upx-5-1-1

name

upx-5-1-1

description

A skill for using UPX (Ultimate Packer for eXecutables) v5.1.1 to compress and decompress executable files across multiple platforms including Windows, Linux, macOS, DOS, Atari, and PlayStation. Achieves 50%-70% file size reduction with very fast decompression and no runtime memory penalty for most formats. Use when reducing distribution size of executables and DLLs, optimizing storage or download costs, compressing bootable Linux kernels, or packing executables for embedded and resource-constrained environments.

UPX 5.1.1

Overview

UPX (the Ultimate Packer for eXecutables) is a free, secure, portable, and high-performance executable compressor. It typically reduces file sizes of programs and DLLs by 50%-70%, reducing disk space, network load times, download times, and other distribution costs.

Programs compressed by UPX are completely self-contained and run exactly as before, with no runtime or memory penalty for most supported formats due to in-place decompression. Decompression speed exceeds 500 MB/s on modern machines.

UPX is written in portable endian-neutral C++, distributed under the GNU General Public License v2+ with special exceptions granting free usage for all binaries including commercial programs.

Version 5.1.1 was released on March 5, 2026 and adds support for ELF MIPS r3000 (32-bit) shared libraries among other bug fixes.

When to Use

• Reducing distribution size of compiled executables and DLLs
• Optimizing storage or bandwidth for software delivery
• Compressing bootable Linux kernel images (vmlinuz)
• Packing executables for embedded systems with limited flash memory
• Creating smaller release builds for Windows, Linux, macOS, or DOS targets
• Any scenario where executable file size matters and the target format is supported by UPX

Core Concepts

• Compression levels: Levels -1 through -9 plus --best. Default is -8 for files under 512 KiB, -7 otherwise. Levels 1-3 are fast, 4-6 balanced, 7-9 favor ratio, and --best may take a long time.