musl is a C standard library intended for operating systems based on the Linux kernel, released under the MIT License.^[3] It was developed by Rich Felker to write a clean, efficient, and standards-conformant libc implementation.^[4]

musl was designed from scratch to allow efficient static linking and to have realtime-quality robustness by avoiding race conditions, internal failures on resource exhaustion, and various other bad worst-case behaviors present in existing implementations.^[4] The dynamic runtime is a single file with stable ABI allowing race-free updates and the static linking support allows an application to be deployed as a single portable binary without significant size overhead.

It claims compatibility with the POSIX 2008 specification and the C11 standard. It also implements most of the widely used non-standard Linux, BSD, and glibc functions.^[5] There is partial ABI compatibility with the part of glibc required by Linux Standard Base.^[6]

Version 1.2.0 has support for (no longer current) Unicode 12.1.0 (while still having full UTF-8 support,^[7] more conformant/strict than glibc), and version 1.2.1 "features the new 'mallocng' malloc implementation, replacing musl's original dlmalloc-like allocator that suffered from fundamental design problems."^[2]

Some Linux distributions that can use musl as the standard C library include Alpine Linux,^[8] Dragora 3,^[9] Gentoo Linux,^[10] OpenWrt,^[11] Sabotage,^[12] Morpheus Linux,^[13] Chimera Linux,^[14] and Void Linux.^[15] The seL4 microkernel^[16] ships with musl. For binaries that have been linked against glibc, gcompat^[17] can be used to execute them on musl-based distros.