Bringup on a New OS or Architecture

CI builders

If you are contributing a port for an operating system or architecture which is not covered by existing CI builders, you will also have to present a plan for testing and contribute a CI builder. See this guide for information on how to add new builders to the LLVM buildbot. You will either have to extend the existing Linux script and/or Windows script or add a new script for your operating system.

An OS specific config directory

If you are starting to bring up LLVM’s libc on a new operating system, the first step is to add a directory for that OS in the libc/config directory. Both Linux and Windows, the two operating systems on which LLVM’s libc is being actively developed, have their own config directory.


Windows development is not as active as the development on Linux. There is a Darwin config also which is in a similar state as Windows.


LLVM’s libc is being brought up on the Fuchsia operating system also. However, there is no config directory for Fuchsia as the bring up is being done in the Fuchsia source tree.

The file

If the Fullbuild Mode is to be supported on the new operating system, then a file named should be added in its config directory. It is written in the LLVM tablegen language. It lists all the relevant macros and type definitions we want in the public libc header files. See the existing Linux file as an example to prepare the file for the new operating system.


In future, LLVM tablegen will be replaced with a different DSL to list config information.

Architecture Subdirectory

There are parts of the libc which are implemented differently for different architectures. The simplest example of this is the syscall function and its internal implementation - its Linux implementation differs for different architectures. Since a large part of the libc makes use of syscalls (or an equivalent on non-Linux like platforms), it might be simpler and convenient to bring up the libc for one architecture at a time. In such cases, wherein the support surface of LLVM’s libc differs for each target architecture, one will have to add a subdirectory (within the config directory os the operating system) for each target architecture, and list the relevant config information separately in those subdirectories. For example, for Linux, the x86_64 and aarch64 configs are in separate directories, named x86_64 and aarch64. The libc CMake machinery looks for subdirectories named after the target architecture.

The entrypoints.txt file

One of the important pieces of config information is listed in a file named entrypoints.txt. This file lists the targets for the entrypoints (see Entrypoints in LLVM libc) you want to include in the static archive of the libc (for the Overlay Mode and/or the Fullbuild Mode.) If you are doing an architecture specific bring up, then an entrypoints.txt file should be created in the architecture subdirectory for each architecture. Else, having a single entrypoints.txt in the operating system directory is sufficient.

The Linux config has an entrypoint.txt for each individual target architecture separately: aarch64, arm32 and x86_64. On the other hand, the Windows config has a single entrypoints.txt file.

A typical bring up procedure will normally bring up a small group of entrypoints at a time. The usual practice is to progressively add the targets for those entrypoints to the entrypoints.txt file as they are being brought up. The same is the case if one is implementing a new entrypoint - the target for the new entrypoint should be added to the relevant entrypoints.txt file. If the implementation of the new entrypoint supports multiple operating systems and target architectures, then multiple entrypoints.txt files will have to be updated.

The headers.txt file

Another important piece of config information is listed in a file named headers.txt. It lists the targets for the set of public headers that are provided by the libc. This is relevant only if the libc is to be used in the Fullbuild Mode on the target operating system and architecture. As with the entrypoints.txt file, one headers.txt file should be listed for each individual target architecture if you are doing an architecture specific bring up. The Linux config has headers.txt file listed separately for the aarch64 config and the x86_64 config.