
> +.PP
> +The following was recorded when applying the program for the file
> +.IR /mnt/user/foo .
> +.SS Example output
> +.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 usage of the ioctl API to
> +display the volume label of a FAT filesystem.
> +.PP
> +The following output was recorded when applying the program for
> +directory
> +.IR /mnt/user .
> +.SS Example output
> +.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 usage of the ioctl API to
> +list a directory.
> +.PP
> +The following was recorded when applying the program for the directory
> +.IR /mnt/user .
> +.SS Example output
> +.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)
> -- 
> 2.1.4
> 
> --
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Cheers, Andreas





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