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UPX is a free, portable, extendable, high-performance executable packer for several executable formats.

Please also see the Wikipedia entry for some more background info.

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Overview

UPX achieves an excellent compression ratio and offers very fast decompression. Your executables suffer no memory overhead or other drawbacks for most of the formats supported, because of in-place decompression. UPX strengths in a nutshell:

excellent compression ratio : typically compresses better than WinZip/zip/gzip, use UPX to decrease the size of your distribution!

: typically compresses better than WinZip/zip/gzip, use UPX to decrease the size of your distribution! very fast decompression : ~10 MB/sec on an ancient Pentium 133, ~200 MB/sec on an Athlon XP 2000+.

: ~10 MB/sec on an ancient Pentium 133, ~200 MB/sec on an Athlon XP 2000+. no memory overhead for your compressed executables because of in-place decompression.

for your compressed executables because of in-place decompression. safe : you can list, test and unpack your executables. Also, a checksum of both the compressed and uncompressed file is maintained internally.

: you can list, test and unpack your executables. Also, a checksum of both the compressed and uncompressed file is maintained internally. universal : UPX can pack a number of executable formats.

: UPX can pack a number of executable formats. portable : UPX is written in portable endian-neutral C++.

: UPX is written in portable endian-neutral C++. extendable : because of the class layout it's very easy to add new executable formats or new compression algorithms.

: because of the class layout it's very easy to add new executable formats or new compression algorithms. free: UPX is distributed with full source code under the GNU General Public License v2+, with special exceptions granting the free usage for commercial programs as stated in the UPX License Agreement.

You probably understand now why we call UPX the "Ultimate Packer for eXecutables".

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