awesome-awesomeness/terminal/c

                                   awesome-c !Awesome (https://cdn.rawgit.com/sindresorhus/awesome/d7305f38d29fed78fa85652e3a63e154dd8e8829/media/badge.svg) (https://github.com/sindresorhus/awesome)!List Status 
                                                         (https://api.travis-ci.org/aleksandar-todorovic/awesome-c.svg?branch=master) (https://travis-ci.org/aleksandar-todorovic/awesome-c)

A curated list of C good stuff. This list contains only free software (https://en.wikipedia.org/wiki/Free_software) for code, and sellers who aren't evil for physical resources.

LOOKING FOR MAINTAINERS: This list is currently pretty much inactive. I'm looking for maintainers that can make it great again. See #26 (https://github.com/aleksandar-todorovic/awesome-c/issues/26) for more info, but the process is simple: add or
fix a couple of things in this list and you'll become a contributor to a 850+ stars list.

This list was previously maintained by @kozross (https://github.com/kozross). He decided to switch the list to a new platform (https://notabug.org/koz.ross/awesome-c), so I've decided to fork it so we could keep it on GitHub.

Contents
========

Contents ##

- Build Systems (#build-systems)
- Compilers (#compilers)
- Compiler libraries (#compiler-libraries)
- Compression (#compression)
- Crypto (#crypto)
- Database (#database)
- Deep Learning (#deep-learning)
- Documentation Generation (#documentation-generation)
- Editors (#editors)
- Embedded Systems (#embedded-systems)
  - RTOS (#rtos)
- Environments (#environments)
- Frameworks (#frameworks)
- Game Programming (#game-programming)
  - Engines (#engines)
  - Resources (#resources)
- Generic Programming (#generic-programming)
- Graphics (#graphics)
  - Graphic APIs (#graphic-apis)
- Graphical User Interface (#graphical-user-interface)
- Hashing (#hashing)
- Image Processing (#image-processing)
- JSON (#json)
- Learning, Reference and Tutorials (#learning-reference-and-tutorials)
  - Online (#online)
  - **Reference** (#reference)                    
    - **Language Standards** (#language-standards)
  - **Beginner** (#beginner)                      
  - **Intermediate** (#intermediate)              
  - **Advanced** (#advanced)                      
  - **Self-study courses** (#self-study-courses)  
  - Physical (#physical)
  - **Reference** (#reference-1)      
  - **Beginner** (#beginner-1)        
  - **Intermediate** (#intermediate-1)
  - **Advanced** (#advanced-1)        
- Macros (#macros)
- Memory Allocators (#memory-allocators)
- Multimedia (#multimedia)
- Networking and Internet (#networking-and-internet)
  - Web Frameworks (#web-frameworks)
- Numerical (#numerical)
- Parallel Programming (#parallel-programming)
- Parsers (#parsers)
- Regex (#regex)
- Serialization (#serialization)
- Source Code Collections (#source-code-collections)
- Standard Libraries (#standard-libraries)
- String Manipulation (#string-manipulation)
- Testing (#testing)
- Text Editor Extensions (#text-editor-extensions)
  - Atom (#atom)
  - Emacs (#emacs)
  - Vim (#vim)
- Tools (#tools)
- Utilities (#utilities)
- XML (#xml)

Build Systems ##

These are tools to automate the building and testing of projects in C.

⟡ aimake (http://nethack4.org/projects/aimake/) - Build tool designed to avoid complex configurations. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ Autoconf (https://www.gnu.org/software/autoconf/autoconf.html) - Extensible package of M4 macros that produce shell scripts to automatically configure software source code packages. Part of the Autotools. GNU GPL3 or later 
(http://www.gnu.org/licenses/gpl-3.0.html)
⟡ Automake (https://www.gnu.org/software/automake/automake.html) - Tool for automatically generating Makefile.in files compliant with the GNU Coding Standards. Requires the use of Autoconf. Part of the Autotools. GNU GPL3 or later 
(http://www.gnu.org/licenses/gpl-3.0.html)
⟡ CMake (https://cmake.org/) - Tools for cross-platform building, testing, and packaging. 3-clause BSD (https://gitlab.kitware.com/cmake/cmake/raw/master/Copyright.txt)
⟡ Jam (https://www.perforce.com/documentation/jam-documentation) - Build system, designed to be easier than make. Understands C build rules implicitly. Jam License (https://en.wikipedia.org/wiki/Perforce_Jam#License)
⟡ Libtool (https://www.gnu.org/software/libtool/) - Generic library support script. Part of the Autotools. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ Meson (https://mesonbuild.com/) - Extremely fast, user-friendly build system. Based on Ninja. Apache 2.0 (https://directory.fsf.org/wiki/License:Apache-2.0)
⟡ Premake (https://github.com/premake/premake-core) - Build script generator for toolsets like Visual Studio, Xcode, or GNU Make. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ SCons (https://www.scons.org/) - An easier, more reliable and faster way to build software. MIT (https://raw.githubusercontent.com/atom/atom/master/LICENSE.md)
⟡ Ninja (https://github.com/ninja-build/ninja) - Small, simple build system with a focus on speed. Apache 2.0 (https://directory.fsf.org/wiki/License:Apache-2.0)
⟡ Bazel (https://bazel.build/) - Build system for various operating systems and targets. Apache 2.0 (https://directory.fsf.org/wiki/License:Apache-2.0)
⟡ Buck (https://buck.build/) - Build system created and used by Facebook. Apache 2.0 (https://directory.fsf.org/wiki/License:Apache-2.0)
⟡ Qbs (https://doc.qt.io/qbs/) - Modern build tool for software projects. LGPL3 (https://code.qt.io/cgit/qbs/qbs.git/tree/LICENSE.LGPLv3)
⟡ qmake (https://doc.qt.io/qt-5/qmake-manual.html) - Build system included with the Qt Framework. GNU GPL3 with Qt Exception (https://github.com/qt/qtbase/blob/5.11/LICENSE.GPL3-EXCEPT)
⟡ xmake (https://xmake.io/) - Cross-platform build utility based on lua. Apache 2.0 (https://github.com/xmake-io/xmake/blob/master/LICENSE.md )

Compilers ##

⟡ Clang (http://clang.llvm.org/) - C compiler for LLVM. Supports C11. NCSA (https://directory.fsf.org/wiki/License:NCSA)
⟡ CompCert (http://compcert.inria.fr/) - Fully-verified C compiler. Supports almost all of C89. GNU GPL2.1 or later (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ GCC (https://gcc.gnu.org/) - Provides a C compiler as part of its compiler set. Supports C11 and OpenMP. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ PCC (http://pcc.ludd.ltu.se/) - Venerable C compiler. Supports C99. Various free licenses (http://pcc.ludd.ltu.se/licenses/)
⟡ TCC (https://bellard.org/tcc/) - Tiny C Compiler; a small, fast C compiler. Supports C99 (except complex types). GNU LGPL2.1 (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html)
⟡ Intel SPMD (http://ispc.github.io/) - Compiler for a variant of the C language, for single program, multiple data programming. Various licenses (https://github.com/ispc/ispc/blob/master/LICENSE.txt)
⟡ ccache (https://ccache.dev/) - Compiler cache designed to speedup recompilation. GNU GPL3 or later (https://ccache.dev/license.html)

Compiler libraries ##

Libraries for use in compilers, interpreters, and low-level based projects.

⟡ yasm (http://yasm.tortall.net/) - Yasm Modular Assembler Project. 2-clause BSD (https://directory.fsf.org/wiki/License:BSD-2-Clause)

Compression ##

⟡ libzip (https://libzip.org/) - A C library for reading, creating, and modifying zip archives. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ lzo (http://www.oberhumer.com/opensource/lzo/) - Very fast data compression library. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ zlib (http://zlib.net/) - Massively-spiffy yet delicately-unobtrusive compression library. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ bzip2 (http://www.bzip.org/) - Patent free, high quality data compressor. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ lz4 (https://lz4.github.io/lz4/) - Fast Compression algorithm.
⟡ snappy (https://github.com/google/snappy) - Fast compression library (implementation in C++, native bindings to C).
⟡ zstd (http://facebook.github.io/zstd/) - Fast real-time compression algorithm 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ brotli (https://github.com/google/brotli) - Generic lossless compression algorithm based on LZ77, Huffman coding and 2nd order context modelling MIT (https://raw.githubusercontent.com/atom/atom/master/LICENSE.md)
⟡ quicklz (http://www.quicklz.com/index.php) - Fast compression library. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ pixz (https://github.com/vasi/pixz) - Pixz (pronounced pixie) is a parallel, indexing version of xz. 2-clause BSD (https://directory.fsf.org/wiki/License:BSD-2-Clause)

Crypto ##

⟡ GnuTLS (https://www.gnutls.org/) - Secure communication library, implementing SSL, TLS and DTLS. GNU LGPL2.1 or later (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html)
⟡ libgcrypt (https://gnupg.org/related_software/libgcrypt/) - General-purpose cryptography library, with a range of available ciphers. GNU LGPL2.1or later (code) (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html) and 
GNU GPL2.1 or later (manual and tools) (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ OpenSSL (https://www.openssl.org/) - Implementation of the SSL and TLS protocols, and also includes a cryptography library. Dual Licensed under the OpenSSL License and the SSLeay License (https://www.openssl.org/source/license.html)
⟡ libsodium (https://download.libsodium.org/doc/) - Modern and easy-to-use crypto library. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ libtomcrypt (https://www.libtom.net/) - Fairly comprehensive, modular and portable cryptographic toolkit. Public Domain (https://creativecommons.org/share-your-work/public-domain/)
⟡ mbed TLS (https://tls.mbed.org/) - Another crypto implementation for C. Apache 2.0 (https://directory.fsf.org/wiki/License:Apache-2.0)
⟡ libressl (https://www.libressl.org/) - Modernized fork of OpenSSL. Various Licenses (https://cvsweb.openbsd.org/cgi-bin/cvsweb/~checkout~/src/lib/libssl/LICENSE?rev=1.12)

Database ##

This lists databases and data stores with C APIs.

⟡ BerkeleyDB (https://www.oracle.com/database/berkeley-db/) - Library for a high-performance embedded database for key-value data. GNU AGPLv3 (https://www.gnu.org/licenses/agpl-3.0.html)
⟡ Hiredis (https://github.com/redis/hiredis) - Minimalistic client library for Redis. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ LMDB (https://github.com/LMDB/lmdb) - Ultra-fast, ultra-compact key-value embedded data store. OpenLDAP License (https://directory.fsf.org/wiki/License:OLDAP-2.7)
⟡ MariaDB (https://mariadb.com/) - Robust, scalable and reliable SQL server, designed to be a drop-in replacement for MySQL. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ mongo-c-driver (http://mongoc.org/) - High-performance client library for MongoDB (https://www.mongodb.com/). Apache 2.0 (https://directory.fsf.org/wiki/License:Apache-2.0)
⟡ PostgreSQL (https://www.postgresql.org/) - Powerful object-relational database system. PostgreSQL licence (https://opensource.org/licenses/postgresql)
⟡ recutils (https://www.gnu.org/software/recutils/) - Set of tools and a C library for accessing human-editable, plaintext database files called recfiles. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ Redis (https://redis.io/) - Advanced key-value store. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ sophia (http://sophia.systems/) - Modern, embeddable key-value database. FreeBSD (https://directory.fsf.org/wiki?title=License:FreeBSD)
⟡ SQLite (https://www.sqlite.org/about.html) - Self-contained, serverless, zero-configuration, transactional SQL database engine with a C interface. Public Domain (https://creativecommons.org/share-your-work/public-domain/)
⟡ UnQLite (https://unqlite.org/) - Self-contained, serverless, zero-configuration, transactional NoSQL engine with a C interface. FreeBSD (https://directory.fsf.org/wiki?title=License:FreeBSD)

Deep Learning ##
⟡ Darknet (https://pjreddie.com/darknet/) - Open source neural network framework written in C and CUDA. It is fast, easy to install, and supports CPU and GPU computation.

Documentation Generation ##

⟡ Cxref (http://www.gedanken.org.uk/software/cxref/) - Generates documentation of C programs in either LaTeX, HTML, RTF or SGML. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ DocOnce (https://hplgit.github.io/doconce/doc/web/index.html) - Modestly-tagged markup language that can be used to generate a range of formats. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ Doxygen (http://www.doxygen.nl/) - The de-facto standard tool for generating C documentation from annotated sources. Can generate a large range of formats. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ GTK-Doc (https://www.gtk.org/gtk-doc/) - Tool for generating C documentation from annotated sources. Has support for the Autotools. GNU GPL2.1 (code) (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html) or GNU FDL1.1 
(https://www.gnu.org/licenses/old-licenses/fdl-1.1.html)

Editors ##

These are specifically fancier, IDE-type editors. If you want a programmer's text editor, look elsewhere. Besides, whatever choice you make most likely supports C anyway.

⟡ Anjuta DevStudio (http://anjuta.org/) - The GNOME IDE. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html)
⟡ Atom (https://atom.io/) - Hackable text editor for the 21st century. MIT (https://raw.githubusercontent.com/atom/atom/master/LICENSE.md)
⟡ Code::Blocks (http://www.codeblocks.org/) - Extensible, configurable IDE supporting C. GNU GPL3 (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ CodeLite (https://codelite.org/) - Cross-platform IDE. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ Eclipse (http://www.eclipse.org/ide/) - IDE written in Java. EPL (https://directory.fsf.org/wiki/License:EPL-1.0)
⟡ Geany (https://www.geany.org/) - Very small and fast IDE. GNU GPL2.1 or later (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ KDevelop (https://www.kdevelop.org/) - The KDE IDE. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ Qt Creator (https://www.qt.io/qt-features-libraries-apis-tools-and-ide/#ide) - Cross-platform IDE written with C++ and Qt, part of the Qt SDK. Supports Clang Code Model. GNU GPL3 with Qt exception 
(https://github.com/qt-creator/qt-creator/blob/master/LICENSE.GPL3-EXCEPT)
⟡ Visual Studio Code (https://code.visualstudio.com/) - Code editing, redefined. Visual Studio Code.

Embedded Systems ##

These are projects that allow one to work with microcontrollers and other resource constrained architectures.

RTOS ###

A real-time operating system (RTOS) is intended to serve real-time applications that process data with low latency delay.

⟡ Amazon FreeRTOS (https://aws.amazon.com/freertos/) - RTOS for microcontrollers that makes small, low-power edge devices easy to program. MIT (https://github.com/aws/amazon-freertos/blob/master/LICENSE)
⟡ ChibiOS (http://www.chibios.org/dokuwiki/doku.php) - RTOS with complete development environment (HAL, drivers, support files, and tools). Various Licenses (http://www.chibios.org/dokuwiki/doku.php?id=chibios:licensing:start)
⟡ Contiki (http://www.contiki-os.org/) - Connect low-cost, low power microcontrollers to the Internet. 3-clause BSD (https://github.com/contiki-os/contiki/blob/master/LICENSE)
⟡ Zephyr Project (https://www.zephyrproject.org/) - Scalable real-time RTOS supporting multiple hardware architectures, optimized for resource constrained devices. Apache 2.0 (https://directory.fsf.org/wiki/License:Apache-2.0)
⟡ RIOT OS (https://www.riot-os.org/) - Real-time multi-threading operating system that supports a range of devices that are typically found in the Internet of Things. LGPL-2.1 (https://www.gnu.org/licenses/old-licenses/lgpl-2.1.en.html)

Environments ##

This is a list of technologies designed to bring Windows into the 21st century with respect to support for C.

⟡ Cygwin (https://cygwin.com/) - Designed to emulate a POSIX-compatible environment extensively under Windows. Various Licenses (https://cygwin.com/licensing.html)
⟡ MinGW-w64 (http://mingw-w64.yaxm.org/doku.php/start) - Minimalist environment for C development on Windows with 64 bit support. Various Licenses (http://mingw.org/license)

Frameworks ##

This section has big libraries that provide data structures and other stuff you expect of a 'modern' standard library.

⟡ APR (http://apr.apache.org/) - Apache Portable Runtime; another library of cross-platform utility functions. Apache 2.0 (https://directory.fsf.org/wiki/License:Apache-2.0)
⟡ C Algorithms (https://fragglet.github.io/c-algorithms/) - Collection of common algorithms and data structures for C. ISC (https://directory.fsf.org/wiki/License:ISC)
⟡ C Macro Collections (https://github.com/LeoVen/C-Macro-Collections) - Header only, macro generated, generic and type-safe Collections in C. MIT (https://spdx.org/licenses/MIT.html)
⟡ CPL (http://www.eso.org/sci/software/cpl/) - The Common Pipeline Library; a set of libraries designed to be a comprehensive, efficient and robust software toolkit. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ EFL (https://www.enlightenment.org/) - Large collection of useful data structures and functions.
⟡ GLib (https://wiki.gnome.org/Projects/GLib) - Library of utility functions and structures, designed to be portable, efficient and powerful. GNU LGPL2.1 (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html)
⟡ GIO (https://developer.gnome.org/gio/) - Modern and easy-to-use VFS API. GNU LGPL2.1 (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html)
⟡ GObject (https://developer.gnome.org/gobject/stable/) - Object-oriented system and object model for C. GNU LGPL2.1 (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html)
⟡ libnih (https://github.com/keybuk/libnih) - Lightweight library of C functions and structures. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ libU (http://www.koanlogic.com/libu/) - Small library of basic utilities, including memory allocation, string manipulation and logging. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ PBL (http://www.mission-base.com/peter/source/) - Large library of utilities, featuring data structures, among other things. GNU LGPL2.1 or later (library) (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html) or 
GNU GPL2.1 or later (test code) (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ qlibc (http://wolkykim.github.io/qlibc/) - Simple and powerful C library, designed as a replacement for GLib while focusing on being small and light. qLib license (https://github.com/wolkykim/qlibc/blob/master/LICENSE) (similar to FreeBSD 
(https://directory.fsf.org/wiki?title=License:FreeBSD))
⟡ stb (https://github.com/nothings/stb) - Range of single-file libraries for C. Public Domain (https://creativecommons.org/share-your-work/public-domain/)
⟡ libsrt (https://faragon.github.io/libsrt.html) - Safe Real-Time library for C. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)


Game Programming ##

Engines ###

These are provided as examples of C game programming code.

⟡ Corange (https://github.com/orangeduck/Corange) - Game engine in pure C. FreeBSD (https://directory.fsf.org/wiki?title=License:FreeBSD)
⟡ Darkplaces (https://icculus.org/twilight/darkplaces/) - Modified version of the Quake2 engine. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ ioquake3 (https://ioquake3.org/) - The Quake3 engine, freed at last. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ Orx (http://orx-project.org/) - Portable, lightweight, plugin-based, data-driven, 2D-oriented game engine. zlib (https://directory.fsf.org/wiki/License:Zlib)
⟡ Quake (https://github.com/id-Software/Quake) - The Quake engine. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ Quake2 (https://github.com/id-Software/Quake-2) - The Quake2 engine. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ Spearmint (https://clover.moe/spearmint/) - Engine designed for FPS games. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)

Resources ###

These are libraries of all sorts that are useful for game programming.

⟡ Allegro (https://liballeg.org/) - Cross-platform, video game development and multimedia library. zlib (https://directory.fsf.org/wiki/License:Zlib)
⟡ Chipmunk2D (http://chipmunk-physics.net/) - Fast and lightweight 2D game physics library. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ CSFML (https://www.sfml-dev.org/download/csfml/) - Binding for SFML (https://www.sfml-dev.org/index.php) in C. zlib (https://directory.fsf.org/wiki/License:Zlib)
⟡ FreeGLUT (http://freeglut.sourceforge.net/) - Alternative to the OpenGL Utility Toolkit. Allows the creation and management of windows with OpenGL contexts. X11 (https://directory.fsf.org/wiki/License:X11)
⟡ GLFW (https://www.glfw.org/) - Multi-platform library for creating windows with OpenGL contexts. zlib (https://directory.fsf.org/wiki/License:Zlib)
⟡ libao (https://xiph.org/ao/) - Cross-platform audio library with a wide variety of outputs. GNU GPL2.1 or later (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ RetroArch (https://www.libretro.com/) - The reference frontend for libretro (https://www.libretro.com/). GNU GPL3 (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ SDL and SDL2 (https://www.libsdl.org/) - Cross-platform development library designed to provide low-level access to audio, keyboard, mouse, joystick and graphics hardware via OpenGL. SDL2 is the most current version. zlib 
(https://directory.fsf.org/wiki/License:Zlib)
⟡ raylib (https://www.raylib.com/) - Simple and easy-to-use library to learn videogames programming. zlib (https://directory.fsf.org/wiki/License:Zlib)

Generic Programming ##

⟡ klib (http://attractivechaos.github.io/klib/#About) - Small and lightweight implementations of common algorithms and data structures. MIT (https://en.wikipedia.org/wiki/MIT_License)

Graphics ##

⟡ Cairo (https://www.cairographics.org/) -2D graphics library. GNU LGPL2.1 (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html) or MPLv1.1 (https://directory.fsf.org/wiki/License:MPL-1.1)
⟡ Cogl (https://github.com/rib/cogl-web/wiki) - GPU graphics and utilities API. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ Clutter (https://blogs.gnome.org/clutter/get-it/) - UI library based on OpenGL. GNU LGPL2.1 (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html)
⟡ giflib (https://sourceforge.net/projects/giflib/) - Library for reading and writing gif images. MIT (https://spdx.org/licenses/MIT.html)
⟡ heman (https://github.com/prideout/heman) - Tiny library of image utilities dealing with height maps, normal maps, distance fields and the like. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ libcaca (https://github.com/cacalabs/libcaca) - ASCII renderer for terminal-based interfaces. WTFPLv2 (http://www.wtfpl.net/txt/copying/)
⟡ libimagequant (https://pngquant.org/lib/) - Small, portable library for high-quality conversion of RGBA images to 8-bit indexed colour images. FreeBSD (https://directory.fsf.org/wiki?title=License:FreeBSD)
⟡ libjpeg-turbo (https://libjpeg-turbo.org/) - Faster library for reading and writing JPEG files. Various Licenses (https://libjpeg-turbo.org/About/License)
⟡ libpng (http://www.libpng.org) - The official PNG reference library. libpng license (http://www.libpng.org/pub/png/src/libpng-LICENSE.txt)
⟡ libxmi (https://www.gnu.org/software/libxmi/) - Function library for rasterizing 2D vector graphics. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ mozjpeg (https://github.com/mozilla/mozjpeg) - Improved JPEG encoder. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)

Graphic APIs ###

⟡ OpenGL (https://www.opengl.org/) - Industry adopted 2D and 3D graphics API. More resources at awesome-opengl (https://github.com/eug/awesome-opengl).
⟡ OpenGL ES (https://www.khronos.org/opengles/) - Industry adopted 2D and 3D graphics API for mobile and embedded devices.
⟡ OpenGL SC (https://www.khronos.org/openglsc/) - Graphic and compute standard for industry requiring system safety certification.
⟡ Vulkan (https://www.khronos.org/vulkan/) - Explicit graphic and compute API for modern cross-platform development. More resources at awesome-vulkan (https://github.com/vinjn/awesome-vulkan).

Graphical User Interface ##

These are specifically widget toolkits (https://en.wikipedia.org/wiki/Widget_toolkit).

⟡ GTK+ (https://www.gtk.org/) - Cross-platform widget toolkit. GNU LGPL2.1 (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html)
⟡ IUP (http://webserver2.tecgraf.puc-rio.br/iup/) - Another cross-platform widget toolkit. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ Tk (http://www.tcl.tk/) - Basic widget toolkit. Part of Tcl/Tk. Tcl/Tk License (http://www.tcl.tk/software/tcltk/license.html)
⟡ XForms Toolkit (http://xforms-toolkit.org/) - Widget toolkit designed for the XWindow system. GNU LGPL2.1 (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html)
⟡ nuklear (https://github.com/vurtun/nuklear) - Single-header ANSI C gui library. Public Domain (https://creativecommons.org/share-your-work/public-domain/)
⟡ libui (https://github.com/andlabs/libui) - Simple and portable (but not inflexible) GUI library. MIT (https://github.com/andlabs/libui/blob/master/LICENSE)
⟡ LCUI (https://github.com/lc-soft/LCUI/) - Small C library for building user interfaces with C, XML and CSS. MIT (https://github.com/lc-soft/LCUI/blob/develop/LICENSE.TXT)

Hashing ##

All hashing related libraries, cryptographic or not.

⟡ jwHash (https://github.com/watmough/jwHash) - Fast hashtable implementation. Apache 2.0 (https://directory.fsf.org/wiki/License:Apache-2.0)
⟡ xxHash (http://cyan4973.github.io/xxHash/) - Extremely fast non-cryptographic hash algorithm. 2-clause BSD (https://directory.fsf.org/wiki/License:BSD-2-Clause)
⟡ libcrc (https://github.com/PeterScott/murmur3) - Multi platform CRC library. MIT (https://raw.githubusercontent.com/atom/atom/master/LICENSE.md)
⟡ murmur (https://github.com/ispc/ispc) - C implementation of MurMur Hashing. Public Domain (https://creativecommons.org/share-your-work/public-domain/)
⟡ t1ha (https://github.com/leo-yuriev/t1ha) - Fast Positive Hash library. zlib (https://directory.fsf.org/wiki/License:Zlib)

Image Processing ##

⟡ libccv (http://libccv.org) - Modern Computer Vision Library. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)

JSON ##

⟡ Jansson (http://www.digip.org/jansson/) - C library for encoding, decoding and manipulating JSON. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ jsmn (https://zserge.com/jsmn.html) - Minimalistic JSON parser. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ json-c (https://github.com/json-c/json-c/wiki) - Library for working with JSON. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ parson (https://github.com/kgabis/parson) - Lightweight JSON library written in C. MIT (https://raw.githubusercontent.com/atom/atom/master/LICENSE.md)
⟡ WJElement (https://github.com/netmail-open/wjelement/wiki) - Advanced JSON manipulation library, with support for JSON Schema. LGPL, any version (https://github.com/netmail-open/wjelement/)
⟡ YAJL (https://lloyd.github.io/yajl/) - Fast C JSON streaming parser library. ISC (https://directory.fsf.org/wiki/License:ISC)

Learning, Reference and Tutorials ##

This is a list of resources for learning C programming in general, or something useful relating to C programming.

Online ###

Reference ####

⟡ SEI CERT C Coding Standard (https://wiki.sei.cmu.edu/confluence/display/c/SEI+CERT+C+Coding+Standard)
⟡ C FAQ - comp.lang.c Frequently Asked Questions (http://c-faq.com/)
⟡ Comparison of C/POSIX standard library implementations for GNU/Linux (http://www.etalabs.net/compare_libcs.html)
⟡ GNU C Reference Manual (https://www.gnu.org/software/gnu-c-manual/)
⟡ Robert Pike's notes on programming in C (http://www.lysator.liu.se/c/pikestyle.html)
⟡ Draft ANSI C Rationale (http://port70.net/~nsz/c/c89/rationale/)
⟡ Draft C99 Rationale (http://port70.net/~nsz/c/c99/C99RationaleV5.10.pdf)

Language Standards #####
⟡ Draft ANSI C (C89) standard (http://port70.net/~nsz/c/c89/c89-draft.txt)
⟡ Draft C99 standard (http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf)
⟡ Draft C11 standard (http://www.open-std.org/JTC1/SC22/WG14/)

Beginner ####

⟡ Tutorial on pointers (http://www.cs.ucsb.edu/~mikec/cs16/misc/ptrtut12/index.htm)
⟡ Building C Projects (http://nethack4.org/blog/building-c.html)
⟡ C Programming Wikibook (https://en.wikibooks.org/wiki/C_Programming)
⟡ Introduction to 'fun' C (https://gist.github.com/eatonphil/21b3d6569f24ad164365)
⟡ Learning C with GDB (https://www.recurse.com/blog/5-learning-c-with-gdb)
⟡ POSIX Threads Programming tutorial (https://computing.llnl.gov/tutorials/pthreads/) (a little dated, but most of it is still valid and useful)
⟡ The GNU C Programming Tutorial (http://www.crasseux.com/books/ctut.pdf) (online PDF)
⟡ Templating in C (http://blog.pkh.me/p/20-templating-in-c.html)

Intermediate ####

⟡ 8 gdb tricks you should know (https://blogs.oracle.com/linux/8-gdb-tricks-you-should-know-v2)
⟡ 10 C99 tricks (https://blog.noctua-software.com/c-tricks.html)
⟡ Diving into concurrency: trying out mutexes and atomics (https://jvns.ca/blog/2014/12/14/fun-with-threads/)
⟡ Introduction to OpenMP (https://www.youtube.com/playlist?list=PLLX-Q6B8xqZ8n8bwjGdzBJ25X2utwnoEG) (video)
⟡ OpenMP tutorial (https://computing.llnl.gov/tutorials/openMP/) (for the OpenMP3 standard)
⟡ memcpy vs memmove (https://web.archive.org/web/20170620131430/https://www.tedunangst.com/flak/post/memcpy-vs-memmove)
⟡ MPI tutorial (https://computing.llnl.gov/tutorials/mpi/)
⟡ Some unknown features or tricks in C language (https://proprogramming.org/some-unknown-features-or-tricks-in-c-language/)
⟡ The lost art of C structure packing (http://www.catb.org/esr/structure-packing/)
⟡ What a C programmer should know about memory (https://marek.vavrusa.com/memory/)
⟡ What every C programmer should know about undefined behaviour (http://blog.llvm.org/2011/05/what-every-c-programmer-should-know.html)

Advanced ####

⟡ Advanced metaprogramming in C (http://250bpm.com/blog:56)
⟡ Quick tutorial on implementing and debugging malloc, free, calloc, and realloc (http://danluu.com/malloc-tutorial/)
⟡ Bit twiddling hacks (https://graphics.stanford.edu/~seander/bithacks.html)
⟡ I do not know C (https://kukuruku.co/post/i-do-not-know-c/)
⟡ Implementing smart pointers for the C programming language (https://snai.pe/c/c-smart-pointers/)
⟡ Inline functions in C (http://www.greenend.org.uk/rjk/tech/inline.html)
⟡ Metaprogramming custom control structures in C (https://www.chiark.greenend.org.uk/~sgtatham/mp/)
⟡ Some dark corners of C (https://docs.google.com/presentation/d/1h49gY3TSiayLMXYmRMaAEMl05FaJ-Z6jDOWOz3EsqqQ/edit?pli=1#slide=id.gaf50702c_0153)
⟡ Writing efficient C and C code optimization (https://www.codeproject.com/articles/6154/writing-efficient-c-and-c-code-optimization)
⟡ What every programmer should know about memory (https://www.akkadia.org/drepper/cpumemory.pdf)

Self-study courses ####

⟡ C Programming Language Certified Associate preparation course (https://cppinstitute.com/study-resources )

Physical ###

Reference ####

⟡ C: A Reference Manual 5E (http://careferencemanual.com/) - Full reference book for C99.
⟡ C Pocket Reference (http://shop.oreilly.com/product/9780596004361.do) - Concise reference book for C99.
⟡ The C Programming Language 2E (https://en.wikipedia.org/wiki/The_C_Programming_Language) - The original book on C, by its creators.

Beginner ####

⟡ C Primer Plus 6E (https://www.pearson.com/us/higher-education/program/Prata-C-Primer-Plus-6th-Edition/PGM4399.html) - Complete tutorial on programming in C11.
⟡ C Programming: A Modern Approach (http://knking.com/books/c2/index.html) - Excellent book to learn the basics of C.
⟡ Head First C (http://shop.oreilly.com/product/0636920015482.do) - 'Head-first' style book for learning C.

Intermediate ####

⟡ 21st Century C (http://shop.oreilly.com/product/0636920033677.do) - Very good second programming book on C.
⟡ Understanding and Using C Pointers (http://shop.oreilly.com/product/0636920028000.do) - In-depth resource on pointers in C.
⟡ ZeroMQ (http://shop.oreilly.com/product/0636920026136.do) - Book for using ZeroMQ with C.

Advanced ####

⟡ Expert C Programming: Deep C Secrets (https://dl.acm.org/citation.cfm?id=179241) - Interesting, in-depth and entertaining look at the innards of C.

Macros ##

⟡ P99 (http://p99.gforge.inria.fr/) - Suite of macros to implement advanced features like default function arguments, scope-bound resources, etc. (Requires C99) Q Public License 
(https://tldrlegal.com/license/q-public-license-1.0-(qpl-1.0)#summary)

Memory Allocators ##

Implementations of memory allocators for various systems and platforms.

⟡ jemalloc (http://jemalloc.net/) - General purpose malloc(3) implementation that emphasizes fragmentation avoidance and scalable concurrency support, commonly used in production systems. FreeBSD 
(https://directory.fsf.org/wiki?title=License:FreeBSD)
⟡ dlmalloc (http://g.oswego.edu/pub/misc/malloc.c) - Doug Lea's malloc(3) implementation, useful for academic and research purposes. Public Domain (https://creativecommons.org/share-your-work/public-domain/)
⟡ Hoard (http://hoard.org/) - A Fast, Scalable, and Memory-efficient Malloc for Linux, Windows, Mac, and Solaris. GNU GPL2 (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ nedmalloc (http://www.nedprod.com/programs/portable/nedmalloc/) - An EXTREMELY FAST portable thread caching malloc(3) implementation written in C. GNU GPL2 (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ rpmalloc (https://github.com/rampantpixels/rpmalloc) - Cross platform, lock-free memory allocator. Public Domain (https://creativecommons.org/share-your-work/public-domain/)


Multimedia ##

⟡ FFMPEG (https://www.ffmpeg.org/) - Complete, cross-platform solution to record, convert and stream audio and video. GNU LGPL2.1 or later (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html) or GNU GPL2.1 or later (some parts) 
(http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ GStreamer (https://gstreamer.freedesktop.org/) - Framework for audio and visual media. GNU LGPL2.1 (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html)
⟡ lodepng (https://lodev.org/lodepng/) - Simple PNG image decoder and encoder, requiring no other dependencies. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ libsoup (https://wiki.gnome.org/action/show/Projects/libsoup?action=show&redirect=LibSoup) - GNOME HTTP client/server library. Uses GObject. GNU LGPL2.1 (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html)
⟡ libmpv (https://mpv.io/) - Music-playing library. Compile with ./waf configure --disable-cplayer --enable-libmpv-shared to not have the music player. GNU GPL2.1 or later (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ libsoundio (http://libsound.io/) - Library for cross-platform, real-time audio input and output. Has a range of back-ends. Expat (https://directory.fsf.org/wiki/License:Expat)


Networking and Internet ##

⟡ asnlc (http://lionet.info/asn1c/compiler.html) - Compiler of ASN.1 specifications into C source code. FreeBSD (https://directory.fsf.org/wiki?title=License:FreeBSD)
⟡ czmq (http://czmq.zeromq.org/) - High-level binding for ZeroMQ. MPL2.0 (https://www.gnu.org/licenses/license-list.html#MPL-2.0)
⟡ GNU adns (https://www.gnu.org/software/adns/) - Advanced, easy-to-use, asynch-capable DNS client library and utilities. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ GNU SASL (https://www.gnu.org/software/gsasl/) - Implementation of the Simple Authentication and Security Layer and a few common SASL mechanism. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ GnuTLS (https://www.gnutls.org/) - Secure communication library, implementing SSL, TLS and DTLS. GNU LGPL2.1 or later (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html)
⟡ gumbo-parser (https://github.com/google/gumbo-parser) - HTML5 parsing library in C99. Apache 2.0 (https://directory.fsf.org/wiki/License:Apache-2.0)
⟡ http-parser (https://github.com/nodejs/http-parser) - HTTP request/response parser. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ ldns (https://www.nlnetlabs.nl/projects/ldns/index.html) - Library to simplify DNS programming. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ libcurl (https://curl.haxx.se/libcurl/) - Client-side URL transfer library, supporting a wide range of formats. curl license (https://curl.haxx.se/docs/copyright.html)
⟡ LibEtPan (http://www.etpan.org/) - Mail library providing an efficient network for IMAP, SMTP, POP and NNTP. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ libev (http://software.schmorp.de/pkg/libev.html) - Yet another event loop. FreeBSD (https://directory.fsf.org/wiki?title=License:FreeBSD)
⟡ libuv (http://libuv.org/) - Cross-platform asynchronous I/O. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ libevent (http://libevent.org/) - Event loop replacement for network servers. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ libgss (https://www.gnu.org/software/gss/) - Generic Security Service. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ libhttpd (https://hughes.com.au/products/libhttpd/) - Library to add basic web server capabilities to an application or embedded device. GNU GPL2 (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ libidn (https://www.gnu.org/software/libidn/) - Implementation of the Stringprep, Punycode and IDNA specifications. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ libmicrohttpd (https://www.gnu.org/software/libmicrohttpd/) - Small C library that makes it easy to run an HTTP server as part of another application. GNU LGPL2.1 or later (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html)
⟡ libsagui (https://risoflora.github.io/libsagui/) - Sagui is a cross-platform C library which helps to develop web servers or frameworks. GNU LGPL3 (http://www.gnu.org/licenses/lgpl-3.0.html)
⟡ libvldmail (https://github.com/dertuxmalwieder/libvldmail) - Your friendly e-mail address validation library. WTFPLv2 (http://www.wtfpl.net/txt/copying/)
⟡ lwan (https://lwan.ws/) - Experimental, scalable, high-performance HTTP server. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ mongoose (https://cesanta.com/) - Embedded web server for C. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ MQTT-C (https://github.com/LiamBindle/MQTT-C) - Portable MQTT C client for embedded systems and PCs alike. MIT (https://raw.githubusercontent.com/LiamBindle/MQTT-C/master/LICENSE)
⟡ nanomsg (https://github.com/nanomsg/nanomsg) - C-based implementation of ZeroMQ. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ OpenSSL (https://www.openssl.org/) - Implementation of the SSL and TLS protocols, and also includes a cryptography library. Dual Licensed under the OpenSSL License and the SSLeay License (https://www.openssl.org/source/license.html)
⟡ oSip (https://www.gnu.org/software/osip/) - SIP implementation in C without additional dependencies. GNU LGPLv2.1 or later (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html)
⟡ pig (https://github.com/rafael-santiago/pig) - Linux packet crafting tool. GPL2 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ s2n (https://github.com/awslabs/s2n) - C99 implementation of the TLS/SSL protocols, designed to be simple, fast and with security as a priority. Apache 2.0 (https://directory.fsf.org/wiki/License:Apache-2.0)
⟡ socket99 (https://github.com/silentbicycle/socket99) - C99 wrapper for the BSD sockets API. ISC (https://directory.fsf.org/wiki/License:ISC)
⟡ Tox (https://tox.chat/) - Communication platform, designed to be a Skype-killer. GNU GPL3 (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ twitc (https://github.com/sinemetu1/twitc) - Mini C library for interacting with the Twitter OAuth API. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ librg (https://github.com/librg/librg) - Library for building simple and elegant cross-platform mmo client-server solutions. Apache 2.0 (https://directory.fsf.org/wiki/License:Apache-2.0)
⟡ dyad (https://github.com/rxi/dyad) - Asynchronous networking library. MIT (https://raw.githubusercontent.com/atom/atom/master/LICENSE.md)
⟡ h2o (https://github.com/h2o/h2o/) - Optimized HTTP/1, HTTP/2 high performance server/library. MIT (https://raw.githubusercontent.com/atom/atom/master/LICENSE.md)

Web Frameworks ###

⟡ balde (https://balde.rgm.io/) - Microframework for C based on GLib. GNU LGPLv2.1 (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html)
⟡ onion (https://www.coralbits.com/libonion/) - C library to create simple HTTP servers and Web Applications. Apache 2.0 (https://directory.fsf.org/wiki/License:Apache-2.0)
⟡ kore (https://kore.io/) - Easy to use, scalable and secure web application framework for writing web APIs in C.
⟡ klone (http://www.koanlogic.com/klone/) - KLone is a fully-featured, multiplatform, web application development framework.
⟡ duda (http://duda.io/) - Duda I/O is an event-driven and high performant web services framework written in C. Apache 2.0 (https://directory.fsf.org/wiki/License:Apache-2.0)

Numerical ##

⟡ apophenia (http://apophenia.info/) - Library for statistical and scientific computing. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ ATLAS (http://math-atlas.sourceforge.net/) - Automatically Tuned Linear Algebra Software. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ BLAS (http://www.netlib.org/blas/) - Basic Linear Algebra Subprograms; a set of routines that provide vector and matrix operations. BLAS license (http://www.netlib.org/blas/#_licensing)
⟡ CDFLIB (https://people.sc.fsu.edu/~jburkardt/c_src/cdflib/cdflib.html) - Library with routines to evaluate cumulative density functions fo a variety of standard probability distributions.  Also can compute one parameter of the CDF given the 
others.  (No license given)
⟡ cmathl (https://scientificc.github.io/cmathl/) - Pure-C Math library with a great variety of mathematical functions and CMake build support. Seeks to be close to C89/C90 compliant for portability. MIT 
(https://github.com/ScientificC/cmathl/blob/master/LICENSE)
⟡ Cuba (http://www.feynarts.de/cuba/) - Library for multidimensional numerical integration. GNU LGPLv3 (http://www.gnu.org/licenses/lgpl-3.0.html)
⟡ FFTW (http://www.fftw.org/) - The Fastest Fourier Transform in the West; a highly-optimized fast Fourier transform routine. GNU GPL2.1 or later (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ FLINT (http://flintlib.org/) - Fast Library for Number Theory; a library supporting arithmetic with numbers, polynomials, power series and matrices, among others. GNU GPL2.1 or later (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ GLPK (https://www.gnu.org/software/glpk/) - GNU Linear Programming Kit; a package designed for solving large-scale linear programming, mixed integer programming and other related problems. GNU GPL3 or later 
(http://www.gnu.org/licenses/gpl-3.0.html)
⟡ GMP (https://gmplib.org/) - GNU Multple Precision Arithmetic Library; a library for arbitrary-precision arithmetic. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html) and GNU LGPLv3 (http://www.gnu.org/licenses/lgpl-3.0.html)
⟡ GNU MPC (http://www.multiprecision.org/mpc/) - Library for complex number arithmetic. GNU LGPL3 or later (http://www.gnu.org/licenses/lgpl-3.0.html)
⟡ GNU MPFR (https://www.mpfr.org/index.html) - Library for arbitrary-precision floating-point arithmetic. GNU LGPL3 or later (http://www.gnu.org/licenses/lgpl-3.0.html) or GNU LGPL2.1 or later (until version 2.4.x) 
(http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html)
⟡ GNU MPRIA (https://www.gnu.org/software/mpria/) - Portable mathematics library for multi-precision rational interval arithmetic. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ GSL (http://www.gnu.org/software/gsl/) - The GNU Scientific Library; a sophisticated numerical library. GNU GPL3 (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ igraph (https://igraph.org/) - Library for creating and manipulating large graphs. GNU GPL2 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ KISS FFT (https://sourceforge.net/projects/kissfft/) - Very simple fast Fourier transform library. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ LAPACKE (http://www.netlib.org/lapack/lapacke.html) - C interface to LAPACK (http://www.netlib.org/lapack/). 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ PARI/GP (http://pari.math.u-bordeaux.fr/) - Computer algebra system for number theory; includes a compiler to C. GNU GPL2.1 or later (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ PETSc (https://www.mcs.anl.gov/petsc/) - Suite of data structures and routines for scalable parallel solution of scientific applications modelled by partial differential equations. FreeBSD (https://directory.fsf.org/wiki?title=License:FreeBSD)
⟡ PROB (https://people.sc.fsu.edu/~jburkardt/c_src/prob/prob.html) - Library that handles various discrete and continuous probability density functions.  GNU LGPL3 (https://people.sc.fsu.edu/~jburkardt/txt/gnu_lgpl.txt)
⟡ Yeppp! (https://bitbucket.org/MDukhan/yeppp) - Very fast, SIMD-optimized mathematical library. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)

Parallel Programming ##

⟡ cchan (http://repo.hu/projects/cchan/) - Small library for channel constructs for inter-thread communication. Public Domain (https://creativecommons.org/share-your-work/public-domain/)
⟡ ck (http://concurrencykit.org/) - Concurrency primitives, safe memory reclamation mechanisms and non-blocking data structures. FreeBSD (https://directory.fsf.org/wiki?title=License:FreeBSD)
⟡ mill (http://libmill.org/) - Go-style concurrency in C. X11 (https://directory.fsf.org/wiki/License:X11)
⟡ libdill (http://libdill.org/) - Structured concurrency in C. X11 (https://directory.fsf.org/wiki/License:X11)
⟡ MPICH (http://www.mpich.org/) - Another implementation of MPI. MPICH licence (http://git.mpich.org/mpich.git/blob_plain/6aab201f58d71fc97f2c044d250389ba86ac1e3c:/COPYRIGHT)
⟡ OpenMP (https://www.openmp.org/) - Set of C pragmas designed to allow for easy parallelization of code. 3-clause BSD (https://opensource.org/licenses/BSD-3-Clause)
⟡ OpenMPI (https://github.com/open-mpi/ompi) - Message passing interface implementation. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ pth (https://www.gnu.org/software/pth/) - Portable implementation for non-preemptive priority-based scheduling for multiple threads of execution. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ pthreads (https://en.wikipedia.org/wiki/POSIX_Threads) - The POSIX thread library.
⟡ SLEPc (https://bitbucket.org/slepc/slepc) - Software library for the solution of large, sparse eigenvalue problems on parallel computers. GNU LGPL3 (http://www.gnu.org/licenses/lgpl-3.0.html)
⟡ TinyCThread (https://tinycthread.github.io/) - Portable, small implementation of the C11 threads API. zlib (https://directory.fsf.org/wiki/License:Zlib)

Parsers ##

⟡ hammer (https://github.com/abiggerhammer/hammer) - Parser combinators for binary formats. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ iniparser (https://github.com/ndevilla/iniparser) - Parser for .ini files. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ libconfini (https://github.com/madmurphy/libconfini) - Yet another INI parser. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ libYAML (https://pyyaml.org/wiki/LibYAML) - YAML 1.1 parser and emitter. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ mpc (https://github.com/orangeduck/mpc) - Parser combinator library. FreeBSD (https://directory.fsf.org/wiki?title=License:FreeBSD)
⟡ libucl (https://github.com/vstakhov/libucl) - Universal configuration library parser. FreeBSD (https://directory.fsf.org/wiki?title=License:FreeBSD)


Regex ##

▐ "Some people, when confronted with a problem, think 'I know, I'll use regular expressions'. Now they have two problems." - Jamie Zawinski.

⟡ PCRE (http://www.pcre.org/) - Implementation of regexes identical to that of Perl 5. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ regexp4 (https://github.com/nasciiboy/recursiveregexpraptor-4) - Simple and complete implementation of regular expressions with its own sintax. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ SLRE (https://github.com/cesanta/slre) - Super Light Regular Expression library; a very small implementation of a subset of Perl regex syntax. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ sregex (https://github.com/openresty/sregex) - Non-backtracking NFA/DFA-based Perl-compatible regex engine library. 3-clause BSD (https://opensource.org/licenses/BSD-3-Clause)
⟡ TRE (https://github.com/laurikari/tre/) - POSIX-compliant, feature-full regex library. FreeBSD (https://directory.fsf.org/wiki?title=License:FreeBSD)

Serialization ##

⟡ binn (https://github.com/liteserver/binn) - Binary serialization format meant to be compact, fast and easy-to-use. Apache 2.0 (https://directory.fsf.org/wiki/License:Apache-2.0)
⟡ c-capnproto (https://github.com/jmckaskill/c-capnproto) - Implementation of the Cap'n Proto serialization protocol. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ cmp (https://github.com/camgunz/cmp) - Implementation of the MessagePack (https://msgpack.org/) serialization protocol. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ libavro (http://avro.apache.org/docs/current/api/c/index.html#_introduction_to_avro_c) - C implementation of the Avro data serialization system. Apache 2.0 (https://directory.fsf.org/wiki/License:Apache-2.0)
⟡ mpack (https://github.com/ludocode/mpack) - Another implementation of the MessagePack (https://msgpack.org/) serialization protocol. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ netstring-c (https://github.com/liteserver/netstring-c) - Netstring encoder and decoder. Public Domain (https://creativecommons.org/share-your-work/public-domain/)
⟡ protobuf-c (https://github.com/protobuf-c/protobuf-c) - Implementation of Google Protocol Buffer in C. FreeBSD (https://directory.fsf.org/wiki?title=License:FreeBSD)
⟡ xdr (https://en.wikipedia.org/wiki/External_Data_Representation) - External Data Representation; a standard for data serialization.

Check also: JSON (#json), XML (#xml)

Source Code Collections ##

This contains collections of small source code. If you want something big and integrated, check the Frameworks section.

⟡ CCAN (http://ccodearchive.net/) - Modelled after Perl's CPAN, this is a big collection of C code that does stuff. The full list is here (http://ccodearchive.net/list.html).
⟡ clib (https://github.com/clibs/clib) - Something of a package manager for C. Comes with a bunch of libraries of its own (https://github.com/clibs/clib/wiki/Packages). Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ gnulib (https://www.gnu.org/software/gnulib/) - Collection of common GNU code.
⟡ libdjb (http://www.fefe.de/djb/) - Collection of libraries doing various things. (Apparently) Public Domain (https://creativecommons.org/share-your-work/public-domain/)

Standard Libraries ##

This contains standard C libraries.

⟡ Bionic (https://github.com/aosp-mirror/platform_bionic) - Google's C standard library, developed for Android. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ dietlibc (http://www.fefe.de/dietlibc/) - C standard library designed for the smallest possible binaries. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ glibc (http://www.gnu.org/software/libc/) - The GNU C Library; an implementation of the C standard library. GNU LGPL2.1 (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html)
⟡ musl (http://www.musl-libc.org/) - Standard C library, compatible with POSIX 2008 and C11. Designed for static linking. Expat (https://directory.fsf.org/wiki/License:Expat)

String Manipulation ##

⟡ bstrlib (http://bstring.sourceforge.net/) - The Better String Library. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause) and GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ ICU (http://site.icu-project.org/) - International Components for Unicode; a library for Unicode support. ICU license (http://source.icu-project.org/repos/icu/icu/tags/latest/LICENSE)
⟡ libunistring (https://www.gnu.org/software/libunistring/) - Library for manipulating Unicode strings in C. GNU LGPL3 (http://www.gnu.org/licenses/lgpl-3.0.html)
⟡ libgiconv (https://www.gnu.org/software/libiconv/) - Text conversion library. GNU LGPL2.1 (library) (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html) or GNU GPL3 (*iconv* program) (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ SDS (https://github.com/antirez/sds) - Simple Dynamic Strings; a library for handling C strings in a simpler way, but one that is compatible with normal C string functions. Available via clib (https://github.com/clibs/clib). FreeBSD 
(https://directory.fsf.org/wiki?title=License:FreeBSD)
⟡ shoco (http://ed-von-schleck.github.io/shoco/) - Compressor for small text strings. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ smaz (https://github.com/antirez/smaz) - Efficient string compression library. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ utf8.h (https://github.com/sheredom/utf8.h) - Single header utf8 string functions. Unlicense (https://unlicense.org/)
⟡ utf8proc (https://github.com/JuliaStrings/utf8proc) - Small, clean library for processing UTF-8 Unicode data. License (https://github.com/JuliaStrings/utf8proc/blob/master/LICENSE.md)

Testing ##

⟡ CHEAT (http://users.jyu.fi/~sapekiis/cheat/) - Very simple unit testing framework. FreeBSD (https://directory.fsf.org/wiki?title=License:FreeBSD)
⟡ Check (https://libcheck.github.io/check/) - Unit testing framework for C. GNU LGPL2.1 (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html)
⟡ CMock (http://www.throwtheswitch.org/) - Mock/stub generator for C. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ cmocka (https://cmocka.org/) - Unit testing framework with support for mock objects. Apache 2.0 (https://directory.fsf.org/wiki/License:Apache-2.0)
⟡ Criterion (https://criterion.readthedocs.io/en/master/) - KISS, non-intrusive C test framework. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ CUnit (http://cunit.sourceforge.net/) - Another unit testing framework for C. GNU LGPL2.0 (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html)
⟡ Cutest (https://github.com/rafael-santiago/cutest) - Library for unit testing with memory leak detection (Linux, freeBSD and Windows). GPL2 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ minunit (https://github.com/siu/minunit) - Minimal unit testing framework for C. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ munit (https://nemequ.github.io/munit/) - Small but full-featured unit testing framework for C with no dependencies. MIT (https://raw.githubusercontent.com/atom/atom/master/LICENSE.md)
⟡ Unity (http://www.throwtheswitch.org/) - Simple unit testing framework for C. Expat (https://directory.fsf.org/wiki/License:Expat)

Text Editor Extensions ##

While practically any decent programmer's text editor supports C, there are some extensions that make it more pleasant. This is categorized by editor.

Atom ###
⟡ linter-clang (https://github.com/AtomLinter/linter-clang) - Lint C code in Atom, using Clang. MIT (https://raw.githubusercontent.com/atom/atom/master/LICENSE.md)
⟡ linter-gcc (https://github.com/hebaishi/linter-gcc) - Lint C code in Atom, using GCC. MIT (https://raw.githubusercontent.com/atom/atom/master/LICENSE.md)

Emacs ###

⟡ CEDET (http://cedet.sourceforge.net/) - Collection of Emacs Development Environment Tools; designed to provide IDE-like features to Emacs. Built-in. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ Flycheck (https://github.com/flycheck/flycheck) - Modern syntax checking. For C, it can use either GCC or Clang as a back-end. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ Yasnippet (https://github.com/joaotavora/yasnippet) - Template system, with C templates for common code snippets. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)

Vim ###

⟡ Syntastic (https://github.com/vim-syntastic/syntastic) - Syntax checking and linting. Do What The Fuck You Want To license (https://github.com/vim-syntastic/syntastic/blob/master/LICENCE)
⟡ YouCompleteMe (http://valloric.github.io/YouCompleteMe/) - Code completion engine for Vim. GNU GPL3 (http://www.gnu.org/licenses/gpl-3.0.html)

Tools ##

This is a list of useful programs to help you write and debug C code which are not editors, libraries or compilers.

⟡ Artistic Style (http://astyle.sourceforge.net/) - Fast and small automatic source code formatter that supports C. GNU LGPL3 (http://www.gnu.org/licenses/lgpl-3.0.html)
⟡ address-sanitizer (https://github.com/google/sanitizers) - Fast memory error detector. Apache 2.0 (https://directory.fsf.org/wiki/License:Apache-2.0)
⟡ biicode (https://biicode.github.io/biicode/) - Modern dependency manager for C. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ c (https://github.com/ryanmjacobs/c) - Compile and execute C "scripts" in one go on the command line. Also has shebang support. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ c99sh (https://github.com/RhysU/c99sh) - Run C files using hash-bang. FreeBSD (https://directory.fsf.org/wiki?title=License:FreeBSD)
⟡ CBMC (http://www.cprover.org/cbmc/) - C Bounded Model Checker; a tool for verification of array bounds, pointer safety and user-specified assertions. Original BSD (https://directory.fsf.org/wiki/License:BSD-4-Clause)
⟡ cdecl (https://cdecl.org/) - Online service to translate C declarations into English and vice versa. Public Domain (https://creativecommons.org/share-your-work/public-domain/)
⟡ cinclude2dot (https://www.flourish.org/cinclude2dot/) - Graphs include dependencies in a C project using Graphviz. Any GNU GPL version (due to underspecification in the file) (https://directory.fsf.org/wiki/Cinclude2dot#tab=Details)
⟡ Complexity (https://www.gnu.org/software/complexity/) - Tool for measuring the complexity of C source code. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ CScout (https://www.spinellis.gr/cscout/) - Source code analyzer and refactoring browser for collections of C programs. GNU GPL3 (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ DDD (https://www.gnu.org/software/ddd/ddd.html) - Graphical front-end for a range of command-line debuggers. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ GDB (http://www.gnu.org/software/gdb/) - The GNU Project debugger; a debugger for C. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ Glade (https://glade.gnome.org/) - RAD tool to enable quick development of GTK+ GUIs. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ GMSL (https://gmsl.sourceforge.io/) - GNU Make Standard Library; a collection of additional functionality for GNU Make. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ GNU Global (https://www.gnu.org/software/global/) - Source code tagging tool which works with C. GNU GPL3 (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ gprof (http://www.gnu.org/software/binutils/) - Performance analysis tool. Part of GNU binutils. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ Highlight (http://www.andre-simon.de/index.php) - Converts source code to formatted text with nice highlighting. GNU GPL3 (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ include-what-you-use (https://github.com/include-what-you-use/include-what-you-use) - Helps find unecessary inclusions and make suggestions for fixing them. Based on LLVM/Clang (and only works with it). NCSA 
(https://directory.fsf.org/wiki/License:NCSA)
⟡ indent (https://www.gnu.org/software/indent/) - Formats C source code automatically to make it easier to read. Also converts from one style of source to another. GNU GPLv3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ rr (https://rr-project.org/) - Debugger that records non-deterministic executions to allow for deterministic debugging. FreeBSD (https://directory.fsf.org/wiki?title=License:FreeBSD)
⟡ tup (http://gittup.org/tup/index.html) - Very fast, file-based, cross-platform build system. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ unifdef (http://dotat.at/prog/unifdef/) - Removes #ifdef and #if directives with their delimited text without touching any other part of the file. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause) and FreeBSD 
(https://directory.fsf.org/wiki?title=License:FreeBSD)
⟡ Valgrind (http://www.valgrind.org/) - Range of dynamic analysis tools, including a leak checker. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)

Utilities ##

This is a 'catch-all' category for anything that doesn't fit well anywhere else.

⟡ ApeTagLibs (https://github.com/jeremyevans/ape_tag_libs/tree/master/c) - C library for working with APEv2 tags. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ bfd (http://sourceware.org/binutils/docs/bfd/) - Library for manipulating binary object files. Part of GNU binutils. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ cf4ocl (https://fakenmc.github.io/cf4ocl/) - The C Framework for OpenCL; a cross-platform object-oriented framework for developing and benchmarking  OpenCL (https://www.khronos.org/opencl/) projects. GNU LGPL3 (library) 
(http://www.gnu.org/licenses/lgpl-3.0.html) or GNU GPL3 (project code) (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ CommonMark (https://github.com/commonmark/commonmark-spec) - C implementation of the CommonMark spec. Various Licenses (https://github.com/commonmark/commonmark-spec/blob/master/LICENSE)
⟡ CException (https://github.com/ThrowTheSwitch/CException) - C implementation of exceptions. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ docopt.c (https://github.com/docopt/docopt.c) - C implementation of a command-line option parser. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ dyncall (http://www.dyncall.org/) - Another foreign function interface library. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ FANN (http://leenissen.dk/fann/wp/) - Fast Artifical Neural Network library; an implementation of neural networks. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ Firm (https://pp.ipd.kit.edu/firm/) - C library that provides a graph-based intermediate representation, optimizations and assembly code generation suitable for use in compilers. Comes with an example C front-end under the same license. 
GNU LGPLv2.1 (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html)
⟡ Genann (https://github.com/codeplea/genann) - Simple neural network library in C. Zlib (https://directory.fsf.org/wiki/License:Zlib)
⟡ gjrand (https://sourceforge.net/projects/gjrand/) - Library of random-number generation routines. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html) or GNU GPLv3 (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ GNU FreeIPMI (https://www.gnu.org/software/freeipmi/index.html) - In-band and out-of-band IPMI implementation. GNU GPL3 (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ GNU gperf (https://www.gnu.org/software/gperf/) - Perfect hash function generator, given a list of strings. Outputs C code. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ GNU Libffcall (https://www.gnu.org/software/libffcall/) - Collection of libraries for building foreign function interfaces. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ gperftools (https://github.com/gperftools/gperftools) - Collection of utilities for measuring and improving performance. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ huffandpuff (https://github.com/adamierymenko/huffandpuff) - Minimal Huffman encoder and decoder. Public Domain (https://creativecommons.org/share-your-work/public-domain/)
⟡ kdtree (https://github.com/jtsiomb/kdtree) - Simple library for working with KD-trees. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ Kitsune (http://kitsune-dsu.com/) - Efficient, general-purpose framework for dynamic software updating. GNU LGPL3 or later (http://www.gnu.org/licenses/lgpl-3.0.html)
⟡ libavl (http://adtinfo.org/libavl.html/index.html) - Library containing a range of self-balancing binary trees. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ libbson (https://github.com/mongodb/libbson) - BSON utility library. Apache 2.0 (https://directory.fsf.org/wiki/License:Apache-2.0)
⟡ libCello (http://libcello.org/) - Library introducing higher-level programming to C. 3-clause BSD (https://directory.fsf.org/wiki/License:BSD-3-Clause)
⟡ libffi (https://github.com/libffi/libffi) - Portable foreign-function interface library. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ libgit2 (https://libgit2.org/) - Pure C implementation of Git. GNU GPL2 only, with a linking exception (https://github.com/libgit2/libgit2/blob/master/COPYING)
⟡ libimobiledevice (https://github.com/libimobiledevice/libimobiledevice) - Cross-platform protocol library to communicate with iThings. GNU LGPLv2.1 or later (library) (http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html) or 
GNU GPL2.1 or later (tools) (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ libnfc (https://github.com/nfc-tools/libnfc) - Platform-independent Near-Field Communication library. GNU LGPL3 (http://www.gnu.org/licenses/lgpl-3.0.html)
⟡ libPhenom (https://github.com/facebookarchive/libphenom) - Deprecated, here for archival purposes. Eventing framework for building high-scalability and high-performance systems. Apache 2.0 (https://directory.fsf.org/wiki/License:Apache-2.0)
⟡ ncurses (https://www.gnu.org/software/ncurses/) - Coloured terminal UI library. GNU GPL3 or later (http://www.gnu.org/licenses/gpl-3.0.html)
⟡ nope.c (https://github.com/riolet/WAFer) - C-language-based, ultra-light software platform for scalable server-side and networking applications (think node.js for C programmers). GNU GPL2.1 
(http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ pbc (https://github.com/cloudwu/pbc) - Protocol buffers library. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ rabbitmq-c (https://github.com/alanxz/rabbitmq-c) - Client library for RabbitMQ (https://www.rabbitmq.com/). Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ Ragel (http://www.colm.net/open-source/ragel/) - DSL for state machines that compiles to C. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ Tinyexpr (https://github.com/codeplea/tinyexpr) - Tiny recursive-descent parser, compiler and evaluation engine for simple mathematical expressions. zlib (https://directory.fsf.org/wiki/License:Zlib)
⟡ uthash (http://troydhanson.github.io/uthash/) - Hash table implementation, allowing existing structures to be stored in a hash table easily. 1-clause BSD (http://troydhanson.github.io/uthash/license.html)
⟡ cpu_features (https://github.com/google/cpu_features) - A cross platform C99 library to get cpu features at runtime. Apache 2.0 (https://directory.fsf.org/wiki/License:Apache-2.0)
⟡ pblog (https://github.com/google/pblog) - Small, low overhead, structured logging library intended for logging formware events.Apache 2.0 (https://directory.fsf.org/wiki/License:Apache-2.0)
⟡ libelf (https://github.com/WolfgangSt/libelf) - ELF object file access library in C. GNU GPL2.1 (http://www.gnu.org/licenses/old-licenses/gpl-2.0.html)
⟡ mcpp (http://mcpp.sourceforge.net/) - Portable C preprocessor. 2-clause BSD (https://opensource.org/licenses/BSD-2-Clause)
⟡ libusb (https://libusb.info/) - Generic access to USB devices. LGPL2.1 (https://github.com/libusb/libusb/blob/master/COPYING)
⟡ alsa-lib (https://www.alsa-project.org/main/index.php/Main_Page ) - Userspace library to interact with ALSA. LGPL2.1 (https://www.gnu.org/licenses/old-licenses/lgpl-2.1.en.html)
⟡ Capstone (https://github.com/aquynh/capstone) - Dissasembly/disassembler framework. BSD 3-clause (https://github.com/aquynh/capstone/blob/master/LICENSE.TXT)

XML ##

▐ "XML is crap. Really. There are no excuses. XML is nasty to parse for humans, and it's a disaster to parse even for computers. There's just no reason for that horrible crap to exist." - Linus Torvalds

⟡ Expat (http://www.libexpat.org/) - Stream-oriented XML parser. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ libxml2 (http://xmlsoft.org/) - Standards-compliant, portable XML parser. Expat (https://directory.fsf.org/wiki/License:Expat)
⟡ mini-xml (https://github.com/michaelrsweet/mxml) - Small XML reading and writing library. No dependencies aside from C standard library. Apache 2.0 with exceptions (https://github.com/michaelrsweet/mxml/blob/master/LICENSE)

c Github: https://github.com/inputsh/awesome-c