GPT fdisk (aka gdisk, cgdisk, and sgdisk) and FixParts
by Roderick W. Smith, rodsmith@rodsbooks.com
Introduction
------------
This package includes the source code for four related disk partitioning
programs:
- gdisk -- This program is modeled after Linux fdisk, but it operates on
GUID Partition Table (GPT) disks rather than the Master Boot Record (MBR)
disks that fdisk modifies. As such, gdisk is an interactive text-mode
tool for manipulating partitions, but it does nothing to the contents of
those partitions (usually filesystems, but sometimes swap space or other
data).
- cgdisk -- This program is modeled after Linux cfdisk, but it operates on
GPT disks rather than the MBR disks that cfdisk modifies. As such, cgdisk
is a curses-based text-mode tool for manipulating partitions, which is to
say that it uses an interface that relies on arrow keys and a dynamic
display rather than the command letters and a scrolling display like
gdisk uses.
- sgdisk -- This program is conceptually similar to the Linux sfdisk and
FreeBSD gpt programs, but its operational details differ. It enables
manipulation of GPT disks using command-line options, so it's suitable
for use in scripts or by experts to perform specific tasks that might
take several commands in gdisk to accomplish.
- fixparts -- This program, unlike the preceding three, operates on MBR
disks. It's intended to fix certain problems that can be created by
various utilities. Specifically, it can fix mis-sized extended partitions
and primary partitions located in the middle of extended partitions. It
also enables changing primary vs. logical partition status (within limits
of what's legal in the MBR scheme) and making a few other minor changes.
It does NOT support creating new partitions; for that, you should use
fdisk, parted, or some other tool.
More details about the abilities of these tools follows.
All four programs rely on the same set of underlying code base; they differ
only in their control interfaces (defined in gdisk.cc, cgdisk.cc,
sgdisk.cc, and fixparts.cc, respectively) and in which support code they
use.
GPT fdisk (gdisk, cgdisk, and sgdisk) Details
---------------------------------------------
The gdisk program is intended as a (somewhat) fdisk-workalike program for
GPT-partitioned disks, cgdisk is similarly a workalike for fdisk, and
sgdisk provides most of gdisk's functionality in a more script-friendly
program. Although libparted and programs that use it (GNU Parted, gparted,
etc.) provide the ability to handle GPT disks, they have certain
limitations that gdisk overcomes. Specific advantages of gdisk, cgdisk, and
sgdisk include:
* The ability to convert MBR-partitioned disks in-place to GPT format,
without losing data
* The ability to convert BSD disklabels in-place to create GPT
partitions, without losing data
* The ability to convert from GPT format to MBR format without data loss
(gdisk and sgdisk only)
* More flexible specification of filesystem type code GUIDs, which
GNU Parted tends to corrupt
* Clear identification of the number of unallocated sectors on a
disk
* A user interface that's familiar to long-time users of Linux
fdisk and cfdisk (gdisk and cgdisk only)
* The MBR boot loader code is left alone
* The ability to create a hybrid MBR, which permits GPT-unaware OSes to
access up to three GPT partitions on the disk (gdisk and sgdisk only)
Of course, GPT fdisk isn't without its limitations. Most notably, it lacks
the filesystem awareness and filesystem-related features of GParted. You
can't resize a partition's filesystem or create a partition with a
filesystem already in place with gdisk, for instance. There's no GUI
version of gdisk.
The GPT fdisk package provides three program files: the interactive
text-mode gdisk, the curses-based interactive cgdisk, and the
command-line-driven sgdisk. The first two are intended for use in manually
partitioning disks or changing partitioning details; sgdisk is intended for
use in scripts to help automate tasks such as disk cloning or preparing
multiple disks for Linux installation.
FixParts Details
----------------
This program's creation was motivated by cries for help I've seen in online
forums from users who have found their partition tables to be corrupted by
various buggy partitioning tools. Although most OSes can handle the
afflicted disks fine, libparted-based tools (GParted, parted, most Linux
installers, etc.) tend to flake out when presented with these disks.
Typically, the symptom is a disk that appears to hold no partitions;
however, sometimes the libparted tool presents partitions other than those
that the OS sees.
I've observed four causes of these symptoms, three of which FixParts can
correct:
* Old GPT data -- If a disk is used as a GPT disk and then re-used as an
MBR disk, the GPT data may be incompletely erased. This happens if the
disk is repartitioned with fdisk or the Microsoft Windows installer, for
instance. (Tools based on libparted correctly remove the old GPT data
when converting from GPT to MBR format.) FixParts checks for this problem
when it starts and offers to correct it. If you opt to erase the GPT
data, this erasure occurs immediately, unlike other changes the program
makes.
* Mis-sized extended partitions -- Some tools create an extended partition
that's too large, typically ending after the last sector of the disk.
FixParts automatically corrects this problem (if you use the 'w' option
to save the partition table).
* Primary partitions inside an extended partition -- Some utilities create
or move primary partitions to within the range covered by the extended
partition. FixParts can usually correct this problem by turning the
primary partition into a logical partition or by changing one or more
other logical partitions into primaries. Such corrections aren't always
possible, though, at least not without deleting or resizing other
partitions.
* Leftover RAID data -- If a disk is used in a RAID array and then re-used
as a non-RAID disk, some utilities can become confused and fail to see
the disk. FixParts can NOT correct this problem. You must destroy the old
RAID data, or possibly remove the dmraid package from the system, to fix
this problem.
When run, FixParts presents an fdisk-like interface, enabling you to adjust
partition types (primary, logical, or omitted), change type codes, change
the bootable flag, and so on. Although you can delete a partition (by
omitting it), you can't create new partitions with the program. If you're
used to partitioning disks, particularly with Linux fdisk, two unusual
features of FixParts require elaboration:
* No extended partitions -- Internally, FixParts reads the partition table
and discards data on any extended partition(s) it finds. When you save
the partition table, the program generates a new extended partition. This
design means that the program automatically corrects many problems
related to the extended partition. It also means that you'll see no
evidence of extended partitions in the FixParts user interface, although
it keeps track of the requirements and prevents you from creating illegal
layouts, such as a primary between two logicals.
* Partition numbering -- In most Linux tools, partitions 1-4 are primaries
and partitions 5 and up are logicals. Although a legal partition table
loaded into FixParts will initially conform to this convention, some
types of damaged table might not, and various changes you make can also
cause deviations. When FixParts writes the partition table, its numbering
will be altered to conform to the standard MBR conventions, but you
should use the explicit labeling of partitions as primary or logical
rather than the partition numbers to determine a partition's status.
Installing
----------
To compile GPT fdisk, you must have appropriate development tools
installed, most notably the GNU Compiler Collection (GCC) and its g++
compiler for C++. I've also tested compilation with Clang, which seems to
work; however, I've not done extensive testing of the resulting binaries,
beyond checking a few basics. Under Windows, Microsoft Visual C++ 2008 can
be used instead. In addition, note these requirements:
* On Linux, FreeBSD, macOS, and Solaris, libuuid must be installed. This is
the standard for Linux and macOS, although you may need to install a
package called uuid-dev or something similar to get the headers. On
FreeBSD, the e2fsprogs-libuuid port must be installed.
* The ICU library (http://site.icu-project.org), which provides support for
Unicode partition names, is optional on all platforms except Windows, on
which it's not supported. Using this library was required to get proper
UTF-16 partition name support in GPT fdisk versions prior to 0.8.9, but
as of that version it should no longer be required. Nonetheless, you can
use it if you're having problems with the new UTF-16 support. This
library is normally installed in Linux and macOS, but you may need to
install the development headers (libicu-dev or something similar in
Linux; or the libicu36-dev Fink package in macOS). To compile with ICU
support, you must modify the Makefile: Look for commented-out lines that
refer to USE_UTF16, -licuuc, -licudata, or -licucore. Uncomment them and
comment out the equivalents that lack these lines.
* The cgdisk program requires the ncurses library and its development files
(headers). Most Linux distributions install ncurses by default, but you
may need to install a package called libncurses5-dev, ncurses-devel, or
something similar to obtain the header files. On my macOS development
system, I installed the nurses Homebrew ("brew") package; however, other
Unix-style software repositories are available and may work for you (see
the next item). If you're having problems installing ncurses, you can
compile gdisk and/or sgdisk without cgdisk by specifying only the targets
you want to compile to make.
* The sgdisk program requires the popt library and its development files
(headers). Most Linux distributions install popt by default, but you may
need to install a package called popt-dev, popt-devel, or something
similar to obtain the header files. MacOS users can find a version of
popt for Mac OS from Darwin Ports (http://popt.darwinports.com), MacPorts
(https://trac.macports.org/browser/trunk/dports/devel/popt/Portfile), Fink
(http://www.finkproject.org), or brew (http://macappstore.org/popt/);
however, you'll first need to install the relevant environment
(instructions exist on the relevant projects' pages). When I re-built my
Mac build environment in February of 2020, I found that brew was, by far,
the easiest of these to install. Some of the others seem to have been
abandoned, but I didn't investigate thoroughly. I'm leaving the references
in case they might be useful in the future. Instead of installing one of
These ports, you can compile gdisk and/or cgdisk alone, without sgdisk;
gdisk and cgdisk don't require popt.
When all the necessary development tools and libraries are installed, you
can uncompress the package and type "make" at the command prompt in the
resulting directory. (You must type "make -f Makefile.mac" on macOS, "make
-f Makefile.freebsd" on FreeBSD, "make -f Makefile.solaris" on Solaris, or
"make -f Makefile.mingw" to compile using MinGW for Windows.) You may also
need to add header (include) directories or library directories by setting
the CXXFLAGS environment variable or by editing the Makefile. The result
should be program files called gdisk, cgdisk, sgdisk, and fixparts. Typing
"make gdisk", "make cgdisk", "make sgdisk", or "make fixparts" will compile
only the requested programs. You can use these programs in place or copy the
files to a suitable directory, such as /usr/local/sbin. You can copy the man
pages (gdisk.8, cgdisk.8, sgdisk.8, and fixparts.8) to /usr/local/man/man8
to make them available.
Caveats
-------
DISK PARTITIONING SOFTWARE IS DANGEROUS! Although the GPT fdisk project has
existed since 2009, I do not claim it is entirely bug-free; in fact a glance
at the revision history shows recent bug fixes. I believe all
data-corruption bugs to be squashed, but I know full well that the odds of
my missing something are high. This is particularly true for large
(over-2TiB) drives and use in exotic environments.
My main development platform is a system running the 64-bit version of
Ubuntu Linux. I've also tested on several other 32- and 64-bit Linux
distributions, Intel-based macOS 10 and 11, 64-bit FreeBSD 7.1, and Windows
7 and 10. Other environments qualify as "exotic," and even macOS and Windows
are borderline exotic in this context, since I use Linux almost exclusively,
and my impression is that GPT fdisk is far more commonly used on Linux than
in other OSes.
Redistribution
--------------
This program is licensed under terms of the GNU GPL (see the file COPYING).
Acknowledgements
----------------
This code is mostly my own; however, I've used three functions from two
other GPLed programs:
- The code used to generate CRCs is taken from the efone program by
Krzysztof Dabrowski and ElysiuM deeZine. (See the crc32.h and
crc32.cc source code files.)
- A function to find the disk size is taken from Linux fdisk by A. V. Le
Blanc. This code has subsequently been heavily modified.
Additional code contributors include:
- Yves Blusseau (1otnwmz02@sneakemail.com)
- David Hubbard (david.c.hubbard@gmail.com)
- Justin Maggard (justin.maggard@netgear.com)
- Dwight Schauer (dschauer@ti.com)
- Florian Zumbiehl (florz@florz.de)
- Guillaume Delacour (contributed the gdisk_test.sh script)
Mr. DellatioNx196 GaLers xh3LL Backd00r 1.0, Coded By Mr. DellatioNx196 - Bogor BlackHat