awesome-c

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

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

This list was previously maintained by @kozross. He decided to switch the list to a new platform, so I've decided to fork it so we could keep it on GitHub.

Contents

Build Systems

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

• aimake - Build tool designed to avoid complex configurations. GNU GPL3 or later
• Autoconf - Extensible package of M4 macros that produce shell scripts to automatically configure software source code packages. Part of the Autotools. GNU GPL3 or later
• Automake - Tool for automatically generating Makefile.in files compliant with the GNU Coding Standards. Requires the use of Autoconf. Part of the Autotools. GNU GPL3 or later
• CMake - Tools for cross-platform building, testing, and packaging. 3-clause BSD
• Jam - Build system, designed to be easier than make. Understands C build rules implicitly. Jam License
• Libtool - Generic library support script. Part of the Autotools. GNU GPL3 or later
• Meson - Extremely fast, user-friendly build system. Based on Ninja. Apache 2.0
• Premake - Build script generator for toolsets like Visual Studio, Xcode, or GNU Make. 3-clause BSD
• Ninja - Small, simple build system with a focus on speed. Apache 2.0
• Bazel - Build system for various operating systems and targets. Apache 2.0
• Buck - Build system created and used by Facebook. Apache 2.0
• Qbs - Modern build tool for software projects. LGPL3
• qmake - Build system included with the Qt Framework. GNU GPL3 with Qt Exception
• xmake - Cross-platform build utility based on lua. Apache 2.0

Compilers

• Clang - C compiler for LLVM. Supports C11. NCSA
• CompCert - Fully-verified C compiler. Supports almost all of C89. GNU GPL2.1 or later
• GCC - Provides a C compiler as part of its compiler set. Supports C11 and OpenMP. GNU GPL3 or later
• PCC - Venerable C compiler. Supports C99. Various free licenses
• TCC - Tiny C Compiler; a small, fast C compiler. Supports C99 (except complex types). GNU LGPL2.1
• Intel SPMD - Compiler for a variant of the C language, for single program, multiple data programming. Various licenses
• ccache - Compiler cache designed to speedup recompilation. GNU GPL3 or later

Compiler libraries

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

• yasm - Yasm Modular Assembler Project. 2-clause BSD

Compression

• libzip - A C library for reading, creating, and modifying zip archives. 3-clause BSD
• lzo - Very fast data compression library. GNU GPL2.1
• zlib - Massively-spiffy yet delicately-unobtrusive compression library. 3-clause BSD
• bzip2 - Patent free, high quality data compressor. 3-clause BSD
• lz4 - Fast Compression algorithm.
• snappy - Fast compression library (implementation in C++, native bindings to C).
• zstd - Fast real-time compression algorithm 3-clause BSD
• quicklz - Fast compression library. GNU GPL2.1
• pixz - Pixz (pronounced pixie) is a parallel, indexing version of xz. 2-clause BSD

Crypto

• GnuTLS - Secure communication library, implementing SSL, TLS and DTLS. GNU LGPL2.1 or later
• libgcrypt - General-purpose cryptography library, with a range of available ciphers. GNU LGPL2.1or later (code) and GNU GPL2.1 or later (manual and tools)
• OpenSSL - Implementation of the SSL and TLS protocols, and also includes a cryptography library. Dual Licensed under the OpenSSL License and the SSLeay License
• libsodium - Modern and easy-to-use crypto library. Expat
• libtomcrypt - Fairly comprehensive, modular and portable cryptographic toolkit. Public Domain
• mbed TLS - Another crypto implementation for C. Apache 2.0
• libressl - Modernized fork of OpenSSL. Various Licenses

Database

This lists databases and data stores with C APIs.

• BerkeleyDB - Library for a high-performance embedded database for key-value data. GNU AGPLv3
• Hiredis - Minimalistic client library for Redis. 3-clause BSD
• LMDB - Ultra-fast, ultra-compact key-value embedded data store. OpenLDAP License
• MariaDB - Robust, scalable and reliable SQL server, designed to be a drop-in replacement for MySQL. 3-clause BSD
• mongo-c-driver - High-performance client library for MongoDB. Apache 2.0
• PostgreSQL - Powerful object-relational database system. PostgreSQL licence
• recutils - Set of tools and a C library for accessing human-editable, plaintext database files called recfiles. GNU GPL3 or later
• Redis - Advanced key-value store. 3-clause BSD
• sophia - Modern, embeddable key-value database. FreeBSD
• SQLite - Self-contained, serverless, zero-configuration, transactional SQL database engine with a C interface. Public Domain
• UnQLite - Self-contained, serverless, zero-configuration, transactional NoSQL engine with a C interface. FreeBSD

Deep Learning

• Darknet - Open source neural network framework written in C and CUDA. It is fast, easy to install, and supports CPU and GPU computation.

Documentation Generation

• Cxref - Generates documentation of C programs in either LaTeX, HTML, RTF or SGML. GNU GPL2.1
• DocOnce - Modestly-tagged markup language that can be used to generate a range of formats. 3-clause BSD
• Doxygen - The de-facto standard tool for generating C documentation from annotated sources. Can generate a large range of formats. GNU GPL2.1
• GTK-Doc - Tool for generating C documentation from annotated sources. Has support for the Autotools. GNU GPL2.1 (code) or GNU FDL1.1

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. GNU GPL2.1
• Code::Blocks - Extensible, configurable IDE supporting C. GNU GPL3
• CodeLite - Cross-platform IDE. GNU GPL2.1
• Eclipse - IDE written in Java. EPL
• Geany - Very small and fast IDE. GNU GPL2.1 or later
• KDevelop - The KDE IDE. GNU GPL2.1
• Qt Creator - Cross-platform IDE written with C++ and Qt, part of the Qt SDK. Supports Clang Code Model. GNU GPL3 with Qt exception
• Visual Studio Code - Code editing, redefined. Visual Studio Code.

Embedded Systems

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

RTOS

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

• Amazon FreeRTOS - RTOS for microcontrollers that makes small, low-power edge devices easy to program. MIT
• ChibiOS - RTOS with complete development environment (HAL, drivers, support files, and tools). Various Licenses
• Contiki - Connect low-cost, low power microcontrollers to the Internet. 3-clause BSD
• Zephyr Project - Scalable real-time RTOS supporting multiple hardware architectures, optimized for resource constrained devices. Apache 2.0
• RIOT OS - Real-time multi-threading operating system that supports a range of devices that are typically found in the Internet of Things. LGPL-2.1

Environments

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

• Cygwin - Designed to emulate a POSIX-compatible environment extensively under Windows. Various Licenses
• MinGW-w64 - Minimalist environment for C development on Windows with 64 bit support. Various Licenses

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 - Collection of common algorithms and data structures for C. ISC
• C Macro Collections - Header only, macro generated, generic and type-safe Collections in C. MIT
• CPL - The Common Pipeline Library; a set of libraries designed to be a comprehensive, efficient and robust software toolkit. GNU GPL2.1
• EFL - Large collection of useful data structures and functions.
• GLib - Library of utility functions and structures, designed to be portable, efficient and powerful. GNU LGPL2.1
• GIO - Modern and easy-to-use VFS API. GNU LGPL2.1
• GObject - Object-oriented system and object model for C. GNU LGPL2.1
• libnih - Lightweight library of C functions and structures. GNU GPL2.1
• libU - Small library of basic utilities, including memory allocation, string manipulation and logging. 3-clause BSD
• PBL - Large library of utilities, featuring data structures, among other things. GNU LGPL2.1 or later (library) or GNU GPL2.1 or later (test code)
• qlibc - Simple and powerful C library, designed as a replacement for GLib while focusing on being small and light. qLib license (similar to FreeBSD)
• stb - Range of single-file libraries for C. Public Domain
• libsrt - Safe Real-Time library for C. 3-clause BSD

Game Programming

Engines

These are provided as examples of C game programming code.

• Corange - Game engine in pure C. FreeBSD
• Darkplaces - Modified version of the Quake2 engine. GNU GPL2.1
• ioquake3 - The Quake3 engine, freed at last. GNU GPL2.1
• Orx - Portable, lightweight, plugin-based, data-driven, 2D-oriented game engine. zlib
• Quake - The Quake engine. GNU GPL2.1
• Quake2 - The Quake2 engine. GNU GPL2.1
• Spearmint - Engine designed for FPS games. GNU GPL3 or later

Resources

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

• Allegro - Cross-platform, video game development and multimedia library. zlib
• Chipmunk2D - Fast and lightweight 2D game physics library. Expat
• CSFML - Binding for SFML in C. zlib
• 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
• libao - Cross-platform audio library with a wide variety of outputs. GNU GPL2.1 or later
• RetroArch - The reference frontend for libretro. GNU GPL3
• SDL and SDL2 - Cross-platform development library designed to provide low-level access to audio, keyboard, mouse, joystick and graphics hardware via OpenGL. SDL2 is the most current version. zlib
• raylib - Simple and easy-to-use library to learn videogames programming. zlib

Generic Programming

• klib - Small and lightweight implementations of common algorithms and data structures. MIT

Graphics

• Cairo -2D graphics library. GNU LGPL2.1 or MPLv1.1
• Cogl - GPU graphics and utilities API. Expat
• Clutter - UI library based on OpenGL. GNU LGPL2.1
• giflib - Library for reading and writing gif images. MIT
• heman - Tiny library of image utilities dealing with height maps, normal maps, distance fields and the like. Expat
• libcaca - ASCII renderer for terminal-based interfaces. WTFPLv2
• libimagequant - Small, portable library for high-quality conversion of RGBA images to 8-bit indexed colour images. FreeBSD
• libjpeg-turbo - Faster library for reading and writing JPEG files. Various Licenses
• libpng - The official PNG reference library. libpng license
• libxmi - Function library for rasterizing 2D vector graphics. GNU GPL3 or later
• mozjpeg - Improved JPEG encoder. 3-clause BSD

Graphic APIs

• OpenGL - Industry adopted 2D and 3D graphics API. More resources at awesome-opengl.
• OpenGL ES - Industry adopted 2D and 3D graphics API for mobile and embedded devices.
• OpenGL SC - Graphic and compute standard for industry requiring system safety certification.
• Vulkan - Explicit graphic and compute API for modern cross-platform development. More resources at awesome-vulkan.

Graphical User Interface

These are specifically widget toolkits.

• GTK+ - Cross-platform widget toolkit. GNU LGPL2.1
• IUP - Another cross-platform widget toolkit. Expat
• Tk - Basic widget toolkit. Part of Tcl/Tk. Tcl/Tk License
• XForms Toolkit - Widget toolkit designed for the XWindow system. GNU LGPL2.1
• nuklear - Single-header ANSI C gui library. Public Domain
• libui - Simple and portable (but not inflexible) GUI library. MIT
• LCUI - Small C library for building user interfaces with C, XML and CSS. MIT

Hashing

All hashing related libraries, cryptographic or not.

• jwHash - Fast hashtable implementation. Apache 2.0
• xxHash - Extremely fast non-cryptographic hash algorithm. 2-clause BSD
• murmur - C implementation of MurMur Hashing. Public Domain
• t1ha - Fast Positive Hash library. zlib

Image Processing

• libccv - Modern Computer Vision Library. 3-clause BSD

JSON

• Jansson - C library for encoding, decoding and manipulating JSON. Expat
• jsmn - Minimalistic JSON parser. Expat
• json-c - Library for working with JSON. Expat
• WJElement - Advanced JSON manipulation library, with support for JSON Schema. LGPL, any version
• YAJL - Fast C JSON streaming 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.

Online

Reference

• SEI CERT C Coding Standard
• C FAQ - comp.lang.c Frequently Asked Questions
• Comparison of C/POSIX standard library implementations for GNU/Linux
• GNU C Reference Manual
• Robert Pike's notes on programming in C
• Draft ANSI C Rationale
• Draft C99 Rationale

Language Standards

• Draft ANSI C (C89) standard
• Draft C99 standard
• Draft C11 standard

Beginner

• Tutorial on pointers
• 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

• 8 gdb tricks you should know
• 10 C99 tricks
• Diving into concurrency: trying out mutexes and atomics
• Introduction to OpenMP (video)
• OpenMP tutorial (for the OpenMP3 standard)
• memcpy vs memmove
• MPI tutorial
• Some unknown features or tricks in C language
• The lost art of C structure packing
• What a C programmer should know about memory
• What every C programmer should know about undefined behaviour

Advanced

• Advanced metaprogramming in C
• 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
• Some dark corners of C
• Writing efficient C and C code optimization
• What every programmer should know about memory

Self-study courses

• C Programming Language Certified Associate preparation course

Physical

Reference

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

Beginner

• 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

• 21^st Century C - Very 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

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

Macros

• P99 - Suite of macros to implement advanced features like default function arguments, scope-bound resources, etc. (Requires C99) Q Public License

Memory Allocators

Implementations of memory allocators for various systems and platforms.

• jemalloc - General purpose malloc(3) implementation that emphasizes fragmentation avoidance and scalable concurrency support, commonly used in production systems. FreeBSD
• dlmalloc - Doug Lea's malloc(3) implementation, useful for academic and research purposes. Public Domain
• Hoard - A Fast, Scalable, and Memory-efficient Malloc for Linux, Windows, Mac, and Solaris. GNU GPL2
• nedmalloc - An EXTREMELY FAST portable thread caching malloc(3) implementation written in C. GNU GPL2
• rpmalloc - Cross platform, lock-free memory allocator. Public Domain

Multimedia

• FFMPEG - Complete, cross-platform solution to record, convert and stream audio and video. GNU LGPL2.1 or later or GNU GPL2.1 or later (some parts)
• GStreamer - Framework for audio and visual media. GNU LGPL2.1
• lodepng - Simple PNG image decoder and encoder, requiring no other dependencies. 3-clause BSD
• libsoup - GNOME HTTP client/server library. Uses GObject. GNU LGPL2.1
• libmpv - Music-playing library. Compile with ./waf configure --disable-cplayer --enable-libmpv-shared to not have the music player. GNU GPL2.1 or later
• libsoundio - Library for cross-platform, real-time audio input and output. Has a range of back-ends. Expat

Networking and Internet

• asnlc - Compiler of ASN.1 specifications into C source code. FreeBSD
• czmq - High-level binding for ZeroMQ. MPL2.0
• GNU adns - Advanced, easy-to-use, asynch-capable DNS client library and utilities. GNU GPL3 or later
• GNU SASL - Implementation of the Simple Authentication and Security Layer and a few common SASL mechanism. GNU GPL3 or later
• GnuTLS - Secure communication library, implementing SSL, TLS and DTLS. GNU LGPL2.1 or later
• gumbo-parser - HTML5 parsing library in C99. Apache 2.0
• http-parser - HTTP request/response parser. Expat
• ldns - Library to simplify DNS programming. 3-clause BSD
• libcurl - Client-side URL transfer library, supporting a wide range of formats. curl license
• LibEtPan - Mail library providing an efficient network for IMAP, SMTP, POP and NNTP. 3-clause BSD
• libev - Yet another event loop. FreeBSD
• libuv - Cross-platform asynchronous I/O. Expat
• libevent - Event loop replacement for network servers. 3-clause BSD
• libgss - Generic Security Service. GNU GPL3 or later
• libhttpd - Library to add basic web server capabilities to an application or embedded device. GNU GPL2
• libidn - Implementation of the Stringprep, Punycode and IDNA specifications. GNU GPL3 or later
• libmicrohttpd - Small C library that makes it easy to run an HTTP server as part of another application. GNU LGPL2.1 or later
• libsagui - Sagui is a cross-platform C library which helps to develop web servers or frameworks. GNU LGPL3
• libvldmail - Your friendly e-mail address validation library. WTFPLv2
• lwan - Experimental, scalable, high-performance HTTP server. GNU GPL2.1
• mongoose - Embedded web server for C. GNU GPL2.1
• nanomsg - C-based implementation of ZeroMQ. Expat
• OpenSSL - Implementation of the SSL and TLS protocols, and also includes a cryptography library. Dual Licensed under the OpenSSL License and the SSLeay License
• oSip - SIP implementation in C without additional dependencies. GNU LGPLv2.1 or later
• pig - Linux packet crafting tool. GPL2
• s2n - C99 implementation of the TLS/SSL protocols, designed to be simple, fast and with security as a priority. Apache 2.0
• socket99 - C99 wrapper for the BSD sockets API. ISC
• Tox - Communication platform, designed to be a Skype-killer. GNU GPL3
• twitc - Mini C library for interacting with the Twitter OAuth API. Expat
• librg - Library for building simple and elegant cross-platform mmo client-server solutions. Apache 2.0

Web Frameworks

• balde - Microframework for C based on GLib. GNU LGPLv2.1
• onion - C library to create simple HTTP servers and Web Applications. Apache 2.0
• kore - Easy to use, scalable and secure web application framework for writing web APIs in C.
• klone - KLone is a fully-featured, multiplatform, web application development framework.
• duda - Duda I/O is an event-driven and high performant web services framework written in C. Apache 2.0

Numerical

• apophenia - Library for statistical and scientific computing. GNU GPL2.1
• ATLAS - Automatically Tuned Linear Algebra Software. 3-clause BSD
• BLAS - Basic Linear Algebra Subprograms; a set of routines that provide vector and matrix operations. BLAS license
• CDFLIB - Library with routines to evaluate cumulative density functions fo a variety of standard probability distributions. Also can compute one parameter of the CDF given the others. (No license given)
• cmathl - Pure-C Math library with a great variety of mathematical functions and CMake build support. Seeks to be close to C89/C90 compliant for portability. MIT
• Cuba - Library for multidimensional numerical integration. GNU LGPLv3
• FFTW - The Fastest Fourier Transform in the West; a highly-optimized fast Fourier transform routine. GNU GPL2.1 or later
• FLINT - Fast Library for Number Theory; a library supporting arithmetic with numbers, polynomials, power series and matrices, among others. GNU GPL2.1 or later
• GLPK - GNU Linear Programming Kit; a package designed for solving large-scale linear programming, mixed integer programming and other related problems. GNU GPL3 or later
• GMP - GNU Multple Precision Arithmetic Library; a library for arbitrary-precision arithmetic. GNU GPL2.1 and GNU LGPLv3
• GNU MPC - Library for complex number arithmetic. GNU LGPL3 or later
• GNU MPFR - Library for arbitrary-precision floating-point arithmetic. GNU LGPL3 or later or GNU LGPL2.1 or later (until version 2.4.x)
• GNU MPRIA - Portable mathematics library for multi-precision rational interval arithmetic. GNU GPL3 or later
• GSL - The GNU Scientific Library; a sophisticated numerical library. GNU GPL3
• igraph - Library for creating and manipulating large graphs. GNU GPL2
• KISS FFT - Very simple fast Fourier transform library. 3-clause BSD
• LAPACKE - C interface to LAPACK. 3-clause BSD
• PARI/GP - Computer algebra system for number theory; includes a compiler to C. GNU GPL2.1 or later
• PETSc - Suite of data structures and routines for scalable parallel solution of scientific applications modelled by partial differential equations. FreeBSD
• PROB - Library that handles various discrete and continuous probability density functions. GNU LGPL3
• Yeppp! - Very fast, SIMD-optimized mathematical library. 3-clause BSD

Parallel Programming

• cchan - Small library for channel constructs for inter-thread communication. Public Domain
• ck - Concurrency primitives, safe memory reclamation mechanisms and non-blocking data structures. FreeBSD
• mill - Go-style concurrency in C. X11
• libdill - Structured concurrency in C. X11
• MPICH - Another implementation of MPI. MPICH licence
• OpenMP - Set of C pragmas designed to allow for easy parallelization of code. 3-clause BSD
• OpenMPI - Message passing interface implementation. 3-clause BSD
• pth - Portable implementation for non-preemptive priority-based scheduling for multiple threads of execution. GNU GPL3 or later
• pthreads - The POSIX thread library.
• SLEPc - Software library for the solution of large, sparse eigenvalue problems on parallel computers. GNU LGPL3
• TinyCThread - Portable, small implementation of the C11 threads API. zlib

Parsers

• hammer - Parser combinators for binary formats. GNU GPL2.1
• iniparser - Parser for .ini files. Expat
• libconfini - Yet another INI parser. GNU GPL3 or later
• libYAML - YAML 1.1 parser and emitter. Expat
• mpc - Parser combinator library. FreeBSD
• libucl - Universal configuration library parser. FreeBSD

Regex

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

• PCRE - Implementation of regexes identical to that of Perl 5. 3-clause BSD
• regexp4 - Simple and complete implementation of regular expressions with its own sintax. GNU GPL3 or later
• SLRE - Super Light Regular Expression library; a very small implementation of a subset of Perl regex syntax. GNU GPL2.1
• sregex - Non-backtracking NFA/DFA-based Perl-compatible regex engine library. 3-clause BSD
• TRE - POSIX-compliant, feature-full regex library. FreeBSD

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. Expat
• cmp - Implementation of the MessagePack serialization protocol. Expat
• libavro - C implementation of the Avro data serialization system. Apache 2.0
• mpack - Another implementation of the MessagePack serialization protocol. Expat
• netstring-c - Netstring encoder and decoder. Public Domain
• protobuf-c - Implementation of Google Protocol Buffer in C. FreeBSD
• xdr - External Data Representation; a standard for data serialization.

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 C code that does stuff. The full list is here.
• clib - Something of a package manager for C. Comes with a bunch of libraries of its own. Expat
• gnulib - Collection of common GNU code.
• libdjb - Collection of libraries doing various things. (Apparently) Public Domain

Standard Libraries

This contains standard C libraries.

• Bionic - Google's C standard library, developed for Android. 3-clause BSD
• dietlibc - C standard library designed for the smallest possible binaries. GNU GPL2.1
• glibc - The GNU C Library; an implementation of the C standard library. GNU LGPL2.1
• musl - Standard C library, compatible with POSIX 2008 and C11. Designed for static linking. Expat

String Manipulation

• bstrlib - The Better String Library. 3-clause BSD and GNU GPL2.1
• ICU - International Components for Unicode; a library for Unicode support. ICU license
• libunistring - Library for manipulating Unicode strings in C. GNU LGPL3
• libgiconv - Text conversion library. GNU LGPL2.1 (library) or GNU GPL3 (*iconv* program)
• SDS - Simple Dynamic Strings; a library for handling C strings in a simpler way, but one that is compatible with normal C string functions. Available via clib. FreeBSD
• shoco - Compressor for small text strings. Expat
• smaz - Efficient string compression library. 3-clause BSD
• utf8.h - Single header utf8 string functions. Unlicense
• utf8proc - Small, clean library for processing UTF-8 Unicode data. License

Testing

• CHEAT - Very simple unit testing framework. FreeBSD
• Check - Unit testing framework for C. GNU LGPL2.1
• CMock - Mock/stub generator for C. Expat
• cmocka - Unit testing framework with support for mock objects. Apache 2.0
• Criterion - KISS, non-intrusive C test framework. Expat
• CUnit - Another unit testing framework for C. GNU LGPL2.0
• Cutest - Library for unit testing with memory leak detection (Linux, freeBSD and Windows). GPL2
• minunit - Minimal unit testing framework for C. Expat
• Unity - Simple unit testing framework for C. Expat

Text Editor Extensions

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

Atom

Emacs

• CEDET - Collection of Emacs Development Environment Tools; designed to provide IDE-like features to Emacs. Built-in. GNU GPL3 or later
• Flycheck - Modern syntax checking. For C, it can use either GCC or Clang as a back-end. GNU GPL3 or later
• Yasnippet - Template system, with C templates for common code snippets. GNU GPL3 or later

Vim

• Syntastic - Syntax checking and linting. Do What The Fuck You Want To license
• YouCompleteMe - Code completion engine for Vim. GNU GPL3

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 - Fast and small automatic source code formatter that supports C. GNU LGPL3
• address-sanitizer - Fast memory error detector. Apache 2.0
• biicode - Modern dependency manager for C. Expat
• c - Compile and execute C "scripts" in one go on the command line. Also has shebang support. Expat
• c99sh - Run C files using hash-bang. FreeBSD
• CBMC - C Bounded Model Checker; a tool for verification of array bounds, pointer safety and user-specified assertions. Original BSD
• cdecl - Online service to translate C declarations into English and vice versa. Public Domain
• cinclude2dot - Graphs include dependencies in a C project using Graphviz. Any GNU GPL version (due to underspecification in the file)
• Complexity - Tool for measuring the complexity of C source code. GNU GPL3 or later
• CScout - Source code analyzer and refactoring browser for collections of C programs. GNU GPL3
• DDD - Graphical front-end for a range of command-line debuggers. GNU GPL3 or later
• GDB - The GNU Project debugger; a debugger for C. GNU GPL3 or later
• Glade - RAD tool to enable quick development of GTK+ GUIs. GNU GPL2.1
• GMSL - GNU Make Standard Library; a collection of additional functionality for GNU Make. 3-clause BSD
• GNU Global - Source code tagging tool which works with C. GNU GPL3
• gprof - Performance analysis tool. Part of GNU binutils. GNU GPL3 or later
• Highlight - Converts source code to formatted text with nice highlighting. GNU GPL3
• 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. GNU GPLv3 or later
• rr - Debugger that records non-deterministic executions to allow for deterministic debugging. FreeBSD
• tup - Very fast, file-based, cross-platform build system. GNU GPL2.1
• unifdef - Removes #ifdef and #if directives with their delimited text without touching any other part of the file. 3-clause BSD and FreeBSD
• Valgrind - Range of dynamic analysis tools, including a leak checker. GNU GPL2.1

Utilities

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

• ApeTagLibs - C library for working with APEv2 tags. Expat
• bfd - Library for manipulating binary object files. Part of GNU binutils. GNU GPL3 or later
• cf4ocl - The C Framework for OpenCL; a cross-platform object-oriented framework for developing and benchmarking OpenCL projects. GNU LGPL3 (library) or GNU GPL3 (project code)
• CommonMark - C implementation of the CommonMark spec. Various Licenses
• CException - C implementation of exceptions. Expat
• docopt.c - C implementation of a command-line option parser. Expat
• dyncall - Another foreign function interface library. Expat
• FANN - Fast Artifical Neural Network library; an implementation of neural networks. GNU GPL2.1
• Firm - C library that provides a graph-based intermediate representation, optimizations and assembly code generation suitable for use in compilers. Comes with an example C front-end under the same license. GNU LGPLv2.1
• Genann - Simple neural network library in C. Zlib
• gjrand - Library of random-number generation routines. GNU GPL2.1 or GNU GPLv3
• GNU FreeIPMI - In-band and out-of-band IPMI implementation. GNU GPL3
• GNU gperf - Perfect hash function generator, given a list of strings. Outputs C code. GNU GPL3 or later
• GNU Libffcall - Collection of libraries for building foreign function interfaces. GNU GPL3 or later
• gperftools - Collection of utilities for measuring and improving performance. 3-clause BSD
• huffandpuff - Minimal Huffman encoder and decoder. Public Domain
• kdtree - Simple library for working with KD-trees. 3-clause BSD
• Kitsune - Efficient, general-purpose framework for dynamic software updating. GNU LGPL3 or later
• libavl - Library containing a range of self-balancing binary trees. GNU GPL3 or later
• libbson - BSON utility library. Apache 2.0
• libCello - Library introducing higher-level programming to C. 3-clause BSD
• libffi - Portable foreign-function interface library. Expat
• libgit2 - Pure C implementation of Git. GNU GPL2 only, with a linking exception
• libimobiledevice - Cross-platform protocol library to communicate with iThings. GNU LGPLv2.1 or later (library) or GNU GPL2.1 or later (tools)
• libnfc - Platform-independent Near-Field Communication library. GNU LGPL3
• libPhenom - Deprecated, here for archival purposes. Eventing framework for building high-scalability and high-performance systems. Apache 2.0
• ncurses - Coloured terminal UI library. GNU GPL3 or later
• nope.c - C-language-based, ultra-light software platform for scalable server-side and networking applications (think node.js for C programmers). GNU GPL2.1
• pbc - Protocol buffers library. Expat
• rabbitmq-c - Client library for RabbitMQ. Expat
• Ragel - DSL for state machines that compiles to C. GNU GPL2.1
• Tinyexpr - Tiny recursive-descent parser, compiler and evaluation engine for simple mathematical expressions. zlib
• uthash - Hash table implementation, allowing existing structures to be stored in a hash table easily. 1-clause BSD
• cpu_features - A cross platform C99 library to get cpu features at runtime. Apache 2.0
• pblog - Small, low overhead, structured logging library intended for logging formware events.Apache 2.0
• libelf - ELF object file access library in C. GNU GPL2.1
• mcpp - Portable C preprocessor. 2-clause BSD
• libusb - Generic access to USB devices. LGPL2.1
• alsa-lib - Userspace library to interact with ALSA. LGPL2.1
• Capstone - Dissasembly/disassembler framework. BSD 3-clause

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 - Stream-oriented XML parser. Expat
• libxml2 - Standards-compliant, portable XML parser. Expat
• mini-xml - Small XML reading and writing library. No dependencies aside from C standard library. Apache 2.0 with exceptions