awesome-c

Awesome C

A curated list of C good stuff. This list contains only freedom-respecting code, and sellers who aren't evil for physical resources.

This is released under a Creative Commons-Attribution-ShareAlike license, version 4. 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
• Build Systems
• Compilers
• Compression
• Concurrency and Parallelism
• Crypto
• Database
• Debugging
• Documentation Generation
• Editors
• Frameworks
• Game Programming
  • Engines
  • Resources
• Generic Programming
• Graphics
• Graphical User Interface
• Hashing
• JSON
• Learning, Reference and Tutorials
  • Reference resources online
  • Beginner resources online
  • Intermediate resources online
  • Advanced resources online
  • Online self-study courses
  • Reference books
  • Beginner books
  • Intermediate books
  • Advanced books
• Lexing and Parsing
• Memory Management
• Multimedia
• Networking and Internet
  • Web Frameworks
• Numerical
• Profiling
• Regex
• Serialization
• Source Code Collections
• Standard Libraries
• String Manipulation
• Testing
• Text Editor Extensions
  • Emacs
  • Neovim/Vim
• Tools
• Utilities
• Windows Environments
• XML

AI

This includes such things as computer vision, neural nets, machine learning, and other such 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 - A portable, header-only ANN library in C99. MIT.
• FANN - Fast Artifical Neural Network library; an implementation of neural networks. GPL-2.0.
• Genann - A simple ANN in C89, without additional dependencies. Zlib.
• libdeep - A deep learning library. BSD-3-Clause.

Build Systems

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

• Autoconf - An extensible package of M4 macros that produce shell scripts to automatically configure software source code packages. Part of the Autotools. GPL-3.0+.
• Automake - A tool for automatically generating Makefile.in files compliant with the GNU Coding Standards. Requires the use of Autoconf. Part of the Autotools. GPL-3.0+.
• GNU Make - A tool which controls the generation of executables and other non-source files of a program. GPL-3.0+.
• Libtool - A generic library support script. Part of the Autotools. GPL-3.0+.
• Meson - An extremely fast, user-friendly build system. Based on Ninja. Apache-2.0.
• Premake - A 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.
• tup - A very fast, file-based, cross-platform build system. GPL-2.0.
• zproject - A project generator and build system support. MPL-2.0.

Compilers

This section also includes compiler-related and compilation-related tooling.

• ccache - A compiler cache designed to speed up recompilation. GPL-3.0+.
• Clang - A compiler for LLVM. Supports C11. NCSA.
• distcc - A program to allow builds to be distributed among several machines. GPL-2.0+.
• GCC - Provides a C compiler as part of its compiler set. Supports C11. GPL-3.0+.
• PCC - A venerable compiler. Supports C99. Various licenses, all free.

Compression

• blosc - An extremely fast, multi-threaded, meta-compressor library. Various licenses, all free.
• clzip - C version of the high-quality data compressor Lzip (LZMA implementation). GPL-2.0+.
• CRoaring - C implementation of Roaring bitmaps. Apache-2.0.
• DENSITY - A super-fast compression library. BSD-3-Clause.
• heatshrink - A 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.
• huffandpuff - A minimal Huffman encoder and decoder. Public domain.
• libbzip2 - A patent-free, high-quality data compression library. BSD-4-Clause.
• lz4 - Library for an extremely fast compression algorithm. BSD-2-Clause.
• lzo - A very fast data compression library. GPL-2.0+.
• shoco - A compressor for small text strings. MIT.
• SIMDComp - A simple library for compressing lists of integers using binary packing. Makes use of SIMD instructions on x86. BSD-3-Clause.
• smaz - An efficient string compression library. BSD-3-Clause.
• squash - A compression abstraction library, complete with some utilities. MIT.
• TurboPFor - Fastest integer compression. GPL-2.0+.
• TurboRLE - Most efficient run-length encoding. GPL-2.0+.
• zip - A really really small zip archive processing library. Unlicense.
• Zlib - A massively-spiffy yet delicately-unobtrusive compression library. BSD-3-Clause.
• Zstandard - A fast, lossless compression algorithm, targeting real-time compression scenarios at zlib-level or better compression ratios. BSD-3-Clause.

Concurrency and Parallelism

• cchan - A small library for channel constructs for inter-thread communication. Public domain.
• cf4ocl - The C Framework for OpenCL; a cross-platform object-oriented framework for developing and benchmarking OpenCL projects. LGPL-3.0.
• checkedthreads - No race condition goes unnoticed! 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.
• libconcurrent - Concurrent programming library, using coroutines, for C11. BSD-3-Clause.
• libdill - A library which makes structured concurrent programming easy. MIT.
• libhl - A 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.
• liburcu - A data synchronization library, which scales linearly with the number of cores. LGPL-2.1+.
• mill - Go-style concurrency. MIT.
• OpenMP - A set of pragmas designed to allow for easy parallelization of code. Standard (licensing not applicable).
• OpenMPI - A message passing interface implementation. BSD-3-Clause.
• pal - An optimized library for maths, parallel processing and data movement. Apache-2.0.
• PETSc - A suite of data structures and routines for scalable parallel solution of scientific applications modelled by partial differential equations. BSD-2-Clause.
• pth - A portable implementation for non-preemptive priority-based scheduling for multiple threads of execution. GPL-3.0+.
• pthreads - The POSIX thread library. Standard (no license applicable).
• TinyCThread - A portable, small implementation of the C11 threads API. Zlib.

Crypto

This is mostly library implementations of well-known cryptographic algorithms or protocols.

• GNU SASL - An implementation of the Simple Authentication and Security Layer and a few common SASL mechanism. GPL-3.0+.
• GnuTLS - A secure communication library, implementing SSL, TLS and DTLS. LGPL-2.1+.
• libgcrypt - A general-purpose cryptography library, with a range of available ciphers. LGPL-2.1+.
• OpenSSL - Implementation of the SSL and TLS protocols. Also includes a cryptography library. Dual Licensed under the OpenSSL License and the SSLeay License.
• liboqs - A library for quantum-resistant cryptographicl algorithms. MIT.
• libsodium - A modern and easy-to-use crypto library. MIT.
• libtomcrypt - A 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-1.0+.
• retter - A collection of hash functions, ciphers, tools, libraries and materials related to cryptography and security. Public domain.
• s2n - A C99 implementation of the TLS/SSL protocols, designed to be simple, fast and with security as a priority. Apache-2.0.
• sphlib - A set of implementations of various hash functions, including several cryptographic ones. MIT.

Database

This lists databases and data stores with C APIs.

• BerkeleyDB - A library for a high-performance embedded database for key-value data. AGPL-1.0.
• Groonga - Columnar store with full-text search. LGPL-2.1.
• Hiredis - A minimalistic client library for Redis. BSD-3-Clause.
• libmongoc - A high-performance client library for MongoDB. Apache-2.0.
• LMDB - An ultra-fast, ultra-compact key-value embedded data store. OLDAP-2.8.
• MariaDB - A robust, scalable and reliable SQL server, designed to be a drop-in replacement for MySQL. BSD-3-Clause.
• PostgreSQL - A powerful object-relational database system. PostgreSQL.
• recutils - A set of tools and a library for accessing human-editable, plaintext database files called recfiles. GPL-3.0+.
• Redis - An advanced key-value store. BSD-3-Clause.
• sophia - A modern, embeddable key-value database. BSD-2-Clause.
• sparkey - A simple constant key/value storage library. Designed for read-heavy loads with infrequent, large bulk inserts. Apache-2.0.
• SQLite - A self-contained, serverless, zero-configuration, transactional SQL database engine. Public domain.
• UnQLite - A self-contained, serverless, zero-configuration, transactional NoSQL engine. BSD-2-Clause.
• WhiteDB - A lightweight database library, operating entirely in main memory. GPL-3.0+.

Debugging

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

• C-Reduce - A 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+.
• Complexity - A tool for measuring the complexity of source code. GPL-3.0+.
• CScout - A source code analyzer and refactoring browser for C programs. GPL-3.0.
• DDD - A graphical front-end for a range of command-line debuggers. GPL-3.0+.
• debug - A one-header library for easier 'printf debugging'. MIT.
• GDB - The GNU Project debugger. GPL-3.0+.
• lldb - The LLVM debugger. NCSA.
• rr - A debugger that records non-deterministic executions to allow for deterministic debugging. BSD-2-Clause.
• Valgrind - A range of dynamic analysis tools, including a leak checker. GPL-2.0.

Documentation Generation

• Cxref - Generates documentation in either LaTeX, HTML, RTF or SGML. GPL-2.0.
• DocOnce - A modestly-tagged markup language that can be used to generate a range of formats. BSD-3-Clause.
• Doxygen - The de-facto standard tool for generating documentation from annotated sources. Can generate a large range of formats. GPL-2.0.
• GTK-Doc - A tool for generating documentation from annotated sources. Has support for the Autotools. GPL-2.0.

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 - The GNOME IDE. GPL-2.0.
• Code::Blocks - An extensible, configurable IDE supporting C. GPL-3.0.
• CodeLite - A cross-platform IDE. GPL-2.0.
• Eclipse - An IDE written in Java. EPL.
• Geany - A very small and fast IDE. GPL-2.0+.
• KDevelop - The KDE IDE. GPL-2.0.

Frameworks

This section has 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 - A collection of common algorithms and data structures. ISC.
• Collections-C - A library of generic data structures. LGPL-3.0+.
• CPL - The Common Pipeline Library; a set of libraries designed to be a comprehensive, efficient and robust software toolkit. GPL-2.0.
• EFL - A large collection of useful data structures and functions. Various licenses, all free.
• GLib - A library of utility functions and structures, designed to be portable, efficient and powerful. LGPL-2.1.
• GIO - A modern and easy-to-use VFS API. LGPL-2.1.
• GObject - An object-oriented system and object model. LGPL-2.1.
• libcork - Utility functions and structures, designed for resource-constrained systems. Can be embedded. BSD-3-Clause.
• libnih - A lightweight library of functions and structures. GPL-2.0.
• libU - A small library of basic utilities, including memory allocation, string manipulation and logging. BSD-3-Clause.
• offbrand - A collection of generic, reference-counted data structures. MIT.
• PBL - A large library of utilities, featuring data structures, among other things. LGPL-2.1+.
• qlibc - A simple and powerful library, designed as a replacement for GLib while focusing on being small and light. BSD-2-Clause.
• stb - A range of single-file libraries. Public domain.
• TBOX - A multi-platform library with a large number of capabilities. LGPL-2.1+.

Game Programming

Engines

These are provided as examples of C game programming code.

• Corange - A game engine in pure C. BSD-2-Clause.
• Darkplaces - A modified version of the Quake2 engine. GPL-2.0.
• ioquake3 - The Quake3 engine, freed at last. GPL-2.0.
• Orx - A portable, lightweight, plugin-based, data-driven, 2D-oriented game engine. Zlib.
• Quake - The Quake engine. GPL-2.0.
• Quake2 - The Quake2 engine. GPL-2.0.
• Spearmint - An engine designed for FPS games. GPL-3.0+.

Resources

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

• Allegro - A cross-platform, video game development and multimedia library. Zlib.
• Chipmunk2D - A fast and lightweight 2D game physics library. MIT.
• CSFML - A binding for SFML. Zlib.
• Epoxy - A library for handling OpenGL function pointer management. MIT.
• FreeGLUT - An alternative to the OpenGL Utility Toolkit. Allows the creation and management of windows with OpenGL contexts. X11.
• GLFW - A multi-platform library for creating windows with OpenGL contexts. Zlib.
• kazmath - A maths library for games. BSD-2-Clause.
• libao - A cross-platform audio library with a wide variety of outputs. GPL-2.0+.
• raylib - A simple and easy-to-use library to learn video game programming. Zlib.
• RetroArch - The reference frontend for libretro. GPL-3.0.
• SDL2 - A cross-platform library designed to provide low-level access to audio, keyboard, mouse, joystick and graphics hardware via OpenGL. Zlib.
• sdl-gpu - A 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 free.

Generic Programming

• klib - Small and lightweight implementations of common algorithms and data structures. MIT.
• SGLIB - A simple generic library. Any OSI-approved license.

Graphics

• Cairo - A 2D graphics library. LGPL-2.1 or MPL-1.1.
• Cogl - A GPU graphics and utilities API. MIT.
• Clutter - A UI library based on OpenGL. LGPL-2.1.
• giflib - A library for reading and writing gif images. MIT.
• graphene - A thin layer of graphical data types. MIT.
• heman - A tiny library of image utilities dealing with height maps, normal maps, distance fields and the like. MIT.
• libcaca - An ASCII renderer for terminal-based interfaces. WTFPL.
• libgd - A 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+.
• libjpeg-turbo - A faster library for reading and writing JPEG files. Various licences.
• libpng - The official PNG reference library. Libpng.
• libRSVG - A library to render SVG files using Cairo. LGPL-2.1+.
• libsixel - A library implementing the SIXEL protocol, allowing beautiful graphics in your terminal. MIT.
• libvips - An image processing library. LGPL-2.1+.
• libxmi - A function library for rasterizing 2D vector graphics. GPL-3.0+.
• lightmapper - A single-file library for lightmap baking, using an existing OpenGL renderer. Public domain.
• mozjpeg - An improved JPEG encoder. BSD-3-Clause.
• nanovg - Anti-aliased 2D vector drawing library on top of OpenGL, for UI and visualizations. Zlib.
• OpenGL - The industry standard for high-performance graphics, with a native C binding. Various licenses.

Graphical User Interface

These are specifically widget toolkits, or things meant to be used in a similar way to them.

• GTK+ - A cross-platform widget toolkit. LGPL-2.1.
• IUP - Another cross-platform widget toolkit. MIT.
• nuklear - A small, C89, single-header widget toolkit. Public domain.
• tinyfiledialogs - A single-file library for simple dialogs. Compatible with many other toolkits and OSes. Zlib.
• Tk - A basic widget toolkit. Part of Tcl/Tk. TCL.
• XForms Toolkit - A widget toolkit designed for the XWindow system. LGPL-2.1.

Hashing

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

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

JSON

• cJSON - Ultra-lightweight JSON parser. MIT.
• Jansson - A library for encoding, decoding and manipulating JSON. MIT.
• jsmn - A minimalistic JSON parser. MIT.
• jfes - A simple JSON engine without any dependencies. MIT.
• json-c - A library for working with JSON. MIT.
• WJElement - Advanced JSON manipulation library, with support for JSON Schema. LGPL-2.0+ or LGPL-2.1+ or LGPL-3.0+.
• YAJL - A fast streaming JSON parser library. ISC

Learning, Reference and Tutorials

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

Reference resources online

• Benchmarks of the Lockless Memory Allocator (compares to other implementations)
• C FAQ - comp.lang.c Frequently Asked Questions
• Comparison of C/POSIX standard library implementations for GNU/Linux
• Draft C11 standard
• Finding the best 64-bit simulation PRNG
• GNU C Reference Manual
• Robert Pike's notes on programming in C
• SEI CERT C Coding Standard

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
• 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

Intermediate resources online

• 8 gdb tricks you should know
• 10 C99 tricks
• A comprehensive MPI tutorial resource
• Diving into concurrency: trying out mutexes and atomics
• Introduction to OpenMP (video)
• OpenMP tutorial (for the OpenMP3 standard)
• memcpy vs memmove
• MPI tutorial
• Scalable C - Writing Large-Scale Distributed C
• Some unknown features or tricks in C language
• What a C programmer should know about memory
• 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
• I do not know C
• 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

Online self-study courses

• C Programming Language Certified Associate preparation course

Reference books

• C: A Reference Manual 5E - A full reference book for C99.
• C in a Nushell 2E - A concise reference book for C11.
• C Pocket Reference - A concise reference book for C99.
• The C Programming Language 2E - The original book on C, by its creators.

Beginner books

• C Primer Plus 6E - A complete tutorial on programming in C11.
• C Programming: A Modern Approach - An excellent book to learn the basics from C from.
• Head First C - A 'head-first' style book for learning C.

Intermediate books

• 21st Century C - A very good second programming book on C.
• Understanding and Using C Pointers - An in-depth resource on pointers in C.
• ZeroMQ - A book for using ZeroMQ with C.

Advanced books

• Expert C Programming: Deep C Secrets - An interesting, in-depth and entertaining look at the innards of C.

Lexing and Parsing

This details libraries specifically for lexical analysis (or lexing) and syntactic analysis (or parsing).

• flex - The fast lexical analyzer generator. BSD-2-Clause.
• GNU Bison - A general-purpose parser generator that converts an annotated context-free grammar into a range of parsers. GPL-3.0+.
• hammer - Parser combinators for binary formats. GPL-2.0.
• mpc - A parser combinator library. BSD-2-Clause.
• re2c - A lexer generator, producing very 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? Don't mind if I do! Various licenses, all free.
• jemalloc - A malloc implementation that emphasizes avoidance of fragmentation and scalable concurrency support. BSD-2-Clause.
• Lockless Memory Allocator - An efficient memory allocator. GPL-3.0+.
• talloc - A hierarchical, reference-counted memory pool system with destructors. LGPL-3.0+.
• 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 - A library for audio and music analysis. GPL-3.0+.
• FFMPEG - A complete, cross-platform solution to record, convert and stream audio and video. LGPL-2.1+
• GStreamer - A framework for audio and visual media. LGPL-2.1+.
• libmpv - A music-playing library. Compile with ./waf configure --disable-cplayer --enable-libmpv-shared to not have the music player. GPL-2.0+.
• libsndfile - A library for reading and writing sound files. Supports many formats. LGPL-2.1 or LGPL-3.0.
• libsoundio - A library for cross-platform, real-time audio input and output. Has a range of back-ends. MIT.
• lodepng - A simple PNG image decoder and encoder, requiring no other dependencies. BSD-3-Clause.
• Soundpipe - A lightweight music DSP library. MIT.

Networking and Internet

• asnlc - A 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.
• czmq - A high-level binding for ZeroMQ. MPL-2.0
• GNU adns - An advanced, easy-to-use, asynch-capable DNS client library and utilities. GPL-3.0+.
• gumbo-parser - An HTML5 parsing library in C99. Apache-2.0.
• http-parser - An HTTP request/response parser. MIT.
• ldns - A library to simplify DNS programming. BSD-3-Clause.
• libcurl - A client-side URL transfer library, supporting a wide range of formats. curl.
• LibEtPan - A mail library providing an efficient network for IMAP, SMTP, POP and NNTP. BSD-3-Clause.
• libev - Yet another event loop. BSD-2-Clause.
• libevent - An event loop replacement for network servers. BSD-3-Clause.
• libhttpd - A library to add basic web server capabilities to an application or embedded device. GPL-2.0.
• libidn - An implementation of the Stringprep, Punycode and IDNA specifications. GPL-3.0+.
• libmicrohttpd - A small library that makes it easy to run an HTTP server as part of another application. LGPL-2.1+.
• libonion - HTTP server library, designed to be easy to use. Apache-2.0.
• libquickmail - A 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+.
• libsoup - A GNOME HTTP client/server library. Uses GObject. LGPL-2.1.
• LibVNCServer - Cross-platform libraries to implement VNC server and/or client functionality. GPL-2.0+.
• libwebsock - An easy-to-use and powerful web socket library. LGPL-3.0.
• lwan - An experimental, scalable, high-performance HTTP server. GPL-2.0.
• mongoose - Embedded web server. GPL-2.0.
• nanomsg - A C-based implementation of ZeroMQ. MIT.
• oSip - A SIP implementation without additional dependencies. LGPL-2.1+.
• socket99 - A C99 wrapper for the BSD sockets API. ISC.
• twitc - A mini library for interacting with the Twitter OAuth API. MIT.
• trezor-crypto - Heavily-optimized crypto algorithms for embedded devices. MIT.
• Wslay - A WebSocket library. Implements version 13 of the WebSocket protocol, as described in RFC 6455. MIT.
• zyre - A framework for proximity-based peer-to-peer applications. MPL-2.0.

Web Frameworks

• balde - A microframework based on GLib. LGPL-2.1.
• facil.io - A mini-framework for web applications. Includes a fast HTTP and Websocket server, and also supports custom protocols. MIT.
• KLone - A fully-featured, multi-platform, web application development framework, targeted especially at embedded systems and appliances. BSD-3-Clause.
• Kore - An easy-to-use web application framework for writing scalable web APIs in C. ISC.

Numerical

• apophenia - A library for statistical and scientific computing. GPL-2.0.
• Arb - Library for arbitrary-precision interval arithmetic. LGPL-2.1+.
• ATLAS - Automatically Tuned Linear Algebra Software. BSD-3-Clause.
• clBLAS - BLAS functions written in OpenCL. Apache-2.0.
• Cuba - A library for multidimensional numerical integration. LGPL-3.0.
• FFTW - The Fastest Fourier Transform in the West; a highly-optimized fast Fourier transform routine. GPL-2.0+.
• FLINT - Fast Library for Number Theory; a library supporting arithmetic with numbers, polynomials, power series and matrices, among others. GPL-2.0+.
• GLPK - GNU Linear Programming Kit; a package designed for solving large-scale linear programming, mixed integer programming and other related problems. GPL-3.0+.
• GMP - GNU Multple Precision Arithmetic Library; a library for arbitrary-precision arithmetic. GPL-2.0 or LGPL-3.0.
• GNU MPC - A library for complex number arithmetic. LGPL-3.0+.
• GNU MPFR - A library for arbitrary-precision floating-point arithmetic. LGPL-3.0+.
• GNU MPRIA - A portable mathematics library for multi-precision rational interval arithmetic. GPL-3.0+.
• GSL - The GNU Scientific Library; a sophisticated numerical library. GPL-3.0.
• KISS FFT - A very simple fast Fourier transform library. BSD-3-Clause.
• LAPACKE - An interface to LAPACK. BSD-3-Clause.
• LibTomMath - A portable, number-theoretic, multiple-precision integer library. Supports algebra, digit manipulation, modular reductions, and various number-theoretic routines. Public domain.
• LibTomPoly - A polynomial-related maths library. Public domain.
• PARI/GP - A computer algebra system for number theory; includes a compiler to C. GPL-2.0+.
• PETSc - A 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 - A software library for the solution of large, sparse eigenvalue problems on parallel computers. LGPL-3.0.
• TomsFastMath - A set of assembly-level-optimized maths operations, suitable for cryptographic use. Public domain.
• Yeppp! - Very fast, SIMD-optimized mathematical library. BSD-3-Clause.

Profiling

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

Regex

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

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

Serialization

• binn - A binary serialization format, meant to be compact, fast and easy-to-use. Apache-2.0.
• c-capnproto - An implementation of the Cap'n Proto serialization protocol. MIT.
• cmp - An implementation of the MessagePack serialization protocol. MIT.
• flatcc - FlatBuffers compiler and library. Apache-2.0.
• libavro - An 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+.
• protobuf-c - An implementation of Google Protocol Buffer. BSD-2-Clause.
• tpl - A small binary serialization library. MIT.
• xdr - External Data Representation; a standard for data serialization. Standard (no license applicable).

Source Code Collections

This contains 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 free.
• clib - Something of a package manager. Comes with a bunch of libraries of its own. MIT.
• gnulib - A collection of common GNU code. Various licenses, all free.
• libdjb - A collection of libraries doing various things. (Apparently) public domain.
• par - A bunch of single-file libraries. MIT.

Standard Libraries

This contains standard C libraries.

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

String Manipulation

• bstrlib - The Better String Library. BSD-3-Clause or GPL-2.0.
• ICU - International Components for Unicode; a library for Unicode support. ICU.
• libunistring - A library for manipulating Unicode strings. LGPL-3.0.
• libgiconv - A text conversion library. LGPL-2.1.
• librope - A 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.
• utf8.h - Single-header UTF-8 library, designed to mimic C-style string functions. Public domain.
• utf8proc - A library for processing UTF-8 data. MIT.

Testing

• CHEAT - A very simple unit testing framework. BSD-2-Clause.
• Check - A unit testing framework. LGPL-2.1.
• clar - A clear and simple unit testing framework. MIT.
• CMock - A mock/stub generator. MIT.
• cmocka - A unit testing framework with support for mock objects. Apache-2.0.
• Criterion - A KISS, non-intrusive test framework. MIT.
• ctest - Yet another unit testing framework. Apache-2.0.
• CUnit - Another unit testing framework. LGPL-2.1+.
• greatest - A unit testing library in one file, with no memory allocation. MIT.
• minctest - A unit testing microlibrary. BSD-3-Clause.
• munit - A small unit testing framework. MIT.
• theft - Property-based testing (similar to Quickcheck). MIT.
• Unity - A simple unit testing framework. MIT.

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.

Emacs

• CEDET - Collection of Emacs Development Environment Tools; designed to provide IDE-like features to Emacs. Built-in. GPL-3.0+.
• Flycheck - Modern syntax checking. For C, it can use either GCC or Clang as a back-end. GPL-3.0+.
• YASnippet - A template system, with C templates for common code snippets. GPL-3.0+.

Neovim/Vim

• Neomake - Async :make and linting framework for Neovim/Vim. MIT.
• Syntastic - Syntax checking and linting. WTFPL.
• YouCompleteMe - A code completion engine for Vim. GPL-3.0.

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 - A fast and small automatic source code formatter that supports C. LGPL-3.0.
• address-sanitizer - A fast memory error detector. Apache-2.0.
• biicode - A modern dependency manager. MIT.
• 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 - An 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 GPL-2.0+ or GPL-3.0+.
• ClangCheck - A static analysis tool, designed to work with Clang. NCSA.
• Cppcheck - A static analysis tool. Despite the name, works well with C. GPL-3.0+.
• Glade - A RAD tool to enable quick development of GTK+ GUIs. GPL-2.0.
• GMSL - GNU Make Standard Library; a collection of additional functionality for GNU Make. BSD-3-Clause.
• GNU Global - A source code tagging tool. GPL-3.0.
• GPP - A general-purpose preprocessor. More versatile than the C preprocessor, but more flexible than m4. LGPL-3.0+.
• Highlight - Converts source code to formatted text with nice highlighting. GPL-3.0.
• include-what-you-use - Helps find unecessary inclusions and make suggestions for fixing them. Based on LLVM/Clang (and only works with it). NCSA.
• indent - Formats C source code automatically to make it easier to read. Also converts from one style of source to another. GPL-3.0+.
• qo - A build system that works without a separate config file. MIT.
• SMACK - A 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

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

• ApeTagLibs - A library for working with APEv2 tags. MIT.
• argparse - A command-line argument parsing library, inspired by Python's argparse module. MIT.
• attr - Commands for manipulating filesystem extended attributes. GPL-2.0+.
• bfd - A library for manipulating binary object files. Part of GNU binutils. GPL-3.0+.
• Caffeine - A library for building daemons and services for Linux and FreeBSD systems. LGPL-2.1+.
• CException - An implementation of exceptions. MIT.
• CommonMark - An implementation of the CommonMark spec. Variety of licenses, all free.
• 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 or LGPL-2.1.
• D-Bus - A simple way for applications to talk to one another. AFL-2.1 or GPL-2.0+.
• Discount - A simple implementation of a Markdown parser. BSD-3-Clause.
• dlx - An implementation of Knuth's Algorithm X, with example solvers. GPL-3.0+.
• docopt.c - An implementation of a command-line option parser. MIT.
• dyncall - Another foreign function interface library. MIT.
• Firm - A 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.
• gjrand - A library of random-number generation routines. GPL-2.0 or GPL-3.0.
• GNU FreeIPMI - An in-band and out-of-band IPMI implementation. GPL-3.0.
• GNU gperf - A perfect hash function generator, given a list of strings. Outputs C code. GPL-3.0+.
• GNU Libffcall - A collection of libraries for building foreign function interfaces. GPL-3.0+.
• Hoedown - A fully-standards-compliant, extension-supporting, UTF-8 aware, fast Markdown parser. MIT.
• iniparser - A parser for .ini files. MIT.
• jwHash - A fast hashtable implementation. Apache-2.0.
• kdtree - A simple library for working with KD-trees. BSD-3-Clause.
• Kitsune - An efficient, general-purpose framework for dynamic software updating. LGPL-3.0+.
• libavl - A library containing a range of self-balancing binary trees. GPL-3.0+.
• libbson - A BSON utility library. Apache-2.0.
• libCello - A library introducing higher-level programming to C. BSD-3-Clause.
• libcmark - A library for parsing the CommonMark dialect of Markdown. BSD-2-Clause.
• libcoap - An implementation of the Constrained Application Protocol. GPL-2.0+ or BSD-2-Clause.
• libconfuse - A small configuration file parser library. ISC.
• libcox - A library which permits cross-platform system calls and standard utilities across different operating systems. BSD-2-Clause.
• libcsv - A simple, streaming CSV parser. LGPL-2.1+.
• libffi - A portable foreign-function interface library. MIT.
• libgeohash - A pure C implementation of the Geohash algorithm. BSD-3-Clause.
• libgit2 - Pure C implementation of Git. GPL-2.0, with a linking exception.
• libgss - Generic Security Service. GPL-3.0+.
• liblfds - A portable lock-free data structure library. Public domain (more exactly, whatever license you want).
• libimobiledevice - A cross-platform protocol library to communicate with iThings. LGPL-2.1+.
• libnfc - A platform-independent Near-Field Communication library. LGPL-3.0.
• libPhenom - An eventing framework for building high-scalability and high-performance systems. Apache-2.0.
• libpostal - A library for parsing and normalization of street addresses around the world. Powered by statistical NLP and open geo data. MIT.
• libtrading - An 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.
• libucl - A universal configuration library parser. BSD-2-Clause.
• libuv - Cross-platform asynchronous I/O. MIT.
• libYAML - A YAML 1.1 parser and emitter. MIT.
• 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.
• linenoise - A small, self-contained alternative to readline and libedit. BSD-2-Clause.
• ncurses - Coloured terminal UI library. MIT.
• netbsd-curses - A simplified and small version of ncurses, with the same interface. BSD-3-Clause.
• nope.c - An ultra-light software platform for scalable server-side and networking applications (think node.js for C programmers). GPL-2.0.
• pbc - A protocol buffers library. MIT.
• progressbar - An easy-to-use library for displaying text progress bars. BSD-3-Clause.
• rabbitmq-c - A client library for RabbitMQ. MIT.
• Ragel - A DSL for state machines that compiles to C. GPL-2.0.
• sort - A collection of sorting routines, which type-specialize at compile-time with a user-defined type. MIT.
• termbox - A library for writing text-based interfaces. MIT.
• tinyexpr - A tiny recursive-descent parser, compiler and evaluation engine for simple mathematical expressions. BSD-3-Clause.
• Tulip Indicators - A library of functions for technical analysis of financial data. LGPL-3.0+.
• vector.h - A header library for typed lists. MIT.
• Viola - A simplification of libCello. MIT.
• XLSX I/O - A cross-platform library for reading and writing .xlsx files. MIT.
• zlog - A reliable, pure C logging library. LGPL-2.1.
• zproto - A protocol framework for ZeroMQ. MIT.

Windows Environments

This is a list of 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 free.
• MinGW-w64 - A minimalist environment for C development on Windows with 64 bit support. Various licenses, all free.
• 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.

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 - A stream-oriented XML parser. MIT.
• libxml2 - A standards-compliant, portable XML parser. MIT.