oz123/awesome-c

Awesome C

A curated list of C good stuff. This list contains only open source code (as defined by the linked Open Source Definition), and sellers who aren't evil for physical resources.

This is released under a Creative Commons-Attribution-ShareAlike license, version 4 (SPDX code CC-BY-SA-4.0). You can find its text in the LICENSE file.

An important note: This project does not index anything C++-related; only pure C stuff is considered.

Note for contributors: If you want to make a pull request, please read CONTRIBUTING.md first.

Contents

• AI
• Benchmarking
• Build Systems
• Compilers
• Compression
• Concurrency and Parallelism
• Crypto
• Database
• Data Structures
• Debugging
• Documentation Generation
• Editors
• Embeddable Scripting Engines
• Frameworks
• Game Programming
• Graphics
• Graphical User Interface
• Hashing
• Learning, Reference and Tutorials
  • Reference resources online
  • Beginner resources online
  • Intermediate resources online
  • Advanced resources online
  • Reference books
  • Beginner books
  • Intermediate books
  • Advanced books
• Lexing and Parsing
• Memory Management
• Multimedia
• Networking and Internet
• Numerical
• Profiling
• Regex
• Serialization
• Source Code Collections
• Standard Libraries
  • Template Libraries
• String Manipulation
• Structured File Processing
  • CSV
  • JSON
  • INI
  • Others
  • XML
  • YAML
• Testing
• Text Editor Extensions
• Tools
• Utilities
• Web Frameworks
• Windows Environments

AI

Computer vision, neural nets, machine learning, and other similar things. Basically, if your university calls it AI, it lives here.

• ccv - C-based/Cached/Core Computer Vision library; modern computer vision. BSD-3-Clause
• Cranium - Portable, header-only ANN library in C99. MIT
• FANN - Fast Artifical Neural Network library; an implementation of neural networks. GPL-2.0-only
• Genann - Simple ANN in C89, without additional dependencies. Zlib
• KANN - Two-file ANN library. MIT
• LibDEEP - Deep learning library. BSD-3-Clause
• m2cgen - A CLI tool to transpile trained classic ML models into a native C code with zero dependencies. MIT
• sod - An Embedded Computer Vision & Machine Learning Library. GPL-3.0-only

Benchmarking

Comparing the performance of various subsystems across different chip/system architectures.

• b63 - Light-weight micro-benchmarking tool for C. Apache-2.0

Build Systems

Tools that automate the building and testing of projects in C.

• Autotools - Also known as the GNU build system (automake, autoconf, libtool...) is one of the most widely used build systems (configure && make). GPL-1.0-or-later
• Autotools project skeleton - A simple autotools skeleton (template) to quickly bootstrap new projects. BSD-2-Clause
• CMake - Cross-platform family of tools designed to build, package and test software. BSD-3-Clause
• GNU Make - Tool which controls the generation of executables and other non-source files of a program. GPL-3.0-or-later
• Meson - Extremely fast, user-friendly build system. Based on Ninja. Apache-2.0
• Premake - Command-line utility which reads a scripted definition of a software project and uses it to generate project files for Visual Studio and GNU Make. Other targets are also being worked on. BSD-3-Clause
• SCons - Software construction tool using Python. MIT
• xmake - Cross-platform build utility. Apache-2.0
• zproject - Project generator and build system support tool. MPL-2.0

Compilers

Compilers, as well as compiler- and compilation-related tooling.

• ccache - Compiler cache designed to speed up recompilation. GPL-3.0-or-later
• Clang - Compiler for LLVM. Supports C11. NCSA
• cproc - A C11 compiler using QBE as a backend. ISC
• distcc - Program that allows builds to be distributed among several machines. GPL-2.0-or-later
• Firm - 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. LGPL-2.1-only
• GCC - Provides a C compiler as part of its compiler set. Supports C11. GPL-3.0-or-later
• PCC - Venerable compiler. Supports C99. Various licenses, all open source.

Compression

• blosc - Extremely fast, multi-threaded, meta-compressor library. Various licenses, all open source.
• Brotli - General-purpose lossless compression algorithm library. Has speeds comparable to DEFLATE, but much higher compression ratios. MIT.
• clzip - C version of the high-quality data compressor Lzip (LZMA implementation). GPL-2.0-or-later
• CRoaring - C implementation of Roaring bitmaps. Apache-2.0
• FiniteStateEntropy - Two highly efficient compression codecs optimized for modern CPUs. BSD-2-Clause
• DENSITY - Super-fast compression library. BSD-3-Clause
• heatshrink - Data compression/decompression library for embedded and real-time systems. ISC
• fast_zlib - Improved zlib, which runs 2 to 10 times faster. BSD-3-Clause
• fastLZ - Lightning-fast lossless compression library (LZ77 type). Embbedable on small target like ARM Cortex-M families. Source code directly embbedable on your project ( a pair of h/c files), no dynamic memory allocation. MIT
• huffandpuff - Minimal Huffman encoder and decoder. Public domain.
• libzip - C library for reading, creating and modifying zip archives. BSD-3-Clause
• libbzip2 - Patent-free, high-quality data compression library. BSD-4-Clause
• Lizard - Formerly LZ5; an efficient compressor with fast decompression. Achieves compression ratios comparable with zip and zlib at decompression speeds of 1000MB/s and faster. BSD-2-Clause
• lz4 - Library for an extremely fast compression algorithm. BSD-2-Clause
• lzo - Fast data compression library. GPL-2.0-or-later
• pixz - Parallel, indexed xz compressor. BSD-2-Clause
• shoco - Compressor for small text strings. MIT
• SIMDComp - Simple library for compressing lists of integers using binary packing. Makes use of SIMD instructions on x86. BSD-3-Clause
• smaz - Efficient string compression library. BSD-3-Clause
• squash - Compression abstraction library, complete with some utilities. MIT
• TurboPFor - Fastest integer compression. GPL-2.0-or-later
• TurboRLE - Most efficient run-length encoding. GPL-2.0-or-later
• zip - Really really small zip archive processing library. Unlicense
• Zlib - Massively spiffy yet delicately unobtrusive compression library. BSD-3-Clause
• libarchive - libarchive is a portable, efficient C library that can read and write streaming archives in a variety of formats. BSD-3-Clause
• zlib-ng - Zlib replacement with optimizations for 'next-generation' systems. BSD-3-Clause
• Zstandard - Fast, lossless compression algorithm, targeting real-time compression scenarios at zlib-level or better compression ratios. BSD-3-Clause

Concurrency and Parallelism

• cchan - Small library for channel constructs for inter-thread communication. Public domain.
• checkedthreads - A simple library for parallelism, with built-in checking for race conditions. BSD-2-Clause
• ck - Concurrency primitives, safe memory reclamation mechanisms and non-blocking data structures. BSD-2-Clause
• FCFS RWLock - First-come first-served Readers/Writers lock for POSIX threads. CC0-1.0
• Libaco - A blazing fast and lightweight C asymmetric coroutine library. Apache-2.0
• libconcurrent - Concurrent programming library, using coroutines, for C11. BSD-3-Clause
• libcsp - High performance concurrency C library influenced by the CSP model. MIT
• libdill - Library which makes structured concurrent programming easy. MIT
• libhl - Library implementing a thread-safe API to manage a range of data structures. Also provides some supporting functions and structures for concurrent and lockfree programming. LGPL-3.0-only
• liburcu - Data synchronization library, which scales linearly with the number of cores. LGPL-2.1-or-later
• mill - Go-style concurrency. MIT
• oclkit - Two-file OpenCL wrapper. GPL-3.0-or-later
• OCL-MLA - OpenCL Mid-Level Abstractions. BSD-3-Clause
• OpenMP - Set of pragmas designed to allow for easy parallelization of code. Standard (licensing not applicable).
• Open MPI - Message passing interface implementation. BSD-3-Clause
• pal - Optimized library for maths, parallel processing and data movement. Apache-2.0
• pth - Portable implementation for non-preemptive priority-based scheduling for multiple threads of execution. GPL-3.0-or-later
• pthreads - POSIX thread library. Standard (no license applicable).
• TinyCThread - Portable, small implementation of the C11 threads API. Zlib

Crypto

Mostly library implementations of well-known cryptographic algorithms or protocols.

• GNU SASL - Implementation of the Simple Authentication and Security Layer and a few common SASL mechanism. GPL-3.0-or-later
• GnuTLS - Secure communication library, implementing SSL, TLS and DTLS. LGPL-2.1-or-later
• libgcrypt - General-purpose cryptography library, with a range of available ciphers. LGPL-2.1-or-later
• OpenSSL - Implementation of the SSL and TLS protocols. Also includes a cryptography library. Dual Licensed under the OpenSSL License and the SSLeay License.
• liboqs - Library for quantum-resistant cryptographicl algorithms. MIT
• libsodium - Modern and easy-to-use crypto library. MIT
• libtomcrypt - Fairly comprehensive, modular and portable cryptographic toolkit. Public domain.
• mbed TLS - Another crypto implementation. Apache-2.0
• MIRACL - Multiprecision Integer and Rational Arithmetic Cryptographic Library; an SDK for elliptic curve cryptography. AGPL-3.0-or-later
• retter - Collection of hash functions, ciphers, tools, libraries and materials related to cryptography and security. Public domain.
• s2n - C99 implementation of the TLS/SSL protocols, designed to be simple, fast and with security as a priority. Apache-2.0
• sphlib - Set of implementations of various hash functions, including several cryptographic ones. MIT
• trezor-crypto - Heavily optimized crypto algorithms for embedded devices. MIT
• bfish - A single include ANSI-C Blowfish ECB crypto library. Public domain.

Database

Databases and data stores with C APIs.

• BerkeleyDB - Library for a high-performance embedded database for key-value data. AGPL-3.0-only
• DuckDB - Fast in-process analytical database, with C API. MIT
• EJDB2 - Embeddable JSON Database engine. MIT
• Groonga - Columnar store with full-text search. LGPL-2.1-only
• Hiredis - Minimalistic client library for Redis. BSD-3-Clause
• libmongoc - High-performance client library for MongoDB. Apache-2.0
• LMDB - Ultra-fast, ultra-compact key-value embedded data store. OLDAP-2.8
• MySQL - The world's most popular open source database. GPL-2.0-only
• PostgreSQL - Powerful object-relational database system. PostgreSQL
• Redis - Advanced key-value store. BSD-3-Clause
• sophia - Modern, embeddable key-value database. BSD-2-Clause
• sparkey - Simple constant key/value storage library. Designed for read-heavy loads with infrequent, large bulk inserts. Apache-2.0
• SQLite - Self-contained, serverless, zero-configuration, transactional SQL database engine. Public domain.
• UnQLite - Self-contained, serverless, zero-configuration, transactional NoSQL engine. BSD-2-Clause
• WhiteDB - Lightweight database library, operating entirely in main memory. GPL-3.0-or-later
• FlashDB - An ultra-lightweight database that supports key-value and time series data. Apache-2.0
• TDengine - High-performance, cloud native time-series database. AGPL-3.0-only

Data Structures

• C-Macro-Collections - Generate simple and generic data structures using macros. MIT
• CLIST - Simple and lightweight dynamic array implementation. BSD-2-Clause
• Collections-C - Library of generic data structures. LGPL-3.0-or-later
• ds - Common Data Structures and Algorithms. MIT
• igraph - A graph processing library. GPL-2.0-or-later
• kdtree - Simple library for working with KD-trees. BSD-3-Clause
• libavl - Library containing a range of self-balancing binary trees. GPL-3.0-or-later
• libcdada - A small, portable, MACRO-less library for basic data structures (list, set, map, queue...) in C (C++ backend). BSD-2-Clause
• liblfds - Portable lock-free data structure library. Public domain (more exactly, whatever license you want).
• libsrt - Soft and hard real-time data structures. BSD-3-Clause.
• list.h - Implementations for singly linked and doubly linked list functions. GPL-3.0-only
• M*LIB - Library for generic, but typesafe C containers. Implemented as header-only. BSD-2-Clause
• offbrand - Collection of generic, reference-counted data structures. MIT
• PackedArray - Random-access array of tightly packed unsigned integers of any desired width. Has a SIMD-optimized implementation. WTFPL
• rb3ptr - Red-Black tree. Exposes almost all implementation primitives, so can be used for scenarios like augmentation, multiple compatible ordering functions, and more. MIT
• uthash - Single-file hash table implementation. BSD-1-Clause
• vector.h - Header library for typed lists. MIT

Debugging

Because we all have to do it sometimes. Various tools for making debugging easier or better, as well as libraries or code that allows better debugging work.

• C-Reduce - Tool that takes a large C file with a property of interest and automatically produces a much smaller C file that has the same property. Intended to help create minimal bug-demonstrating cases in complex code. BSD-3-Clause
• CBMC - C Bounded Model Checker; a tool for verification of array bounds, pointer safety and user-specified assertions. BSD-4-Clause
• cflow - Analyzes a collection of source files and prints a graph charting control flow in the program. GPL-3.0-or-later
• Complexity - Tool for measuring the complexity of source code. GPL-3.0-or-later
• CScout - Source code analyzer and refactoring browser for C programs. GPL-3.0-only
• DDD - Graphical front-end for a range of command-line debuggers. GPL-3.0-or-later
• debug - One-header library for easier 'printf debugging'. MIT
• ESBMC - Efficient SMT-based Bounded Model Checker; a tool for verification of single and multithread programs, user assertions, overflow, and pointer/memory safety. Apache-2.0
• GDB - GNU Project debugger. GPL-3.0-or-later
• lldb - LLVM debugger. NCSA
• Owi - A symbolic execution tool (paper) AGPL-3.0-or-later
• rr - Debugger that records non-deterministic executions to allow for deterministic debugging. BSD-2-Clause
• Valgrind - Range of dynamic analysis tools, including a leak checker. GPL-2.0-only
• vfs-pipe - One-header library to expose internal C variables as a virtual filesystem for live inspection and tuning. MIT

Documentation Generation

• Cxref - Generates documentation in either LaTeX, HTML, RTF or SGML. GPL-2.0-only
• DocOnce - Modestly tagged markup language that can be used to generate a range of formats. BSD-3-Clause
• Doxygen - De-facto standard tool for generating documentation from annotated sources. Can generate a large range of formats. GPL-2.0-only

Editors

Fancier, IDE-type editors. If you want a programmer's text editor, look elsewhere. Besides, whatever you use most likely supports C anyway.

• Anjuta DevStudio - GNOME IDE. GPL-2.0-only
• Code::Blocks - Extendable, configurable IDE supporting C. GPL-3.0-only
• CodeLite - Cross-platform IDE. GPL-2.0-only
• Geany - Small and fast IDE. GPL-2.0-or-later
• KDevelop - KDE IDE. GPL-2.0-only
• zinjaI - Simple, lightweight and feature-rich IDE. GPL-3.0-only

Embeddable Scripting Engines

When C isn't the right solution, or when you need a more dynamic configration engine:

• Duktape - Embeddable Javascript engine with a focus on portability and compact footprint MIT.
• MetaCall - Cross-platform Polyglot Runtime which supports NodeJS, JavaScript, TypeScript, Python, Ruby, C#, Wasm, Java, Cobol and more. Apache-2.0

Frameworks

Big libraries that provide data structures and other stuff you expect of a 'modern' standard library.

• APR - Apache Portable Runtime; another library of cross-platform utility functions. Apache-2.0
• C Algorithms - Collection of common algorithms and data structures. ISC
• CPL - The Common Pipeline Library; a set of libraries designed to be a comprehensive, efficient and robust software toolkit. GPL-2.0-only
• EFL - Large collection of useful data structures and functions. Various licenses, all open source.
• GLib - Library of utility functions and structures, designed to be portable, efficient and powerful. LGPL-2.1-only
• klib - Small and lightweight implementations of common algorithms and data structures. MIT
• libcork - Utility functions and structures, designed for resource-constrained systems. Can be embedded. BSD-3-Clause
• libnih - Lightweight library of functions and structures. GPL-2.0-only
• libU - Small library of basic utilities, including memory allocation, string manipulation and logging. BSD-3-Clause
• PBL - Large library of utilities, featuring data structures, among other things. LGPL-2.1-or-later
• plibsys - Cross-platform system C library. Zero third-party dependencies, uses only native system calls. MIT
• qlibc - Simple and powerful library, designed as a replacement for GLib while focusing on being small and light. BSD-2-Clause
• sc - Common libraries and data structures for C. MIT
• TBOX - Multi-platform library with a large number of capabilities. Apache-2.0
• pspsdk - An open-source SDK for PSP homebrew development. Various licences.

Game Programming

Engines, libraries and other helpful things specifically for making games.

• Allegro - Cross-platform, video game development and multimedia library. Zlib
• AssetKit 🎨 3D asset importer/exporter/util library based on COLLADA/glTF specs MIT
• astera - C99 Cross Platform 2D Game Library MIT
• cglm - 📽 Optimized OpenGL/Graphics Math (glm) for C. MIT
• Chipmunk2D - Fast and lightweight 2D game physics library. MIT
• cmt - 🎮 C Bindings/Wrappers for Apple's METAL Graphics Framework. MIT
• Corange - Game engine in pure C. BSD-2-Clause
• CSFML - Binding for SFML. Zlib
• Darkplaces - Modified version of the Quake2 engine. GPL-2.0-only
• Epoxy - Library for handling OpenGL function pointer management. MIT
• exengine - 3D game engine in C99 with a starting template. MIT
• Flecs - A Multithreaded Entity Component System written for C89 & C99 MIT
• Freecell Solver - Set of libraries and command-line programs for automatically solving FreeCell and some similar variants of card Solitaire. MIT
• FreeGLUT - Alternative to the OpenGL Utility Toolkit. Allows the creation and management of windows with OpenGL contexts. X11
• GLFW - Multi-platform library for creating windows with OpenGL contexts. Zlib
• ioquake3 - Quake3 engine, freed at last. GPL-2.0-only
• kazmath - Maths library for games. BSD-2-Clause
• libao - Cross-platform audio library with a wide variety of outputs. GPL-2.0-or-later
• librg - Pure C99 game networking library for building simple and elegant cross-platform multiplayer client-server solutions. Apache-2.0
• MATHC - Math library for 2D and 3D programming. ZLib
• Orx - Portable, lightweight, plugin-based, data-driven, 2D-oriented game engine. Zlib
• Quake - Quake engine. GPL-2.0-only
• Quake2 - Quake2 engine. GPL-2.0-only
• raylib - Simple and easy-to-use library to learn video game programming. Zlib
• RetroArch - Reference frontend for libretro. GPL-3.0-only
• SDL2 - Cross-platform library designed to provide low-level access to audio, keyboard, mouse, joystick and graphics hardware via OpenGL. Zlib
• sdl-gpu - Library for high-performance, modern 2D graphics. Based on SDL. MIT
• SIGIL - Sound, Input and Graphics Integration Library; a simple alternative to other libraries for doing all those things. Various licenses, all open source.
• uastar - Minimal A* implementation. ZLib

Graphics

Programmatic manipulation of graphics in C; if you want to make a GUI, the Graphical User Interface section has what you need.

• AssetKit 🎨 3D asset importer/exporter/util library based on COLLADA/glTF specs MIT
• Cairo - 2D graphics library. LGPL-2.1-only or MPL-1.1.
• cmt - 🎮 C Bindings/Wrappers for Apple's METAL Graphics Framework. MIT
• giflib - Library for reading and writing gif images. MIT
• graphene - Thin layer of graphical data types. MIT
• heman - Tiny library of image utilities dealing with height maps, normal maps, distance fields and the like. MIT
• libcaca - ASCII renderer for terminal-based interfaces. WTFPL
• libgd - Library for the dynamic creation of images by programmers. MIT
• libimagequant - Small, portable library for high-quality conversion of RGBA images to 8-bit indexed colour images. GPL-3.0-or-later
• libjpeg-turbo - Faster library for reading and writing JPEG files. Various licences.
• libpng - Official PNG reference library. Libpng
• libRSVG - Library to render SVG files using Cairo. LGPL-2.1-or-later
• libsixel - Library implementing the SIXEL protocol, allowing beautiful graphics in your terminal. MIT
• libspng - A simpler interface for reading and writing PNG files. BSD-2-Clause
• libvips - Image processing library. LGPL-2.1-or-later
• libxmi - Function library for rasterizing 2D vector graphics. GPL-3.0-or-later
• lightmapper - Single-file library for lightmap baking, using an existing OpenGL renderer. Public domain.
• little CMS - A Color Management System. It provides fast transforms between ICC profiles. MIT
• mozjpeg - Improved JPEG encoder. BSD-3-Clause
• nanovg - Anti-aliased 2D vector drawing library on top of OpenGL, for UI and visualizations. Zlib
• OpenGL - Industry standard for high-performance graphics, with a native C binding. Various licenses.
• PlutoVG - A standalone 2D vector graphics library in C MIT
• SAIL - ⛵ The missing small and fast image decoding library for humans (not for machines) MIT

Graphical User Interface

Widget toolkits, or things meant to be used in a similar way to them.

• GTK+ - Cross-platform widget toolkit. LGPL-2.1-only
• IUP - Another cross-platform widget toolkit. MIT
• microui - Tiny immediate-mode UI library written in portable ANSI C. MIT
• nappgui - Professional SDK to build cross-platform desktop applications using C. MIT
• nuklear - Small, C89, single-header widget toolkit. Public domain.
• tinyfiledialogs - Single-file library for simple dialogs. Compatible with many other toolkits and OSes. Zlib
• Tk - Basic widget toolkit. Part of Tcl/Tk. TCL
• XForms Toolkit - Widget toolkit designed for the XWindow system. LGPL-2.1-only
• LVGL - embedded GUI with easy-to-use graphical elements, beautiful visuals and a low memory footprint. MIT
• luigi - A barebones single-header GUI library for Win32, X11 and Essence. MIT

Hashing

Hash function implementations for non-crypto purposes. Cryptographic hashes can be found in the Crypto section.

• CLHash - Library implementing the ridiculously fast CLHash hashing function. Only works on Intel Haswell or newer. Apache-2.0
• HighwayHash - Fast, strong, SIMD-using hash function. Also contains an implementation of SipHash (although this is slower). Apache-2.0
• SpookyHash - Extremely fast hash function. BSD-3-Clause
• t1ha - Fast Positive Hash - a portable, fast hash function. BSD-3-Clause
• xxHash - Extremely fast hashing algorithm. Comes in 32 and 64-bit varieties. BSD-2-Clause

Learning, Reference and Tutorials

Resources for learning C programming in general, or something useful relating to C programming.

Reference resources online

• Benchmarks of the Lockless Memory Allocator
• C FAQ - comp.lang.c Frequently Asked Questions
• Comparison of C/POSIX standard library implementations for Linux
• Draft C89 standard
• Draft C99 standard
• Draft C11 standard
• Finding the best 64-bit simulation PRNG
• SEI CERT C Coding Standard
• tinyc.game - Tiny C games you can compile and run RIGHT NOW http://tinyc.games MIT

Beginner resources online

• A tutorial on pointers
• A tutorial on portable Makefiles
• Building C Projects
• C Programming Wikibook
• Introduction to `fun' C
• Learning C with GDB
• memcpy vs memmove
• POSIX Threads Programming tutorial (a little dated, but most of it is still valid and useful)
• The GNU C Programming Tutorial (online PDF)
• Templating in C
• What a C programmer should know about memory
• CodeforWin: Learn C Programming, Data Structures Tutorials and Exercises online
• Learn C: Free and Open-Source Interactive C Tutorial
• How to program a text adventure in C

Intermediate resources online

• 8 gdb tricks you should know
• 10 C99 tricks
• A comprehensive MPI tutorial resource
• Build Your Own Text Editor
• Diving into concurrency: trying out mutexes and atomics
• Generic C reference counting
• How to write portable C without complicating your build
• Introduction to OpenMP (video)
• OpenMP tutorial (for the OpenMP3 standard)
• MPI tutorial
• Scalable C - Writing Large-Scale Distributed C
• Some unknown features or tricks in C language
• What every C programmer should know about undefined behaviour

Advanced resources online

• Advanced metaprogramming in C
• A quick tutorial on implementing and debugging malloc, free, calloc, and realloc
• Bit twiddling hacks
• Implementing smart pointers for the C programming language
• Inline functions in C
• Metaprogramming custom control structures in C
• Solving the temporary storage problem of C macros
• Some dark corners of C
• Writing efficient C and C code optimization
• Compiling Algebraic Data Types in Pure C99

Reference books

• C: A Reference Manual 5E - Full reference book for C99.
• C in a Nutshell 2E - Concise reference book for C11.
• C Pocket Reference - Concise reference book for C99.
• The C Programming Language 2E - Original book on C, by its creators.

Beginner books

• C Primer Plus 6E - Complete tutorial on programming in C11.
• C Programming: A Modern Approach - Excellent book to learn the basics of C.
• Head First C - 'Head-first' style book for learning C.

Intermediate books

• 21st Century C - Good second programming book on C.
• Understanding and Using C Pointers - In-depth resource on pointers in C.
• ZeroMQ - Book for using ZeroMQ with C.

Advanced books

• Expert C Programming: Deep C Secrets - Interesting, in-depth and entertaining look at the innards of C.
• Modern C, Third Edition: Covers the C23 standard - In Modern C, Third Edition you’ll learn to harness C’s full potential using the latest tools and techniques.

Lexing and Parsing

Libraries specifically for lexical analysis (or lexing) and syntactic analysis (or parsing).

• flex - Fast lexical analyzer generator. BSD-2-Clause
• GNU Bison - General-purpose parser generator that converts an annotated context-free grammar into a range of parsers. GPL-3.0-or-later
• hammer - Parser combinators for binary formats. GPL-2.0-only
• mpc - Parser combinator library. BSD-2-Clause
• owl - A parser generator for visibly pushdown languages. MIT
• re2c - Lexer generator, producing fast lexers, with access to its internals. Public domain.

Memory Management

Whether a different, faster malloc or outright garbage collection, anything to do with managing C memory lives here.

• Boehm GC - Garbage collection for C. Various licenses, all open source.
• jemalloc - Malloc implementation that emphasizes avoidance of fragmentation and scalable concurrency support. BSD-2-Clause
• Lockless Memory Allocator - Efficient memory allocator. GPL-3.0-or-later
• libcsptr - Smart pointers for C. MIT
• rpmalloc - Thread-caching, fast memory allocator, naturally aligned on 32-byte boundaries. Public domain.
• talloc - Hierarchical, reference-counted memory pool system with destructors. LGPL-3.0-or-later
• tlsf - Two-Level Segregated Fit allocator; a general-purpose, dynamic memory allocator designed to meet real-time requirements. Up-to-date implementation. BSD-3-Clause

Multimedia

• aubio - Library for audio and music analysis. GPL-3.0-or-later
• FFMPEG - Complete, cross-platform solution to record, convert and stream audio and video. LGPL-2.1-or-later
• GStreamer - Framework for audio and visual media. LGPL-2.1-or-later
• libmpv - Music-playing library. GPL-2.0-or-later
• libsndfile - Library for reading and writing sound files. Supports many formats. LGPL-2.1-only or LGPL-3.0-only
• libsoundio - Library for cross-platform, real-time audio input and output. Has a range of back-ends. MIT
• libVLC - Complete multimedia library for audio and video encoding, decoding, playing and streaming. GPL-2.0-only
• lodepng - Simple PNG image decoder and encoder, requiring no other dependencies. BSD-3-Clause
• minimp3 - Lightweight MP3 decoder single header library. CC0-1.0
• Soundpipe - Lightweight music DSP library. MIT

Networking and Internet

Low-level networking and internet-related stuff. If you want something more comprehensive and high-level, you may want the Web Frameworks section.

• asnlc - Compiler of ASN.1 specifications into C source code. BSD-2-Clause
• CHL - C Hypertext Library - A library for writing web applications in C. GPL-3.0-only
• czmq - High-level binding for ZeroMQ. MPL-2.0
• Dyad.c - Lightweight, easy, asynchronous networking library. MIT
• GNU adns - Advanced, easy-to-use, asynch-capable DNS client library and utilities. GPL-3.0-or-later
• gumbo-parser - HTML5 parsing library in C99. Apache-2.0
• H20 - A new-generation HTTP server. MIT
• llhttp - HTTP request/response parser. MIT
• ldns - Library to simplify DNS programming. BSD-3-Clause
• libcurl - Client-side URL transfer library, supporting a wide range of formats. curl
• LibEtPan - Mail library providing an efficient network for IMAP, SMTP, POP and NNTP. BSD-3-Clause
• libev - Yet another event loop. BSD-2-Clause
• libevent - Event loop replacement for network servers. BSD-3-Clause
• libhttpd - Library to add basic web server capabilities to an application or embedded device. GPL-2.0-only
• libhv - Cross platform event loop library. BSD-3-Clause
• libidn - Implementation of the Stringprep, Punycode and IDNA specifications. GPL-3.0-or-later
• libmicrohttpd - Small library that makes it easy to run an HTTP server as part of another application. LGPL-2.1-or-later
• libnl - libnl is a collection of libraries to provie APIs to the Netlink protocol (replacement for ioctl). It's primary use is to communicate with the Linux kernel, to modify networking state (interfaces, routing etc...). LGPL-2.1-only
• libonion - HTTP server library, designed to be easy to use. Apache-2.0
• libpcap - API provides to various kernel packet capture mechanism. BSD-3-Clause
• libquickmail - Library intended to give developers a way to send email from their applications. Supports multiple To/Cc/Bcc recipients and attachments without size limits. GPL-3.0-or-later
• libsagui - Library for cross-platform HTTP servers. LGPL-3.0-or-later
• libuhttpd - A very flexible, lightweight and fully asynchronous HTTP server library based on libev and http-parser for Embedded Linux. MIT
• LibVNCServer - Cross-platform libraries to implement VNC server and/or client functionality. GPL-2.0-or-later
• libwebsock - Easy-to-use and powerful web socket library. LGPL-3.0-only
• libzmq - Core ZeroMQ library, a high-performance asynchronous messaging library, aimed at use in distributed or concurrent applications. C API (backend C++) GPL-3.0-or-later with static linking exception
• lwan - Experimental, scalable, high-performance HTTP server. GPL-2.0-only
• mongoose - Embedded web server. GPL-2.0-only
• MQTT-C - A portable MQTT C client for embedded systems and PCs alike. MIT
• nanomsg - C-based implementation of ZeroMQ. MIT
• NNG - nanomsg-next-generation - lightweight brokerless messaging. MIT
• oSip - SIP implementation without additional dependencies. LGPL-2.1-or-later
• silgy - Asynchronous HTTP(S) engine for C/C++ projects. MIT
• socket99 - C99 wrapper for the BSD sockets API. ISC
• twitc - Mini library for interacting with the Twitter OAuth API. MIT
• uriparser - Strictly RFC 3986-compliant URI parsing and handling library. BSD-3-Clause
• Wslay - WebSocket library. Implements version 13 of the WebSocket protocol, as described in RFC 6455. MIT
• zyre - Framework for proximity-based peer-to-peer applications. MPL-2.0

Numerical

• apophenia - Library for statistical and scientific computing. GPL-2.0-only
• Arb - Library for arbitrary-precision interval arithmetic. LGPL-2.1-or-later
• ATLAS - Automatically Tuned Linear Algebra Software. BSD-3-Clause
• clBLAS - BLAS functions written in OpenCL. Apache-2.0
• cmathl - Math library with a great variety of mathematical functions with CMake build support. Seeks to be close to C89/C90 compliant for portability. MIT
• Cuba - Library for multidimensional numerical integration. LGPL-3.0-only
• fft-c - A high-performance Fourier Transform from netlib's fftpack; wrapped in a user-friendly format MIT
• FFTW - The Fastest Fourier Transform in the West; a highly optimized fast Fourier transform routine. GPL-2.0-or-later
• FLINT - Fast Library for Number Theory; a library supporting arithmetic with numbers, polynomials, power series and matrices, among others. GPL-2.0-or-later
• GLPK - GNU Linear Programming Kit; a package designed for solving large-scale linear programming, mixed integer programming and other related problems. GPL-3.0-or-later
• GMP - GNU Multple Precision Arithmetic Library; a library for arbitrary-precision arithmetic. GPL-2.0-only or LGPL-3.0-only
• GNU MPC - Library for complex number arithmetic. LGPL-3.0-or-later
• GNU MPFR - Library for arbitrary-precision floating-point arithmetic. LGPL-3.0-or-later
• GNU MPRIA - Portable mathematics library for multi-precision rational interval arithmetic. GPL-3.0-or-later
• GSL - The GNU Scientific Library; a sophisticated numerical library. GPL-3.0-only.
• KISS FFT - Simple fast Fourier transform library. BSD-3-Clause
• LAPACKE - Interface to LAPACK. BSD-3-Clause
• LibTomMath - Portable, number-theoretic, multiple-precision integer library. Supports algebra, digit manipulation, modular reductions, and various number-theoretic routines. Public domain.
• LibTomPoly - Polynomial-related maths library. Public domain.
• PARI/GP - Computer algebra system for number theory; includes a compiler to C. GPL-2.0-or-later
• PETSc - Suite of data structures and routines for scalable parallel solution of scientific applications modelled by partial differential equations. BSD-2-Clause
• SCS - Splitting Conic Solver; a numerical optimization package for solving large-scale convex cone problems. MIT
• SLEPc - Library for the solution of large, sparse eigenvalue problems on parallel computers. LGPL-3.0-only
• TomsFastMath - Set of optimized maths operations (in assembly), suitable for cryptographic use. Public domain.
• Yeppp! - Fast, SIMD-optimized mathematical library. BSD-3-Clause

Profiling

• gperftools - Collection of utilities for measuring and improving performance. BSD-3-Clause
• gprof - Performance analysis tool. Part of GNU binutils. GPL-3.0-or-later
• OProfile - Statistical profiler for Linux. Can profile any code (including the kernel!) with low overhead and without recompilation. GPL-2.0-only
• perf - Linux kernel-based profiler with a lot of functionality. GPL-2.0-only

PDF

• pdfio - PDFio is a simple C library for reading and writing PDF files.Apache-2.0

Regex

• Onigmo - Fork of Oniguruma, supporting more advanced regexps. BSD-2-Clause
• Oniguruma - Regex library supporting a wide range of encodings, and incorporating many security-oriented fixes. BSD-2-Clause
• PCRE - Implementation of regexes identical to that of Perl 5. BSD-3-Clause
• SLRE - Super Light Regular Expression library; a small implementation of a subset of Perl regex syntax. GPL-2.0-only
• TRE - POSIX-compliant, feature-full regex library. BSD-2-Clause

Serialization

• binn - Binary serialization format, meant to be compact, fast and easy-to-use. Apache-2.0
• c-capnproto - Implementation of the Cap'n Proto serialization protocol. MIT
• cmp - Implementation of the MessagePack serialization protocol. MIT
• flatcc - FlatBuffers compiler and library. Apache-2.0
• libavro - Implementation of the Avro data serialization system. Apache-2.0
• mpack - Another implementation of the MessagePack serialization protocol. MIT
• OPIC - Object Persistence in C; a revolutionary serialization framework, with matching on-disk and in-memory representations. GPL-3.0-or-later
• protobuf-c - Implementation of Google Protocol Buffer. BSD-2-Clause
• tpl - Small binary serialization library. MIT
• xdr - External Data Representation; a standard for data serialization. Standard (no license applicable).
• pbtools - Google Protocol Buffers C source code generator. MIT

Source Code Collections

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

• CCAN - Modelled after Perl's CPAN, this is a big collection of code that does stuff. The full list is here. Various licenses, all open source.
• clib - Something of a package manager. Comes with a bunch of libraries of its own. MIT
• gnulib - Collection of common GNU code. Various licenses, all open source.
• libdjb - Collection of libraries doing various things. (Apparently) public domain.
• mmx - Collection of single-header libraries. Various licenses, all open source.
• par - Bunch of single-file libraries. MIT
• Snippets - Useful code snippets and header-only libraries. Public domain.
• stb - Range of single-file libraries. Public domain.
• tinyheaders - Collection of header-only libraries, primarily oriented toward game development. Zlib
• zpl - C99 cross-platform header-only library with many goodies. [BSD-3-Clause, Unlicense][BSD-3-Clause, Unlicense]

Standard Libraries

Implementations of the (standard-mandated) C standard library.

• Bionic - Google's standard library, developed for Android. BSD-3-Clause
• cloudlibc - Standard library based on the concept of capability-based security. BSD-2-Clause
• dietlibc - Standard library designed for the smallest possible binaries. GPL-2.0-only
• glibc - The GNU C Library; an implementation of the standard library. LGPL-2.1-only.
• musl - Standard library, compatible with POSIX 2008 and C11. Designed for static linking. MIT
• PDCLib - The Public Domain C Library. Implements most of C99 and some of C11. CC0-1.0
• uClibc-ng - Small C library for developing embedded systems. LGPL-2.1-or-later

Template libraries

• CTL - C CONTAINER TEMPLATE LIBRARY (CTL) MIT

String Manipulation

• bstring - The Better String Library. BSD-3-Clause
• ICU - International Components for Unicode; a library for Unicode support. ICU
• levenstein.c - Levenstein distance algorithm implementation. MIT.
• libunistring - Library for manipulating Unicode strings. LGPL-3.0-only
• libgiconv - Text conversion library. LGPL-2.1-only
• librope - UTF-8 rope ('heavy' string) library. MIT
• SDS - Simple Dynamic Strings; a library for handling strings in a simpler way, but one that is compatible with normal C string functions. Available via clib. BSD-2-Clause
• stmr.c - Porter Stemmer algorithm implementation. MIT
• str - Yet another string library for C language. BSD-3-Clause
• StringZilla - Up to 10x faster SIMD and SWAR-accelerated string search, sort, hashes, edit distances, alignments, and generators. Apache-2.0
• utf8.h - Single-header UTF-8 library, designed to mimic C-style string functions. Public domain.
• utf8proc - Library for processing UTF-8 data. MIT

Structured File Processing

This includes libraries for things like XML, JSON, CSV, and other similar formats.

CSV

• libcsv - Simple, streaming CSV parser. LGPL-2.1-or-later

JSON

• Jansson - Library for encoding, decoding and manipulating JSON. MIT
• jfes - JSON For Embedded Systems; simple JSON engine without any dependencies. MIT
• jsmn - Minimalistic JSON parser. MIT
• json - Simple, low-memory-use JSON parser. MIT
• json-c - Easily work with JSON in C. Comes with a reference-counted object model, and aims for conformance with RFC 7159. MIT
• json.h - Single-file non-streaming JSON parser. Unlicense
• parson - Two-file, C89-compatible JSON parser. MIT
• WJElement - Advanced JSON manipulation library, with support for JSON Schema. LGPL-2.0-or-later or LGPL-2.1-or-later or LGPL-3.0-or-later
• YAJL - Fast streaming JSON parser library. ISC

INI

• inih - Small and simple INI file parser, good for embedded systems. BSD-3-Clause
• iniparser - Parser for .ini files. MIT
• libconfini - Yet another INI parser. GPL-3.0-only
• minIni - Small and portable INI parser. Apache-2.0

Others

• libbson - BSON utility library. Apache-2.0
• libcbor: CBOR protocol implementation for C and others. MIT
• libconfuse - Small configuration file parser library. ISC
• libelf - Simple library for parsing ELF files. MIT
• libucl - Universal configuration library parser. BSD-2-Clause
• libxo - Allows an application to generate plain text, XML, JSON and HTML output using a common set of function calls. The application decides at runtime what output style should be produced. BSD-2-Clause

XML

• Expat - Stream-oriented XML parser. MIT
• libxml2 - Standards-compliant, portable XML parser. MIT
• xml - Simple, low-memory-use XML parser / tokenizer. MIT

YAML

• libYAML - YAML 1.1 parser and emitter. MIT

Signal Processing

• libsigrok - signal analysis software suite that supports various device types (such as logic analyzers, oscilloscopes, multimeters, and more). [GPL][GPL]

Testing

• CHEAT - Simple unit testing framework. BSD-2-Clause
• Check - Unit testing framework. LGPL-2.1-only
• ciut - A modern minimal hassle unit test framework. MIT
• clar - Clear and simple unit testing framework. MIT
• CMock - Mock/stub generator. MIT
• cmocka - Unit testing framework with support for mock objects. Apache-2.0
• Criterion - KISS, non-intrusive test framework. MIT
• ctest - Yet another unit testing framework. Apache-2.0
• CUnit - Another unit testing framework. LGPL-2.1-or-later
• greatest - Unit testing library in one file, with no memory allocation. ISC
• minctest - Unit testing microlibrary. BSD-3-Clause
• munit - Small unit testing framework. MIT
• Nala - A test framework for C projects. MIT
• Owi - A bug-finding tool built on symbolic execution (paper) AGPL-3.0-or-later
• Rexo - Framework for C89/C++ featuring automatic registration of tests and a polished API. Unlicense
• Tau - A Micro Unit testing framework for C/C++ (~1k lines of code). Includes a rich set of assertion macros, supports automatic test registration and can output to multiple formats, like the TAP format or JUnit XML. Supported on Linux, macOS, FreeBSD, and Windows. MIT
• theft - Property-based testing (similar to Quickcheck). MIT
• Unity - Simple unit testing framework. MIT
• utest - Single-header unit testing library. Unlicense

Text Editor Extensions

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

• CCompletion - Notepad++ autocompletion plugin. Works with all identifiers recognized by Ctags. This is a download link. GPL-2.0-or-later
• CEDET - Collection of Emacs Development Environment Tools; designed to provide IDE-like features to Emacs. Built-in. GPL-3.0-or-later
• Flycheck - Modern syntax checking for Emacs. For C, it can use either GCC or Clang as a back-end. GPL-3.0-or-later
• Neomake - Async :make and linting framework for Neovim/Vim. MIT
• Syntastic - Syntax checking and linting for Vim. WTFPL
• YASnippet - Emacs code template system, with C templates for common snippets. GPL-3.0-or-later
• YouCompleteMe - Code completion engine for Vim. GPL-3.0-only

Tools

Useful programs to help you write and debug C code which are not editors, libraries or compilers.

• Artistic Style - Fast and small automatic source code formatter that supports C. LGPL-3.0-only
• address-sanitizer - Fast memory error detector. Apache-2.0
• bcc - A byte array generator to import binary files directy from C in the spirit of xxd. ISC
• c - Compile and execute C "scripts" in one go on the command line. Also has shebang support. MIT
• c99sh - Run C files using hash-bang. BSD-2-Clause
• cdecl - Online service to translate C declarations into English and vice versa. Public domain.
• cinclude2dot - Graphs include dependencies in a project using Graphviz. GPL-1.0-or-later or GPL-2.0-or-later or GPL-3.0-or-later
• ClangCheck - Static analysis tool, designed to work with Clang. NCSA
• conan.io - Something of a package manager for C. MIT.
• Cppcheck - Static analysis tool. Despite the name, works well with C. GPL-3.0-or-later
• Glade - RAD tool to enable quick development of GTK+ GUIs. GPL-2.0-only
• GMSL - GNU Make Standard Library; a collection of additional functionality for GNU Make. BSD-3-Clause
• GNU Global - Source code tagging tool. GPL-3.0-only
• GPP - General-purpose preprocessor. More versatile than the C preprocessor, but more flexible than m4. LGPL-3.0-or-later
• Highlight - Converts source code to formatted text with nice highlighting. GPL-3.0-only
• include-what-you-use - Helps find unecessary inclusions and make suggestions for fixing them. Based on LLVM/Clang (and only works with it). NCSA
• incbin - Include binary files in your C/C++ applications with ease Unlicense
• indent - Formats C source code automatically to make it easier to read. Also converts from one style of source to another. GPL-3.0-or-later
• SMACK - Modular software verification toolchain and a self-contained software verifier. Currently only works with programs compiled using Clang. MIT
• unifdef - Removes #ifdef and #if directives with their delimited text without touching any other part of the file. BSD-3-Clause or BSD-2-Clause

Utilities

A 'catch-all' category for anything that doesn't fit well anywhere else.

• ApeTagLibs - Library for working with APEv2 tags. MIT
• argparse - Command-line argument parsing library, inspired by Python's argparse module. MIT
• attr - Commands for manipulating filesystem extended attributes. GPL-2.0-or-later
• bfd - Library for manipulating binary object files. Part of GNU binutils. GPL-3.0-or-later
• Caffeine - Library for building daemons and services for Linux and FreeBSD systems. LGPL-2.1-or-later
• CException - Implementation of exceptions. MIT
• CommonMark - Implementation of the CommonMark spec.
• cosmopolitan - fast portable static native textmode containers (build C programs for Linux\Mac\Windows in one go) Variety of licenses, all open source.
• cpu_features - Get CPU features at runtime. Apache-2.0.
• CRIU - Checkpoint/Restore In Userspace; a software tool (with a C API) for 'freezing' a running application to disk, then restoring it. GPL-2.0-only or LGPL-2.1-only
• D-Bus - Simple way for applications to talk to one another. AFL-2.1 or GPL-2.0-or-later
• Discount - Simple implementation of a Markdown parser. BSD-3-Clause
• dlx - Implementation of Knuth's Algorithm X, with example solvers. GPL-3.0-or-later
• docopt.c - Implementation of a command-line option parser. MIT
• dyncall - Another foreign function interface library. MIT
• GNU FreeIPMI - In-band and out-of-band IPMI implementation. GPL-3.0-only
• GNU gperf - Perfect hash function generator, given a list of strings. Outputs C code. GPL-3.0-or-later
• GNU Libffcall - Collection of libraries for building foreign function interfaces. GPL-3.0-or-later
• Hoedown - Fully standards-compliant, extension-supporting, UTF-8 aware, fast Markdown parser. MIT
• Kitsune - Efficient, general-purpose framework for dynamic software updating. LGPL-3.0-or-later
• libCello - Library introducing higher-level programming to C. BSD-3-Clause
• libcmark - Library for parsing the CommonMark dialect of Markdown. BSD-2-Clause
• libcoap - Implementation of the Constrained Application Protocol. GPL-2.0-or-later or BSD-2-Clause
• libcox - Library which permits cross-platform system calls and standard utilities across different operating systems. BSD-2-Clause
• libffi - Portable foreign-function interface library. MIT
• libgeohash - Pure C implementation of the Geohash algorithm. BSD-3-Clause
• libgit2 - Portable implementation of the Git core methods, provided as a re-entrant linkable library. Custom license.
• libgss - Generic Security Service. GPL-3.0-or-later
• libimobiledevice - Cross-platform protocol library to communicate with iThings. LGPL-2.1-or-later
• libnfc - Platform-independent Near-Field Communication library. LGPL-3.0-only
• libpostal - Library for parsing and normalization of street addresses around the world. Powered by statistical NLP and open geo data. MIT
• libtrading - Implementation of network protocols for communicating with exchanges, dark pools and other trading venues. Supports FIX, FIX/FAST and many proprietary protocols. BSD-2-Clause
• libusb - Provides generic access to USB devices. LGPL-2.1-or-later
• libuv - Cross-platform asynchronous I/O. MIT
• libvldmail - Your friendly email validation library. No external dependencies (not even regexps). WTFPL
• linenoise - Small, self-contained alternative to readline and libedit. BSD-2-Clause
• libXDGdirs - An implementation of XDG Base Directory Specification MIT
• MegaMimes - Library for getting the MIME types of a file. MIT
• ncurses - Coloured terminal UI library. MIT
• netbsd-curses - Simplified and small version of ncurses, with the same interface. BSD-3-Clause
• nope.c - Ultra-light software platform for scalable server-side and networking applications (think node.js for C programmers). GPL-2.0-only
• obj.h - A single-header supports OOP in pure C. MIT
• parg - A single-file reimplementation of getopt with better defaults. CC0-1.0
• pbc - Protocol buffers library. MIT
• progressbar - Easy-to-use library for displaying text progress bars. BSD-3-Clause
• rabbitmq-c - Client library for RabbitMQ. MIT
• Ragel - DSL for state machines that compiles to C. GPL-2.0-only
• rmw - safe-remove utility for the command line that can purge items from your waste directories after x number of days. GPL-3.0-or-later
• Rogueutil - Cross-platform library for creating text-based user interfaces (TUI) Apache-2.0
• sort - Collection of sorting routines, which type-specialize at compile-time with a user-defined type. MIT
• termbox - Library for writing text-based interfaces. MIT
• tinyexpr - Tiny recursive-descent parser, compiler and evaluation engine for simple mathematical expressions. BSD-3-Clause
• tm - ⏱ Timer and Timeline Utils for C. MIT
• Tulip Indicators - Library of functions for technical analysis of financial data. LGPL-3.0-or-later
• whereami - One-file library for locating the current executable on the file system. WTFPL
• XLSX I/O - Cross-platform library for reading and writing .xlsx files. MIT
• xlsx_drone - Fast Microsoft Excel's *.xlsx reader. MIT
• zlog - Reliable, pure C logging library. LGPL-2.1-only
• zproto - Protocol framework for ZeroMQ. MIT
• Metalang99 - Full-blown preprocessor metaprogramming. MIT
• Datatype99 - Algebraic data types for C99. MIT
• EasyLogger - An ultra-lightweight, high-performance C/C++ log library. MIT
• microlog - Extensible and configurable logging library with topics for embedded and desktop MIT

Web Frameworks

Comprehensive and integrated solutions for building the next brilliant web application in C.

• Concord - A Discord API wrapper library written in C. MIT
• facil.io - Mini-framework for web applications. Includes a fast HTTP and Websocket server, and also supports custom protocols. MIT
• kcgi - CGI and FastCGI library for C ISC.
• KLone - Fully featured, multi-platform, web application development framework, targeted especially at embedded systems and appliances. BSD-3-Clause
• Kore - Easy-to-use web application framework for writing scalable web APIs in C. ISC

Windows Environments

Technologies designed to bring Windows into the 21st century with respect to support for C.

• Cygwin - Designed to emulate a POSIX-compatible environment extensively under Windows. Various licenses, all open source.
• MinGW-w64 - Minimalist environment for C development on Windows with 64 bit support. Various licenses, all open source.
• MSYS2 - Minimal SYStem 2; aims to provide support for a POSIX environment on Windows, with a package manager based on Arch Linux's pacman. Packages have individual licenses, otherwise, as MinGW and Cygwin.