“llvm-libc” C Standard Library


This is a proposal to start llvm-libc, an implementation of the C standard library targeting C17 and above, as part of the LLVM project. llvm-libc will also provide platform specific extensions as relevant. For example, on Linux it also provides pthreads, librt and other POSIX extension libraries.


llvm-libc will be developed to have a certain minimum set of features:

  • C17 and upwards conformant.

  • A modular libc with individual pieces implemented in the “as a library” philosophy of the LLVM project.

  • Ability to layer this libc over the system libc if possible and desired for a platform.

  • Provide C symbols as specified by the standards, but take advantage and use C++ language facilities for the core implementation.

  • Provides POSIX extensions on POSIX compliant platforms.

  • Provides system-specific extensions as appropriate. For example, provides the Linux API on Linux.

  • Vendor extensions if and only if necessary.

  • Designed and developed from the start to work with LLVM tooling and testing like fuzz testing and sanitizer-supported testing.

  • ABI independent implementation as far as possible.

  • Use source based implementations as far possible rather than assembly. Will try to fix the compiler rather than use assembly language workarounds.

  • Extensive unit testing and standards conformance testing. If relevant and possible, differential testing: We want to be able to test llvm-libc against another battle-tested libc. This is essentially to understand how we differ from other libcs. Also if relevant and possible, test against the testsuite of an another battle-tested libc implementation.

Why a new C Standard Library?

Implementing a libc is no small task and is not be taken lightly. A natural question to ask is, “why a new implementation of the C standard library?” There is no single answer to this question, but some of the major reasons are as follows:

  • Most libc implementations are monolithic. It is a non-trivial porting task to pick and choose only the pieces relevant to one’s platform. The llvm-libc will be developed with sufficient modularity to make picking and choosing a straightforward task.

  • Most libc implementations break when built with sanitizer specific compiler options. The llvm-libc will be developed from the start to work with those specialized compiler options.

  • The llvm-libc will be developed to support and employ fuzz testing from the start.

  • Most libc implementations use a good amount of assembly language, and assume specific ABIs (may be platform dependent). With the llvm-libc implementation, we want to use normal source code as much as possible so that compiler-based changes to the ABI are easy. Moreover, as part of the LLVM project, we want to use this opportunity to fix performance related compiler bugs rather than using assembly workarounds.

  • A large hole in the LLVM toolchain will be plugged with llvm-libc. With the broad platform expertise in the LLVM community, and the strong license and project structure, we think that llvm-libc will be more tunable and robust, without sacrificing the simplicity and accessibility typical of the LLVM project.

Platform Support

We envision that llvm-libc will support a variety of platforms in the coming years. Interested parties are encouraged to participate in the design and implementation, and add support for their favorite platforms.

ABI Compatibility

As llvm-libc is new, it will not offer ABI stability in the initial stages. However, as we’ve heard from other LLVM contributors that they are interested in having ABI stability, llvm-libc code will be written in a manner which is amenable to ABI stability. We are looking for contributors interested in driving the design in this space to help us define what exactly does ABI stability mean for llvm-libc.

Layering Over Another libc

When meaningful and practically possible on a platform, llvm-libc will be developed in a fashion that it will be possible to layer it over the system libc. This does not mean that one can mix llvm-libc with the system-libc. Also, it does not mean that layering is the only way to use llvm-libc. What it means is that, llvm-libc can optionally be packaged in a way that it can delegate parts of the functionality to the system-libc. The delegation happens internal to llvm-libc and is invisible to the users. From the user’s point of view, they only call into llvm-libc.

There are a few problems one needs to be mindful of when implementing such a delegation scheme in llvm-libc. Examples of such problems are:

1. One cannot mix data structures from llvm-libc with those from the system-libc. A translation from one set of data structures to the other should happen internal to llvm-libc. 2. The delegation mechanism has to be implemented over a related set of functions. For example, one cannot delegate just the fopen function to the system-libc. One will have to delegate all FILE related functions to the system-libc.

Current Status

llvm-libc development is still in the planning phase.

Build Bots

Once the development starts, there will be llvm-libc focused builders added to the LLVM BuildBot.