UPX (Ultimate Packer for Executables) is a
free and open source
Free and open-source software (FOSS) is a term used to refer to groups of software consisting of both free software and open-source software where anyone is freely licensed to use, copy, study, and change the software in any way, and the source ...
executable packer supporting a number of file formats from different operating systems.
Compression
UPX uses a
data compression algorithm
In information theory, data compression, source coding, or bit-rate reduction is the process of encoding information using fewer bits than the original representation. Any particular compression is either lossy or lossless. Lossless compression ...
called UCL, which is an
open-source implementation of portions of the proprietary NRV (''Not Really Vanished'') algorithm.
UCL has been designed to be simple enough that a decompressor can be implemented in just a few hundred
byte
The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable uni ...
s of code. UCL requires no additional memory to be allocated for decompression, a considerable advantage that means that a UPX packed executable usually requires no additional memory.
UPX (since 2.90 beta) can use
LZMA on most platforms; however, this is disabled by default for 16-bit due to slow decompression speed on older computers (use
--lzma to force it on).
Starting with version 3.91, UPX also supports 64-Bit (x64)
PE files on the
Windows
Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for ser ...
platform.
[UPX News]
/ref> This feature is currently declared as ''experimental''.
Decompression
UPX supports two mechanisms for decompression: an in-place technique and extraction to temporary file
A temporary file is a file created to store information temporarily, either for a program's intermediate use or for transfer to a permanent file when complete. It may be created by computer programs for a variety of purposes, such as when a program ...
.
The in-place technique, which decompresses the executable into memory, is not possible on all supported platforms. It has the advantage of being more efficient in terms of memory, and that the environment set up by the OS remains correct.
The rest uses extraction to temporary file. This procedure involves additional overhead and other disadvantages; however, it allows any executable file format to be packed. The extraction to temporary file method has several disadvantages:
* Special permissions are ignored, such as suid.
* argv /code> will not be meaningful.
* Multiple running instances of the executable are unable to share common segments.
Unmodified UPX packing is often detected and unpacked by antivirus software
Antivirus software (abbreviated to AV software), also known as anti-malware, is a computer program used to prevent, detect, and remove malware.
Antivirus software was originally developed to detect and remove computer viruses, hence the name. ...
scanners. UPX also has a built-in feature for unpacking unmodified executables packed with itself.
Supported formats
UPX supports the following formats:
* Portable Executable (PE, EXE and Dynamic-link library, DLL files):
**ARM architecture, ARM (Windows CE)
** 32-bit x86 (Windows Desktop)
** 64-bit x86-64 (Windows Desktop, still experimental)
** RTM32 (DOS extender, as generated by Borland Turbo C, C/Turbo Pascal, Pascal compilers)
Notes
References
External links
*
{{compression software implementations
Free data compression software
Free software programmed in C++
EXE packers
1998 software
Assembly language software