mlibc

mlibc is a fully featured C standard library designed with portability in mind.

We support a number of architectures (x86-64, AArch64, RISC-V, IA-32, m68k), and provide a clean syscall abstraction layer for new operating system ports to plug into.

Unlike other portable C standard libraries like newlib, we aim for feature parity with glibc/musl, i.e full pthread support and GNU extensions.

mlibc is capable enough to run a range of software, including Xorg, several Wayland compositors, Mesa, and web browsers like WebKitGTK – though some features are still missing.

Individual operating systems can opt in or out of certain features as desired; for example POSIX APIs like pthread are gated behind the POSIX ‘option’, Linux APIs like epoll are gated behind the Linux option, etc.

Official Discord server: https://discord.gg/7WB6Ur3

AUR package (provides mlibc-gcc): https://aur.archlinux.org/packages/mlibc

Design of the library

Directory	Purpose
options/	(More or less) OS-independent headers and code. options/ is divided into subdirectories that can be enabled or disabled by ports.
sysdeps/	OS-specific headers and code. sysdeps/ is divided into per-port subdirectories. Exactly one of those subdirectories is enabled in each build.
abis/	OS-specific interface headers (“ABI headers”). Those contain the constants and structs of the OS interface. For example, the numerical values of SEEK_SET or O_CREAT live here, as well as structs like struct stat. ABI headers are only allowed to contain constants, structs and unions but no function declarations or logic. abis/ is divided into per-OS subdirectories but this division is for organizational purposes only. Ports can still mix headers from different abis/ subdirectories.

Porting mlibc to a new OS

Ports to new OSes are welcome. To port mlibc to another OS, the following changes need to be made:

1. Add new sysdeps/ subdirectory sysdeps/some-new-os/ and a meson.build to compile it. Integrate sysdeps/some-new-os/meson.build into the toplevel meson.build.
2. Create ABI headers in abis/some-new-os/. Add symlinks in sysdeps/some-new-os/include/abi-bits to your ABI headers. Look at existing ports to figure out the ABI headers required for the options enabled by sysdeps/some-new-os/meson.build.
3. In sysdeps/some-new-os/, add code to implement (a subset of) the functions from options/internal/include/mlibc/internal-sysdeps.hpp. Which subset you need depends on the options that sysdeps/some-new-os/meson.build enables.

We recommend that new ports do not build from master as we occasionally make internal changes that cause out-of-tree sysdeps to break. Instead we recommend you pin a specific release (or commit), or to upstream your changes to this repository so that we can build them on our CI and thus any breakages will be fixed by us in-tree.

Build Configuration

The following custom meson options are accepted, in addition to the built-in options. The options below are booleans which default to false (see meson_options.txt).

• headers_only: Only install headers; don’t build libc.so or ld.so.
• no_headers: Don’t install headers; only build libc.so and ld.so.
• build_tests: Build the test suite (see below).
• x_option: Enable x component of mlibc functionality. See meson_options.txt for a full list of possible values for x. This may be used to e.g disable POSIX and glibc extensions.
• linux_kernel_headers: Allows for directing mlibc to installed linux headers. These can be obtained easily, placed in a directory and this option set to the corresponding path. This is required if the linux option is enabled, i.e. when the linux option is not disabled.
• debug_allocator: Replace the normal allocator with a debug allocator (see mlibc/options/internal/generic/allocator.cpp for implementation details).
• use_freestnd_hdrs: Use freestnd-c{,xx}-hdrs instead of looking for compiler headers. Useful if not using a compiler with the correct target triple.

The type of library to be built (static, shared, or both) is controlled by meson’s default_library option. Passing -Ddefault_library=static effectively disables the dynamic linker.

We also support building with -Db_sanitize=undefined to use UBSan inside mlibc. Note that this does not enable UBSan for external applications which link against libc.so, but it can be useful during development to detect internal bugs (e.g when adding new sysdeps).

Running Tests

The mlibc test suite can be run under a Linux host. To do this, first install a set of kernel headers (as described here), then run from the project root:

meson setup -Dbuild_tests=true -Dlinux_kernel_headers=/path/to/kernel/headers/include build

This will create a build directory. Then, cd build and run the tests (showing output) with:

meson test -v