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system/lib/libc/README.md

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This folder contains the musl version of libc at /musl. The upstream version can be found at http://www.musl-libc.org/.

Most of the source comes from musl v1.2.5, with some exceptions listed below. We track these changes from upstream in https://github.com/emscripten-core/musl and use a script (system/lib/update_musl.py) to pull in updates.

Some changes have been made to the version that was taken from upstream, including:

  • Emscripten-specific changes (from before this readme existed). These should be marked with XXX EMSCRIPTEN in the source, or ifdefed with #if __EMSCRIPTEN__. They are mostly in pthreads code and hopefully temporary.
  • Backporting an operator-precedence warning fix from 6e76e1540fc58a418494bf5eb832b556f9c5763e in the upstream version.
  • Switch to using the wasi fd_write syscall instead of writev.
  • Simplify stdout stream handling: do not support seeking, terminal handling, etc., as it just increases code size and Emscripten doesn't have those features anyhow.
  • Setting _POSIX_REALTIME_SIGNALS and _POSIX_SPAWN macros to -1, to exclude unsupported functions.
  • Handling trailing % in strftime and wcsftime format strings.

Copy log.c and log2.c from earlier version of musl which result in smaller binary size since they do not rely on data tables in log_data.c and log2_data.c. See https://github.com/emscripten-core/emscripten/issues/15483.

Verifying upstream musl behaviour

Occasionally when working on libc/musl it can be useful to verify to behavior of upstream musl. For example, when trying to determine if a certain behavior is a bug in emscripten, or an upstream bug, or just expected musl behavior.

When I need to do this I use the Alpine linux docker image. Alpine linux is a distro where the sysmtem libc is musl so by default any program you build within the contains will be using musl libc.

$ docker run --rm -it -v "$(pwd):/data" alpine /bin/sh

Then from inside the new container you can run tests against Alpine's musl libc. For example:

$ apk add build-base
$ cd /data
$ gcc -pthread test/pthread/test_pthread_cancel_async.c
$ ./a.out