musl libc
A compact, standards-minded C library in place of glibc. Programs are smaller and start faster; software written only for glibc sometimes needs a hand.
the C library
Built on Alpine
Small, simple and secure — Alpine's own three words, and the reason a desktop this complete can start from a Pi Zero.
01What Alpine is
Alpine Linux is an independent Linux distribution built around a few deliberate choices, each smaller than the usual one:
BusyBox
The everyday commands — shell, file tools, networking — as one small program, instead of a separate package for each.
the userland
OpenRC
A dependency-based init system of plain shell scripts, which a person can read end to end.
the init system
apk
A fast package manager with a stable main repository, a community one, and edge/testing for newer software.
packages
By Alpine's own count, a container needs no more than 8 MB and a minimal install to disk about 130 MB. Stable releases come twice a year, and each is supported for years. It is the base of a great many container images for the same reasons it suits a Raspberry Pi: little is running, and what is running can be read.
02Diskless, then set hard
Alpine can run diskless: it boots from read-only media
into RAM, and nothing is written anywhere unless you save it. That is how it
first comes up on a Copal card, and it is where the name comes from — resin
before it hardens. Stage 3 moves the running system onto the disk in Alpine's
ordinary sys mode, keeping the small system and giving it a
disk to remember with. Everything Copal does is done with Alpine's own tools:
setup-alpine for the base, apk for packages, OpenRC
for services.
03What building for musl takes
Copal compiles programs Alpine does not carry, or carries in an older release, and most of them were written on glibc. The differences are small, specific, and worth writing down. From the programs built for Copal Apps:
| The difference | What it broke | What Copal does |
|---|---|---|
| a thread gets 128 KB of stack, not 8 MB | Naev crashed loading textures on a worker thread; DDNet's renderer overflowed | links every build for glibc-sized thread stacks |
no execinfo.h | crash handlers that print a stack trace | stubs that report no frames; the crash is still logged |
| glibc's own extensions | Ardour's bundled GTK used nftw flags musl lacks | takes the portable path the code already had |
| build probes that test glibc's macros | darktable's image library could not learn the page size | gives it the machine's page and cache-line sizes |
| large-file names are opt-in | code calling stat64 and friends | asks for them in every build |
Each fix sits in the playbook of the program that needed it, with a comment saying why — so the next version of the program can be built without rediscovering it.
04Aggregated, not forked
Copal downloads stock Alpine from Alpine's mirrors and checks it against Alpine's published checksum before anything is written. It replaces no package. What it installs comes from Alpine's own repositories — edge/testing only for the packages named for it, tagged so nothing else follows — or is compiled on the machine from a pinned release whose checksum is written in its playbook. The exceptions are named: the Brave browser, from Flathub, and Claude Code, from npm. Copal is not affiliated with or endorsed by the Alpine project — it is built on its work, with thanks.