libc

Embedded Artistry libc

Embedded Artistry’s libc is a stripped-down C standard library implementation targeted for microcontroller-based embedded systems.

In order to conserve precious memory resources, this library does not supply the complete C standard library implementation. Instead, a subset of functions which are useful on bare-metal embedded systems has been selected. If you have a bare metal or RTOS-based embedded system, this library is for you!

Unlike many other C libraries that I’ve come across, this library implements unit tests and has addressed long-standing flaws in open-source implementations of the C standard library functions. We’re continually adding tests and making additional improvements over the baseline implementations.

malloc and free are not included in this library. If you need dynamic memory allocation support, you will need to couple this library with something like Embedded Artistry’s libmemory, which contains implementations of malloc and free.

If you are interested in contributing to this project, please read the CONTRIBUTING guide.

Table of Contents

1. About the Project
2. Project Status
3. Getting Started
   1. Requirements
      1. git-lfs
      2. Meson Build System
   2. Getting the Source
   3. Building 2. Disabling Position Independent Code 2. Enabling Link-time Optimization
   4. Installation
   5. Usage
4. Configuration Options
5. Stack Overflow Protection
6. Documentation
7. Need Help?
8. Contributing
9. Further Reading
10. Authors
11. License
12. Acknowledgments

About the Project

Embedded Artistry’s libc is intended to provide a portable set of useful C standard library functions that allows quick bring-up of new bare-metal and RTOS-based embedded systems.

Additionally, we want to provide a high-quality libc implementation by ensuring that each function has unit test coverage and addresses flaws exposed by the static analyzer. Many C library function implementations remain untested and contain errors. We are fighting back against poor implementations.

In order to conserve memory, this library does not supply the complete C standard library functionality. Instead, a subset of functions which are useful on bare-metal embedded systems has been selected. This selection has primarily been driven by my own experience in microcontroller-focused development. If you need additional features, please file an issue and make a feature request.

The functional implementations in this library have been selected for portability and quick bring-up of new systems. There may be more efficient implementations for these functions, but often they are architecture specific implementations. If you have suggestions for improving performance, we are always happy to hear them.

malloc and free are not included in this library. Because memory allocation schemes vary greatly with embedded systems (some not even allowing dynamic memory), you will need to supply your own implementations based on your system’s needs. You can couple this library with the Embedded Artistry libmemory, which contains implementations of malloc and free.

Project Status

This library provides a complete-enough implementation to compile and link clang’s libc++ and libc++abi (see Embedded Artistry’s libcpp project). In order to achieve this, some functions are only defined in the headers but do not have an implementation. Unsupported-but-defined functions can be removed using a build option (hide-unimplemented-libc-apis).

The following portions of the C library have been implemented:

• assert
• Basic C runtime support (crt.c, exit, atexit, etc.)
• ctype
• math (via openlibm)
• string functions
• stdlib
  • atoX
  • abs, labs, llabs
  • bsearch
  • calloc
  • div, ldiv, lldiv
  • heapsort, heapsort_r
  • imaxabs, imaxdiv
  • qsort, qsort_r
  • rand family
  • realloc
  • strtoX functions (many via gdtoa)
• Basic stdio
  • printf family (most via embeddedartistry/printf, based on eyalroz/printf and originally mpaland/printf)
  • putchar
  • puts
• time types and asctime()
• wchar type definitions and wctype functions

In addition, this library provides implementations for __stack_chk_guard and __stack_chk_fail.

The following architectures are currently supported:

• x86
• x86_64
• ARM
• ARM64

The following unit tests need to be added:

• realloc
• rand family

These are not implemented by may be added in the future:

• wchar functions
• errno support (enabled as a compile-time switch)
• getopt support
• time support
• FILE and additional stdio functions

We are currently not planning full support for:

• locale

Getting Started

Requirements

This project uses Embedded Artistry’s standard Meson build system, and dependencies are described in detail on our website.

At a minimum you will need:

• git-lfs, which is used to store binary files in this repository
• Meson is the build system
• Some kind of compiler for your target system.
  • This repository has been tested with:
    • gcc-7, gcc-8, gcc-9
    • arm-none-eabi-gcc
    • Apple clang
    • Mainline clang

git-lfs

This project stores some files using git-lfs.

To install git-lfs on Linux:

sudo apt install git-lfs

To install git-lfs on MacOS:

brew install git-lfs

Additional installation instructions can be found on the git-lfs website.

Meson Build System

The Meson build system depends on python3 and ninja-build.

To install on Linux:

sudo apt-get install python3 python3-pip ninja-build

To install on OSX:

brew install python3 ninja

Meson can be installed through pip3:

pip3 install meson

If you want to install Meson globally on Linux, use:

sudo -H pip3 install meson

Getting the Source

This project uses git-lfs, so please install it before cloning. If you cloned prior to installing git-lfs, simply run git lfs pull after installation.

This project is hosted on GitHub. You can clone the project directly using this command:

git clone --recursive [email protected]:embeddedartistry/libc.git

If you don’t clone recursively, be sure to run the following command in the repository or your build will fail:

git submodule update --init

Building

If Make is installed, the library can be built by issuing the following command:

make

This will build all targets for your current architecture.

You can clean builds using:

make clean

You can eliminate the generated buildresults folder using:

make distclean

You can also use meson directly for compiling.

Create a build output folder:

meson buildresults

And build all targets by running

ninja -C buildresults

Cross-compilation is handled using meson cross files. Example files are included in the build/cross folder. You can write your own cross files for your specific processor by defining the toolchain, compilation flags, and linker flags. These settings will be used to compile libc. (or open an issue and we can help you).

Cross-compilation must be configured using the meson command when creating the build output folder. For example:

meson buildresults --cross-file build/cross/gcc_arm_cortex-m4.txt

Following that, you can run make (at the project root) or ninja to build the project.

Tests will not be cross-compiled. They will only be built for the native platform.

Full instructions for building the project, using alternate toolchains, and running supporting tooling are documented in Embedded Artistry’s Standardized Meson Build System on our website.

Disabling Position Independent Code

Position Independent Code (PIC) is enabled by default, but can be disabled during the Meson configuration stage by setting the built-in option b_staticpic to false:

meson buildresults -Db_staticpic=false

Enabling Link-time Optimization

Link-time Optimization (LTO) can be enabled during the meson configuration stage by setting the built-in option b_lto to true:

meson buildresults -Db_lto=true

This can be combined with other build options.

Installation

If you don’t use meson for your project, the best method to use this project is to build it separately and copy the headers and library contents into your source tree.

• Copy the include/ directory contents into your source tree.
• Library artifacts are stored in the buildresults/src folder
• Copy the desired library to your project and add the library to your link step.

Example linker flags:

-Lpath/to/libc.a -lc

If you’re using meson, you can use libc as a subproject. Place it into your subproject directory of choice and add a subproject statement:

libc = subproject('libc')

You will need to promote the desired subproject dependency variable to your project:

libc_dep = libc.get_variable('libc_dep')

You can use the dependency for your target library configuration in your executable declarations(s) or other dependencies. For example:

fwdemo_sim_platform_dep = declare_dependency(
	include_directories: fwdemo_sim_platform_inc,
	dependencies: [
		fwdemo_simulator_hw_platform_dep,
		posix_os_dep,
		libmemory_native_dep,
		libc_dep, # <----- libc added here
		libcxxabi_native_dep,
		libcxx_full_native_dep,
		logging_subsystem_dep
	],
	sources: files('platform.cpp'),
)

Testing

The tests for this library are written with CMocka, which is included as a subproject and does not need to be installed on your system. You can run the tests by issuing the following command:

make test

By default, test results are generated for use by the CI server and are formatted in JUnit XML. The test results XML files can be found in buildresults/test/.

Configuration Options

The following meson project options can be set for this library when creating the build results directory with meson, or by using meson configure:

• enable-pedantic: Turn on pedantic warnings
• enable-pedantic-error: Turn on pedantic warnings and errors
• hide-unimplemented-libc-apis: Hides the header definitions for functions which are not actually implemented
• enable-gnu-extensions will enable GNU libc extensions that are implemented in this library
• disable-builtins will tell the compiler not to generate built-in functions, forcing it to use the library functions
• disable-stack-protection will tell the compiler not to insert stack protection calls
• stack-canary-value enables you to customize the canary value for your application. Supply a hexadecimal string (e.g., '0xdeadbeef') with the same length as your processor’s word size.
• disable-stk-guard-runtime-config disables the code that is used to configure __stk_chk_guard during program initialization. When this option is true, the program will revert to using a hard-coded definition for the guard value.

Options can be specified using -D and the option name:

meson buildresults -Ddisable-builtins=false

The same style works with meson configure:

cd buildresults
meson configure -Ddisable-builtins=false

Stack Overflow Protection

This library provides an implementation of __stack_chk_guard and __stack_chk_fail, which enables it to be used with GCC and Clang’s stack protection code.

The default value for __stack_chk_guard can be overridden with the stack-canary-value build option.

Documentation

[Documentation for the latest release can always be found here]https://embeddedartistry.github.io/libc/index.html.

Documentation can be built locally by running the following command:

make docs

Documentation can be found in buildresults/docs, and the root page is index.html.

Need help?

If you need further assistance or have any questions, please file a GitHub Issue or send us an email using the Embedded Artistry Contact Form.

You can also reach out on Twitter: mbeddedartistry.

Contributing

If you are interested in contributing to this project, please read our contributing guidelines.

Authors

• Phillip Johnston - original library author - Embedded Artistry

License

Copyright © 2022 Embedded Artistry LLC

This project is licensed under the MIT License - see LICENSE file for details.

For other open-source licenses, please see the Software Inventory.

Acknowledgments

I’d like to thank the following individuals for their direct contributions on this project:

• @seekaddo (Documentation)
• @mpaland (Original author of the printf library)
• @eyalroz (Picked up mpaland’s printf library and continued development)

Many of the open-source function implementations used in this library have been pulled from two primary sources:

• Apple Open Source libc
  • abs, atof, bsearch, div family, heapsort/qsort family, abs family, imax family, strtol/ll/ull
  • fls, flsl, flsll
  • strstr, strtok. strnstr, strnlen, strndup, strncpy, strncat, strlen, strdup, strcpy, strcat
  • memmem, memcpy
• musl libc
  • All ctype member functions (locale support removed)
  • strrchr, strchrnul, strchr
  • memset, memrchr

I have also used and improved the open-source gdtoa library.

The initial groundwork of testing was implemented by referencing the libc-test project.

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