Disk images
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A disk image, in computing, is a
computer file A computer file is a System resource, computer resource for recording Data (computing), data in a Computer data storage, computer storage device, primarily identified by its Filename, file name. Just as words can be written to paper, so can data ...
containing the contents and structure of a disk
volume Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). ...
or of an entire
data storage Data storage is the recording (storing) of information (data) in a storage medium. Handwriting, Phonograph record, phonographic recording, magnetic tape, and optical discs are all examples of storage media. Biological molecules such as RNA a ...
device, such as a
hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating hard disk platter, platters ...
,
tape drive A tape drive is a Computer data storage, data storage device that digital recording, reads and writes data on a magnetic tape. Magnetic tape data storage is typically used for offline, archival data storage. Tape media generally has a favorable un ...
,
floppy disk A floppy disk or floppy diskette (casually referred to as a floppy, or a diskette) is an obsolescent type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined wi ...
,
optical disc In computing and optical disc recording technologies, an optical disc (OD) is a flat, usually circular disc that encodes binary data (bits) in the form of Compact disk#Physical details, pits and lands on a special material, often aluminum, ...
, or
USB flash drive Universal Serial Bus (USB) is an technical standard, industry standard that establishes specifications for cables, connectors and communication protocol, protocols for connection, communication and power supply (Interface (computing), interfa ...
. A disk image is usually made by creating a sector-by-sector copy of the source medium, thereby perfectly replicating the structure and contents of a storage device independent of the
file system In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is Computer data storage, stored and retrieved. Without a file system, data placed in a storage me ...
. Depending on the disk image format, a disk image may span one or more computer files. The
file format A file format is a Computer standard, standard way that information is encoded for storage in a computer file. It specifies how bits are used to encode information in a digital storage medium. File formats may be either proprietary format, pr ...
may be an
open standard An open standard is a standard that is openly accessible and usable by anyone. It is also a prerequisite to use open license, non-discrimination and extensibility. Typically, anybody can participate in the development. There is no single definiti ...
, such as the ISO image format for optical disc images, or a disk image may be unique to a particular software application. The size of a disk image can be large because it contains the contents of an entire disk. To reduce storage requirements, if an imaging utility is filesystem-aware it can omit copying unused space, and it can compress the used space.


History

Disk images were originally (in the late 1960s) used for
backup In information technology, a backup, or data backup is a copy of computer data taken and stored elsewhere so that it may be used to restore the original after a data loss event. The verb form, referring to the process of doing so, is " back up ...
and disk cloning of mainframe disk media. The early ones were as small as 5
megabyte The megabyte is a multiple of the unit byte for digital information. Its recommended unit symbol is MB. The unit prefix ''mega'' is a multiplier of (106) in the International System of Units (SI). Therefore, one megabyte is one million bytes o ...
s and as large as 330 megabytes, and the copy medium was
magnetic tape Magnetic tape is a medium for magnetic storage made of a thin, magnetizable coating on a long, narrow strip of plastic film. It was developed in Germany in 1928, based on the earlier magnetic wire recording from Denmark. Devices that use mag ...
, which ran as large as 200 megabytes per reel. Disk images became much more popular when floppy disk media became popular, where replication or storage of an exact structure was necessary and efficient, especially in the case of copy protected floppy disks.


Uses

Disk images are used for duplication of optical media including DVDs, Blu-ray discs, etc. It is also used to make perfect clones of hard disks. A
virtual disk Virtual disk and virtual drive are software components that Emulator, emulate an actual disk storage device. Virtual disks and virtual drives are common components of virtual machines in hardware virtualization, but they are also widely used for ...
may emulate any type of physical drive, such as a hard disk drive,
tape drive A tape drive is a Computer data storage, data storage device that digital recording, reads and writes data on a magnetic tape. Magnetic tape data storage is typically used for offline, archival data storage. Tape media generally has a favorable un ...
, key drive,
floppy drive A floppy disk or floppy diskette (casually referred to as a floppy, or a diskette) is an obsolescent type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined wi ...
, CD/
DVD The DVD (common abbreviation for Digital Video Disc or Digital Versatile Disc) is a digital optical disc data storage format. It was invented and developed in 1995 and first released on November 1, 1996, in Japan. The medium can store any k ...
/ BD/
HD DVD HD DVD (short for High Definition Digital Versatile Disc) is an obsolete high-density optical disc format for storing data and playback of high-definition video. Supported principally by Toshiba, HD DVD was envisioned to be the successor to the ...
, or a network share among others; and of course, since it is not physical, requires a virtual reader device matched to it (see below). An emulated drive is typically created either in RAM for fast read/write access (known as a RAM disk), or on a hard drive. Typical uses of virtual drives include the mounting of disk images of CDs and DVDs, and the mounting of virtual hard disks for the purpose of on-the-fly
disk encryption Disk encryption is a technology which protects information by converting it into unreadable code that cannot be deciphered easily by unauthorized people. Disk encryption uses disk encryption software or disk encryption hardware, hardware to encrypt ...
("OTFE"). Some operating systems such as
Linux Linux ( or ) is a family of free and open-source software, open-source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linu ...
and
macOS macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac (computer), Mac computers. Within the market of ...
Although macOS's built-in DiskImageMounter software does not emulate a physical drive have virtual drive functionality built-in (such as the
loop device In Unix-like operating systems, a loop device, vnd (vnode disk), or lofi (loop file interface) is a Device file#Pseudo-devices, pseudo-device that makes a computer file accessible as a Device file system#Block devices, block device. Before use, a l ...
), while others such as older versions of
Microsoft Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft Microsoft Corporation is an American multinational corporation, multinational technology company, technology corporatio ...
require additional software. Starting from
Windows 8 Windows 8 is a major release of the Windows NT operating system developed by Microsoft. It was Software release life cycle#Release to manufacturing (RTM), released to manufacturing on August 1, 2012; it was subsequently made available for downl ...
, Windows includes native virtual drive functionality. Virtual drives are typically read-only, being used to mount existing disk images which are not modifiable by the drive. However some software provides virtual CD/DVD drives which can produce new disk images; this type of virtual drive goes by a variety of names, including "virtual burner".


Enhancement

Using disk images in a virtual drive allows users to shift data between technologies, for example from CD optical drive to hard disk drive. This may provide advantages such as speed and noise (hard disk drives are typically four or five times faster than optical drives, are quieter, suffer from less wear and tear, and in the case of
solid-state drive A solid-state drive (SSD) is a solid-state storage device that uses integrated circuit assemblies to store data Persistence (computer science), persistently, typically using flash memory, and functioning as secondary storage in the Computer ...
s, are immune to some physical trauma). In addition it may reduce power consumption, since it may allow just one device (a hard disk) to be used instead of two (hard disk plus optical drive). Virtual drives may also be used as part of emulation of an entire machine (a
virtual machine In computing, a virtual machine (VM) is the virtualization/emulator, emulation of a computer system. Virtual machines are based on computer architectures and provide functionality of a physical computer. Their implementations may involve special ...
).


Software distribution

Since the spread of broadband, CD and DVD images have become a common medium for
Linux distribution A Linux distribution (often abbreviated as distro) is an operating system made from a software collection that includes the Linux kernel and, often, a package management system. Linux users usually obtain their operating system by downloading one ...
s. Applications for
macOS macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac (computer), Mac computers. Within the market of ...
are often delivered online as an Apple Disk Image containing a file system that includes the application, documentation for the application, and so on. Online data and bootable recovery CD images are provided for customers of certain commercial software companies. Disk images may also be used to distribute software across a company network, or for portability (many CD/DVD images can be stored on a hard disk drive). There are several types of software that allow software to be distributed to large numbers of networked machines with little or no disruption to the user. Some can even be scheduled to update only at night so that machines are not disturbed during business hours. These technologies reduce end-user impact and greatly reduce the time and man-power needed to ensure a secure corporate environment. Efficiency is also increased because there is much less opportunity for human error. Disk images may also be needed to transfer software to machines without a compatible physical disk drive. For computers running
macOS macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac (computer), Mac computers. Within the market of ...
, disk images are the most common file type used for
software Software is a set of computer programs and associated software documentation, documentation and data (computing), data. This is in contrast to Computer hardware, hardware, from which the system is built and which actually performs the work. ...
downloads, typically downloaded with a
web browser A web browser is application software for accessing websites. When a User (computing), user requests a web page from a particular website, the browser retrieves its Computer file, files from a web server and then displays the page on the user' ...
. The images are typically compressed Apple Disk Image (.dmg suffix) files. They are usually opened by directly mounting them without using a real disk. The advantage compared with some other technologies, such as Zip and RAR archives, is they do not need redundant drive space for the unarchived data. Software packages for
Windows Windows is a group of several Proprietary software, proprietary graphical user interface, graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, W ...
are also sometimes distributed as disk images including
ISO image An optical disc image (or ISO image, from the ISO 9660 file system used with CD-ROM media) is a disk image that contains everything that would be written to an optical disc, disk sector by disc sector, including the optical disc file system. IS ...
s. While Windows versions prior to
Windows 7 Windows 7 is a major release of the Windows NT operating system developed by Microsoft. It was Software release life cycle#Release to manufacturing (RTM), released to manufacturing on July 22, 2009, and became generally available on October 22, ...
do not natively support mounting disk images to the files system, several software options are available to do this; see
Comparison of disc image software This article is a comparison of notable software applications that can access or manipulate disk image files. It compares their disk image handling features. Comparison criteria This article compares two features: Supported file formats and ca ...
.


Security

Virtual hard disks are often used in on-the-fly
disk encryption Disk encryption is a technology which protects information by converting it into unreadable code that cannot be deciphered easily by unauthorized people. Disk encryption uses disk encryption software or disk encryption hardware, hardware to encrypt ...
("OTFE") software such as FreeOTFE and
TrueCrypt TrueCrypt is a discontinued source-available freeware utility software, utility used for on-the-fly encryption (OTFE). It can create a virtual encrypted disk within a file, or encrypt a Disk partitioning, partition or the whole Data storage dev ...
, where an encrypted "image" of a disk is stored on the computer. When the disk's password is entered, the disk image is "mounted", and made available as a new volume on the computer. Files written to this virtual drive are written to the encrypted image, and never stored in cleartext. The process of making a computer disk available for use is called "mounting", the process of removing it is called "dismounting" or "unmounting"; the same terms are used for making an encrypted disk available or unavailable.


Virtualization

A
hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating hard disk platter, platters ...
or
solid-state drive A solid-state drive (SSD) is a solid-state storage device that uses integrated circuit assemblies to store data Persistence (computer science), persistently, typically using flash memory, and functioning as secondary storage in the Computer ...
in a
virtual machine In computing, a virtual machine (VM) is the virtualization/emulator, emulation of a computer system. Virtual machines are based on computer architectures and provide functionality of a physical computer. Their implementations may involve special ...
is implemented as a disk image, stored either as a collection of flat files, collectively called a ''split'' flat file, where each one is typically 2GB in size, or as a single, large ''monolithic'' flat file. Disk image formats include the VHD format used by Microsoft's
Hyper-V Microsoft Hyper-V, codenamed Viridian, and briefly known before its release as Windows Server Virtualization, is a native hypervisor; it can create virtual machines on x86-64 systems running Microsoft Windows, Windows. Starting with Windows 8, Hyp ...
, the VDI format used by
Oracle Corporation Oracle Corporation is an American Multinational corporation, multinational computer Technology company, technology corporation headquartered in Austin, Texas. In 2020, Oracle was the List of the largest software companies, third-largest softw ...
's
VirtualBox Oracle VM VirtualBox (formerly Sun VirtualBox, Sun xVM VirtualBox and Innotek VirtualBox) is a type-2 hypervisor for x86 virtualization developed by Oracle Corporation. VirtualBox was originally created by Innotek GmbH, which was acquired by S ...
, the VMDK format used for
VMware VMware, Inc. is an American cloud computing and virtualization technology company with headquarters in Palo Alto, California. VMware was the first commercially successful company to virtualize the x86 architecture. VMware's desktop software ru ...
virtual machines, and the QCOW format used by
QEMU QEMU is a free and open-source emulator (Quick EMUlator). It emulates the machine's central processing unit, processor through dynamic binary translation and provides a set of different hardware and device models for the machine, enabling it t ...
.


Forensic imaging

Forensic imaging is the process that involves copying the contents and recording an image of the entire drives contents (imaging) into a single file (or a very small number of files). A component of forensic imaging involves verification of the values imaged to ensure the integrity of the file(s) imaged. Forensic images are created using software tools that can be acquired. Some tools have added forensic functionality previously mentioned; it is typically used to replicate the contents of the hard drive for use in another system. This can typically be done by software programs as it only structure are files themselves. Forensic images are typically acquired using software tools compatible with their system. Note that some forensic imaging software tools may have limitations in terms of the software's ability to communicate, diagnose, or repair storage mediums that (often times) are experiencing errors or even a failure of some internal component.


Data recovery

Data recovery imaging is the process of imaging each sector, systematically, on the source drive to another destination storage medium from which required files can then be retrieved. In data recovery situations, one cannot always rely on the integrity of their particular file structure and therefore a complete sector copy is mandatory to imaging end there though.


System backup

Some backup programs only
back up In information technology, a backup, or data backup is a copy of computer data taken and stored elsewhere so that it may be used to restore the original after a data loss event. The verb form, referring to the process of doing so, is "wikt:back u ...
user files;
boot A boot is a type of footwear. Most boots mainly cover the foot and the ankle, while some also cover some part of the lower calf. Some boots extend up the human leg, leg, sometimes as far as the knee or even the hip. Most boots have a heel (shoe ...
information and files locked by the operating system, such as those in use at the time of the backup, may not be saved on some operating systems. A disk image contains all files, faithfully replicating all data, including file attributes and the file fragmentation state. For this reason, it is also used for backing up
optical media In computing and optical disc recording technologies, an optical disc (OD) is a flat, usually circular disc that encodes binary data (bits) in the form of Compact disk#Physical details, pits and lands on a special material, often aluminum, ...
( CDs and
DVD The DVD (common abbreviation for Digital Video Disc or Digital Versatile Disc) is a digital optical disc data storage format. It was invented and developed in 1995 and first released on November 1, 1996, in Japan. The medium can store any k ...
s, etc.), and allows the exact and efficient recovery after experimenting with modifications to a system or
virtual machine In computing, a virtual machine (VM) is the virtualization/emulator, emulation of a computer system. Virtual machines are based on computer architectures and provide functionality of a physical computer. Their implementations may involve special ...
, in one go. There are benefits and drawbacks to both "file-based" and "bit-identical" image backup methods. Files that don't belong to installed programs can usually be backed up with file-based backup software, and this is preferred because file-based backup usually saves more time or space because they never copy unused space (as a ''bit-identical'' image does), they usually are capable of incremental backups, and generally have more flexibility. But for files of installed programs, file-based backup solutions may fail to reproduce all necessary characteristics, particularly with Windows systems. For example, in Windows certain registry keys use short filenames, which are sometimes not reproduced by file-based backup, some
commercial software Commercial software, or seldom payware, is a computer software that is produced for Selling, sale or that serves Commerce, commercial purposes. Commercial software can be proprietary software or free and open-source software. Background and chal ...
uses
copy protection Copy protection, also known as content protection, copy prevention and copy restriction, describes measures to enforce copyright A copyright is a type of intellectual property that gives its owner the exclusive right to copy, distribute, ...
that will cause problems if a file is moved to a different
disk sector In computer disk storage, a sector is a subdivision of a Track (disk drive), track on a magnetic disk or optical disc. Each sector stores a fixed amount of user-accessible data, traditionally 512 bytes for hard disk drives (HDDs) and 2048 bytes ...
, and file-based backups do not always reproduce
metadata Metadata is "data that provides information about other data", but not the content of the data, such as the text of a message or the image itself. There are many distinct types of metadata, including: * Descriptive metadata – the descriptive ...
such as security attributes. Creating a bit-identical disk image is one way to ensure the system backup will be exactly as the original. Bit-identical images can be made in
Linux Linux ( or ) is a family of free and open-source software, open-source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linu ...
with dd, available on nearly all
live CD A live CD (also live DVD, live disc, or live operating system) is a complete booting, bootable computer installation including operating system which runs directly from a CD-ROM or similar storage device into a computer's memory, rather than lo ...
s. Most commercial imaging software is "user-friendly" and "automatic" but may not create bit-identical images. These programs have most of the same advantages, except that they may allow restoring to partitions of a different size or file-allocation size, and thus may ''not'' put files on the same exact sector. Additionally, if they do not support
Windows Vista Windows Vista is a major release of the Windows NT operating system developed by Microsoft. It was the direct successor to Windows XP, which was released five years before, at the time being the longest time span between successive releases of ...
, they may slightly move or realign partitions and thus make Vista unbootable (see Windows Vista startup process).


Rapid deployment of clone systems

Large enterprises often need to buy or replace new computer systems in large numbers. Installing operating system and programs into each of them one by one requires a lot of time and effort and has a significant possibility of human error. Therefore, system administrators use disk imaging to quickly clone the fully prepared software environment of a reference system. This method saves time and effort and allows administrators to focus on each systems unique idiosyncrasies they must bear. There are several types of disk imaging software available that use single instancing technology to reduce the time, bandwidth, and storage required to capture and archive disk images. This makes it possible to rebuild and transfer information-rich disk images at lightning speeds, which is a significant improvement over the days when programmers spent hours configuring each machine within an organization.


Legacy hardware emulation

Emulator In computing, an emulator is Computer hardware, hardware or software that enables one computer system (called the ''host'') to behave like another computer system (called the ''guest''). An emulator typically enables the host system to run so ...
s frequently use disk images to simulate the floppy drive of the computer being emulated. This is usually simpler to program than accessing a real floppy drive (particularly if the disks are in a format not supported by the host operating system), and allows a large library of software to be managed.


Copy protection circumvention

A ''mini image'' is an optical disc image file in a format that fakes the disk's content to bypass CD/DVD copy protection. Because they are the full size of the original disk, Mini Images are stored instead. Mini Images are small, on the order of
kilobyte The kilobyte is a multiple of the unit byte for Computer data storage, digital information. The International System of Units (SI) defines the prefix ''kilo-, kilo'' as 1000 (103); per this definition, one kilobyte is 1000 bytes.International S ...
s, and contain just the information necessary to bypass CD-checks. Therefore; the Mini Image is a form of a No-CD crack, for unlicensed games, and legally backed up games. Mini images do not contain the real data from an image file, just the code that is needed to satisfy the CD-check. They cannot provide CD or DVD backed data to the computer program such as on-disk image or video files.


Creation

Creating a disk image is achieved with a suitable program. Different disk imaging programs have varying capabilities, and may focus on hard drive imaging (including
hard drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating hard disk platter, platters ...
backup In information technology, a backup, or data backup is a copy of computer data taken and stored elsewhere so that it may be used to restore the original after a data loss event. The verb form, referring to the process of doing so, is " back up ...
, restore and rollout), or
optical media In computing and optical disc recording technologies, an optical disc (OD) is a flat, usually circular disc that encodes binary data (bits) in the form of Compact disk#Physical details, pits and lands on a special material, often aluminum, ...
imaging (CD/DVD images). A ''virtual disk writer'' or ''virtual burner'' is a computer program that emulates an actual disc authoring device such as a CD writer or DVD writer. Instead of writing data to an actual disc, it creates a virtual disk image. A virtual burner, by definition, appears as a disc drive in the system with writing capabilities (as opposed to conventional disc authoring programs that can create virtual disk images), thus allowing software that can burn discs to create virtual discs.


File formats

*
IMG (file format) IMG, in computing, refers to binary files with the .img filename extension that store raw disk images of floppy disks, hard drives, and optical discs or a bitmap, bitmap image – .img. Overview The .img filename extension is used by disk imag ...


Utilities

RawWrite and WinImage are examples of floppy disk image file writer/creator for
MS-DOS MS-DOS ( ; acronym for Microsoft Disk Operating System, also known as Microsoft DOS) is an operating system for x86-based personal computers mostly developed by Microsoft. Collectively, MS-DOS, its rebranding as IBM PC DOS, and a few oper ...
and
Microsoft Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft Microsoft Corporation is an American multinational corporation, multinational technology company, technology corporatio ...
. They can be used to create raw image files from a
floppy disk A floppy disk or floppy diskette (casually referred to as a floppy, or a diskette) is an obsolescent type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined wi ...
, and write such image files to a floppy. In
Unix Unix (; trademarked as UNIX) is a family of Computer multitasking, multitasking, multiuser computer operating systems that derive from the original AT&T Corporation, AT&T Unix, whose development started in 1969 at the Bell Labs research center ...
or similar systems the dd program can be used to create disk images, or to write them to a particular disk. It is also possible to mount and access them at block level using a
loop device In Unix-like operating systems, a loop device, vnd (vnode disk), or lofi (loop file interface) is a Device file#Pseudo-devices, pseudo-device that makes a computer file accessible as a Device file system#Block devices, block device. Before use, a l ...
. Apple Disk Copy can be used on
Classic Mac OS Mac OS (originally System Software; retronym: Classic Mac OS) is the series of operating systems developed for the Macintosh family of personal computers by Apple Computer from 1984 to 2001, starting with System 1 and ending with Mac OS 9. The ...
and
macOS macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac (computer), Mac computers. Within the market of ...
systems to create and write disk image files. Authoring software for CDs/DVDs such as Nero Burning ROM can generate and load disk images for optical media.


See also

* Boot image * Card image *
Comparison of disc image software This article is a comparison of notable software applications that can access or manipulate disk image files. It compares their disk image handling features. Comparison criteria This article compares two features: Supported file formats and ca ...
* Disk cloning * El Torito (CD-ROM standard) *
ISO image An optical disc image (or ISO image, from the ISO 9660 file system used with CD-ROM media) is a disk image that contains everything that would be written to an optical disc, disk sector by disc sector, including the optical disc file system. IS ...
, an archive file of an optical media volume *
Loop device In Unix-like operating systems, a loop device, vnd (vnode disk), or lofi (loop file interface) is a Device file#Pseudo-devices, pseudo-device that makes a computer file accessible as a Device file system#Block devices, block device. Before use, a l ...
* Mtools * no-CD crack * Protected Area Run Time Interface Extension Services (PARTIES) *
ROM image A ROM image, or ROM file, is a computer file which contains a copy of the data from a read-only memory Computer chip, chip, often from a ROM cartridge, video game cartridge, or used to contain a computer's firmware, or from an arcade game's arc ...
* Software cracking


References


External links


Software repository including RAWRITE2
{{Film piracy Archive formats Compact Disc and DVD copy protection Computer file formats Disk image emulators Hacker culture Hardware virtualization Optical disc authoring Warez