awesome-c

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.

This is released under a Creative Commons-Attribution-ShareAlike license, version 4. You can find its text in the LICENSE file.

This project is part of Peers - a community for growing our freedom. Support us today!

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

• Build Systems
• Compilers
• Compression
• Concurrent and Parallel
• Database
• Documentation Generation
• Editors
• Frameworks
• Game Programming
  • Engines
  • Resources
• Generic Programming
• Graphics
• Graphical User Interface
• 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
• Multimedia
• Networking and Internet
  • Web Frameworks
• Numerical
• Regex
• Security
• Serialization
• Source Code Collections
• Standard Libraries
• String Manipulation
• Testing
• Text Editor Extensions
  • Emacs
  • Vim
• Tools
• Utilities
• Windows Environments
• XML

Build Systems

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

• aimake - A build tool designed to avoid complex configurations. GNU GPLv3 or later.
• Autoconf - An extensible package of M4 macros that produce shell scripts to automatically configure software source code packages. Part of the Autotools. GNU GPLv3 or later.
• Automake - A tool for automatically generating Makefile.in files compliant with the GNU Coding Standards. Requires the use of Autoconf. Part of the Autotools. GNU GPLv3 or later.
• fab - A build system that helps build optimally every time. GNU GPLv3 only.
• GNU Make - A tool which controls the generation of executables and other non-source files of a program. GNU GPLv3 or later.
• Jam - A build system, designed to be easier than make. Understands C build rules implicitly. Jam License.
• Libtool - A generic library support script. Part of the Autotools. GNU GPLv3 or later.
• Meson - An extremely fast, user-friendly build system. Based on Ninja. Apache2.0.
• tup - A very fast, file-based, cross-platform build system. GNU GPLv2.1 only.
• zproject - A project generator and build system support. MPLv2.0

Compilers

• Clang - A compiler for LLVM. Supports C11. NCSA.
• GCC - Provides a C compiler as part of its compiler set. Supports C11. GNU GPLv3 or later.
• PCC - A venerable compiler. Supports C99. Various licenses, all free.

Compression

• clzip - C version of the high-quality data compressor Lzip (LZMA implementation). GNU GPLv2.1 or later.
• libbzip2 - A patent-free, high-quality data compression library. Original BSD.
• lzo - A very fast data compression library. GNU GPLv2.1 or later.
• shoco - A compressor for small text strings. Expat.
• smaz - An efficient string compression library. 3-clause BSD.
• squash - A compression abstraction library, complete with some utilities. Expat.
• zlib - A massively-spiffy yet delicately-unobtrusive compression library. 3-clause BSD.

Concurrent and Parallel

• cchan - A small library for channel constructs for inter-thread communication. Public domain.
• ck - Concurrency primitives, safe memory reclamation mechanisms and non-blocking data structures. FreeBSD.
• libconcurrent - Concurrent programming library, using coroutines, for C11. zlip.
• mill - Go-style concurrency. X11.
• MPICH - Another implementation of MPI. MPICH licence.
• OpenMP - A set of pragmas designed to allow for easy parallelization of code. Standard (licensing not applicable).
• OpenMPI - A message passing interface implementation. 3-clause BSD.
• PETSc - A suite of data structures and routines for scalable parallel solution of scientific applications modelled by partial differential equations. FreeBSD.
• pth - A portable implementation for non-preemptive priority-based scheduling for multiple threads of execution. GNU GPLv3 or later.
• pthreads - The POSIX thread library. Standard (no license applicable).
• SLEPc - A software library for the solution of large, sparse eigenvalue problems on parallel computers. GNU LGPLv3 only.
• TinyCThread - A portable, small implementation of the C11 threads API. zlib.

Database

This lists databases and data stores with C APIs.

• BerkeleyDB - A library for a high-performance embedded database for key-value data. GNU AGPLv3 only.
• Hiredis - A minimalistic client library for Redis. 3-clause BSD.
• LMDB - An ultra-fast, ultra-compact key-value embedded data store. newOpenLDAP.
• MariaDB - A robust, scalable and reliable SQL server, designed to be a drop-in replacement for MySQL. 3-clause BSD.
• mongo-c-driver - A high-performance client library for MongoDB. Apache2.0.
• PostgreSQL - A powerful object-relational database system. PostgreSQL licence.
• recutils - A set of tools and a library for accessing human-editable, plaintext database files called recfiles. GNU GPLv3 or later.
• Redis - An advanced key-value store. 3-clause BSD.
• sophia - A modern, embeddable key-value database. FreeBSD.
• SQLite - A self-contained, serverless, zero-configuration, transactional SQL database engine. Public domain.
• UnQLite - A self-contained, serverless, zero-configuration, transactional NoSQL engine. FreeBSD.

Documentation Generation

• Cxref - Generates documentation in either LaTeX, HTML, RTF or SGML. GNU GPLv2.1 only.
• DocOnce - A 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 documentation from annotated sources. Can generate a large range of formats. GNU GPLv2.1 only.
• GTK-Doc - A tool for generating documentation from annotated sources. Has support for the Autotools. GNU GPLv2.1 only (code), GNU FDL1.1 only (documentation).

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 GPLv2.1 only.
• Code::Blocks - An extensible, configurable IDE supporting C. GNU GPLv3 only.
• CodeLite - A cross-platform IDE. GNU GPLv2.1 only.
• Eclipse - An IDE written in Java. EPL.
• Geany - A very small and fast IDE. GNU GPLv2.1 or later.
• KDevelop - The KDE IDE. GNU GPLv2.1 only.

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. Apache2.0.
• C Algorithms - A collection of common algorithms and data structures. ISC.
• Collections-C - A library of generic data structures. GNU LGPLv3 or later.
• CPL - The Common Pipeline Library; a set of libraries designed to be a comprehensive, efficient and robust software toolkit. GNU GPLv2.1 only.
• 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. GNU LGPLv2.1 only.
• GIO - A modern and easy-to-use VFS API. GNU LGPLv2.1 only.
• GObject - An object-oriented system and object model. GNU LGPLv2.1 only.
• libnih - A lightweight library of functions and structures. GNU GPLv2.1 only.
• libU - A small library of basic utilities, including memory allocation, string manipulation and logging. 3-clause BSD.
• PBL - A large library of utilities, featuring data structures, among other things. GNU LGPLv2.1 or later (library), GNU GPLv2.1 or later (test code).
• qlibc - A simple and powerful library, designed as a replacement for GLib while focusing on being small and light. qLib license (similar to FreeBSD).
• stb - A range of single-file libraries. Public domain.
• TBOX - A multi-platform library with a large number of capabilities. GNU LGPLv2.1 or later.

Game Programming

Engines

These are provided as examples of C game programming code.

• Corange - A game engine in pure C. FreeBSD.
• Darkplaces - A modified version of the Quake2 engine. GNU GPLv2.1 only.
• ioquake3 - The Quake3 engine, freed at last. GNU GPLv2.1 only.
• Orx - A portable, lightweight, plugin-based, data-driven, 2D-oriented game engine. zlib.
• Quake - The Quake engine. GNU GPLv2.1 only.
• Quake2 - The Quake2 engine. GNU GPLv2.1 only.
• Spearmint - An engine designed for FPS games. GNU GPLv3 or later.

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. Expat.
• CSFML - A binding for SFML. zlib.
• Epoxy - A library for handling OpenGL function pointer management. Expat.
• 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.
• libao - A cross-platform audio library with a wide variety of outputs. GNU GPLv2.1 or later.
• RetroArch - The reference frontend for libretro. GNU GPLv3 only.
• SDL2 - A cross-platform library designed to provide low-level access to audio, keyboard, mouse, joystick and graphics hardware via OpenGL. zlib.
• 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. Expat.
• SGLIB - A simple generic library. Any OSI-approved license.

Graphics

• Cairo - A 2D graphics library. GNU LGPLv2.1 only or MPLv1.1.
• Cogl - A GPU graphics and utilities API. Expat (dependent on 3-clause BSD and possibly LGPLv2.1 only libs).
• Clutter - A UI library based on OpenGL. GNU LGPLv2.1 only.
• giflib - A library for reading and writing gif images. Expat.
• heman - A tiny library of image utilities dealing with height maps, normal maps, distance fields and the like. Expat.
• libcaca - An ASCII renderer for terminal-based interfaces. WTFPLv2.
• libgd - A library for the dynamic creation of images by programmers. Expat.
• libimagequant - Small, portable library for high-quality conversion of RGBA images to 8-bit indexed colour images. FreeBSD.
• libjpeg-turbo - A faster library for reading and writing JPEG files. Various licences.
• libpng - The official PNG reference library. libpng license.
• libRSVG - A library to render SVG files using Cairo. GNU LGPLv2.1 or later.
• libsixel - A library implementing the SIXEL protocol, allowing beautiful graphics in your terminal. Expat.
• libxmi - A function library for rasterizing 2D vector graphics. GNU GPLv3 or later.
• mozjpeg - An improved JPEG encoder. 3-clause BSD.
• 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. GNU LGPLv2.1 only.
• IUP - Another cross-platform widget toolkit. Expat.
• 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/Tk License.
• XForms Toolkit - A widget toolkit designed for the XWindow system. GNU LGPLv2.1 only.

JSON

• Jansson - A library for encoding, decoding and manipulating JSON. Expat.
• jsmn - A minimalistic JSON parser. Expat.
• json-c - A library for working with JSON. Expat.
• WJElement - Advanced JSON manipulation library, with support for JSON Schema. LGPL, any version.
• 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

• SEI CERT C Coding Standard
• C FAQ - comp.lang.c Frequently Asked Questions
• Comparison of C/POSIX standard library implementations for GNU/Linux
• Draft C11 standard
• GNU C Reference Manual
• Robert Pike's notes on programming in C

Beginner resources online

• A 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 resources online

• 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
• 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.

Multimedia

• FFMPEG - A complete, cross-platform solution to record, convert and stream audio and video. GNU LGPLv2.1 or later, with some parts under GNU GPLv2.1 or later.
• GStreamer - A framework for audio and visual media. GNU LGPLv2.1 only.
• libmpv - A music-playing library. Compile with ./waf configure --disable-cplayer --enable-libmpv-shared to not have the music player. GNU GPLv2.1 or later.
• libsoundio - A library for cross-platform, real-time audio input and output. Has a range of back-ends. Expat.
• lodepng - A simple PNG image decoder and encoder, requiring no other dependencies. 3-clause BSD.

Networking and Internet

The Security section also contains useful resources for making stuff relating to networks and the Internet.

• asnlc - A compiler of ASN.1 specifications into C source code. FreeBSD.
• CHL - C Hypertext Library - A library for writing web applications in C. GNU GPLv3 only.
• czmq - A high-level binding for ZeroMQ. MPLv2.0
• GNU adns - An advanced, easy-to-use, asynch-capable DNS client library and utilities. GNU GPLv3 or later.
• gumbo-parser - An HTML5 parsing library in C99. Apache2.0.
• http-parser - An HTTP request/response parser. Expat.
• ldns - A library to simplify DNS programming. 3-clause BSD.
• libcurl - A client-side URL transfer library, supporting a wide range of formats. curl license.
• LibEtPan - A mail library providing an efficient network for IMAP, SMTP, POP and NNTP. 3-clause BSD.
• libev - Yet another event loop. FreeBSD.
• libevent - An event loop replacement for network servers. 3-clause BSD.
• libhttpd - A library to add basic web server capabilities to an application or embedded device. GNU GPLv2 only.
• libidn - An implementation of the Stringprep, Punycode and IDNA specifications. GNU GPLv3 or later.
• libmicrohttpd - A small library that makes it easy to run an HTTP server as part of another application. GNU LGPLv2.1 or later.
• libonion - HTTP server library, designed to be easy to use. Apache2.0.
• 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. GNU GPLv3 or later.
• libsoup - A GNOME HTTP client/server library. Uses GObject. GNU LGPLv2.1 only.
• lwan - An experimental, scalable, high-performance HTTP server. GNU GPLv2.1 only.
• mongoose - Embedded web server. GNU GPLv2.1 only.
• nanomsg - A C-based implementation of ZeroMQ. Expat.
• oSip - A SIP implementation without additional dependencies. GNU LGPLv2.1 or later.
• socket99 - A C99 wrapper for the BSD sockets API. ISC.
• Tox - A communication platform, designed to be a Skype-killer. GNU GPLv3 only.
• twitc - A mini library for interacting with the Twitter OAuth API. Expat.
• zyre - A framework for proximity-based peer-to-peer applications. MPLv2.0.

Web Frameworks

• balde - A microframework based on GLib. GNU LGPLv2.1 only.
• KLone - A fully-featured, multi-platform, web application development framework, targeted especially at embedded systems and appliances. 3-clause BSD.
• 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. GNU GPLv2.1 only.
• 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.
• Cuba - A library for multidimensional numerical integration. GNU LGPLv3 only.
• FFTW - The Fastest Fourier Transform in the West; a highly-optimized fast Fourier transform routine. GNU GPLv2.1 or later.
• FLINT - Fast Library for Number Theory; a library supporting arithmetic with numbers, polynomials, power series and matrices, among others. GNU GPLv2.1 or later.
• GLPK - GNU Linear Programming Kit; a package designed for solving large-scale linear programming, mixed integer programming and other related problems. GNU GPLv3 or later.
• GMP - GNU Multple Precision Arithmetic Library; a library for arbitrary-precision arithmetic. Dual-licensed GNU GPLv2.1 only and GNU LGPLv3 only.
• GNU MPC - A library for complex number arithmetic. GNU LGPLv3 or later.
• GNU MPFR - A library for arbitrary-precision floating-point arithmetic. GNU LGPLv3 or later (most recent versions), GNU LGPLv2.1 or later (until version 2.4.x).
• GNU MPRIA - A portable mathematics library for multi-precision rational interval arithmetic. GNU GPLv3 or later.
• GSL - The GNU Scientific Library; a sophisticated numerical library. GNU GPLv3 only.
• KISS FFT - A very simple fast Fourier transform library. 3-clause BSD.
• LAPACKE - An interface to LAPACK. 3-clause BSD.
• PARI/GP - A computer algebra system for number theory; includes a compiler to C. GNU GPLv2.1 or later.
• PETSc - A suite of data structures and routines for scalable parallel solution of scientific applications modelled by partial differential equations. FreeBSD.
• SLEPc - A software library for the solution of large, sparse eigenvalue problems on parallel computers. GNU LGPLv3 only.
• Yeppp! - Very fast, SIMD-optimized mathematical library. 3-clause BSD.

Regex

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

• PCRE - An implementation of regexes identical to that of Perl 5. 3-clause BSD.
• SLRE - Super Light Regular Expression library; a very small implementation of a subset of Perl regex syntax. GNU GPLv2.1 only.
• TRE - A POSIX-compliant, feature-full regex library. FreeBSD.

Security

This includes various cryptographical and secure communications libraries and frameworks, as well as various security-related things.

• acl - Commands for manipulating POSIX access control lists. GNU GPLv2.1 or later.
• GNU SASL - An implementation of the Simple Authentication and Security Layer and a few common SASL mechanism. GNU GPLv3 or later.
• GnuTLS - A secure communication library, implementing SSL, TLS and DTLS. GNU LGPLv2.1 or later.
• libgcrypt - A general-purpose cryptography library, with a range of available ciphers. GNU LGPLv2.1 or later (code), GNU GPLv2.1 or later (manual and tools).
• libgss - Generic Security Service. GNU GPLv3 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.
• libsodium - A modern and easy-to-use crypto library. Expat.
• libtomcrypt - A fairly comprehensive, modular and portable cryptographic toolkit. Public domain.
• mbed TLS - Another crypto implementation. Apache2.0.
• s2n - A C99 implementation of the TLS/SSL protocols, designed to be simple, fast and with security as a priority. Apache2.0.

Serialization

• binn - A binary serialization format, meant to be compact, fast and easy-to-use. Apache2.0.
• c-capnproto - An implementation of the Cap'n Proto serialization protocol. Expat.
• cmp - An implementation of the MessagePack serialization protocol. Expat.
• libavro - An implementation of the Avro data serialization system. Apache2.0.
• mpack - Another implementation of the MessagePack serialization protocol. Expat.
• protobuf-c - An implementation of Google Protocol Buffer. FreeBSD.
• 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. Expat.
• gnulib - A collection of common GNU code. Various licenses, all free.
• libdjb - A collection of libraries doing various things. (Apparently) public domain.

Standard Libraries

This contains standard C libraries.

• Bionic - Google's standard library, developed for Android. 3-clause BSD.
• dietlibc - A standard library designed for the smallest possible binaries. GNU GPLv2.1 only.
• glibc - The GNU C Library; an implementation of the standard library. GNU LGPLv2.1 only.
• musl - A standard library, compatible with POSIX 2008 and C11. Designed for static linking. Expat.

String Manipulation

• bstrlib - The Better String Library. Dual-licensed under 3-clause BSD or GNU GPLv2.1 only.
• ICU - International Components for Unicode; a library for Unicode support. ICU license.
• libunistring - A library for manipulating Unicode strings. GNU LGPLv3 only.
• libgiconv - A text conversion library. GNU LGPLv2.1 only (library), GNU GPLv3 only (iconv program).
• 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. FreeBSD.

Testing

• CHEAT - A very simple unit testing framework. FreeBSD.
• Check - A unit testing framework. GNU LGPLv2.1 only.
• CMock - A mock/stub generator. Expat.
• cmocka - A unit testing framework with support for mock objects. Apache2.0.
• Criterion - A KISS, non-intrusive test framework. Expat.
• CUnit - Another unit testing framework. GNU LGPL2.0 only.
• minctest - A unit testing microlibrary. 3-clause BSD.
• minunit - Minimal unit testing framework. Expat.
• munit - A small unit testing framework. Expat.
• theft - Property-based testing (similar to Quickcheck). Expat.
• Unity - A simple unit testing framework. 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.

Emacs

• CEDET - Collection of Emacs Development Environment Tools; designed to provide IDE-like features to Emacs. Built-in. GNU GPLv3 or later.
• Flycheck - Modern syntax checking. For C, it can use either GCC or Clang as a back-end. GNU GPLv3 or later.
• YASnippet - A template system, with C templates for common code snippets. GNU GPLv3 or later.

Vim

• Syntastic - Syntax checking and linting. Do What The Fuck You Want To license.
• YouCompleteMe - A code completion engine for Vim. GNU GPLv3 only.

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. GNU LGPLv3 only.
• address-sanitizer - A fast memory error detector. Apache2.0.
• biicode - A modern dependency manager. Expat.
• c - Compile and execute C "scripts" in one go on the command line. Also has shebang support. Expat.
• 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. 3-clause BSD.
• CScout - A source code analyzer and refactoring browser for C programs. GNU GPLv3 only.
• 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 - An online service to translate C declarations into English and vice versa. Public domain.
• cflow - Analyzes a collection of source files and prints a graph charting control flow in the program. GNU GPLv3 or later.
• cinclude2dot - Graphs include dependencies in a project using Graphviz. Any GNU GPL version (due to underspecification in the file).
• Complexity - A tool for measuring the complexity of source code. GNU GPLv3 or later.
• DDD - A graphical front-end for a range of command-line debuggers. GNU GPLv3 or later.
• GDB - The GNU Project debugger. GNU GPLv3 or later.
• Glade - A RAD tool to enable quick development of GTK+ GUIs. GNU GPLv2.1 only.
• GMSL - GNU Make Standard Library; a collection of additional functionality for GNU Make. 3-clause BSD.
• GNU Global - A source code tagging tool. GNU GPLv3 only.
• GPP - A general-purpose preprocessor. More versatile than the C preprocessor, but more flexible than m4. GNU LGPLv3 or later.
• gprof - A performance analysis tool. Part of GNU binutils. GNU GPLv3 or later.
• Highlight - Converts source code to formatted text with nice highlighting. GNU GPLv3 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.
• 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.
• qo - A build system that works without a separate config file. Expat.
• rr - A debugger that records non-deterministic executions to allow for deterministic debugging. FreeBSD.
• unifdef - Removes #ifdef and #if directives with their delimited text without touching any other part of the file. 3-clause BSD and FreeBSD.
• Valgrind - A range of dynamic analysis tools, including a leak checker. GNU GPLv2.1 only.

Utilities

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

• ApeTagLibs - A library for working with APEv2 tags. Expat.
• argparse - A command-line argument parsing library, inspired by Python's argparse module. Expat.
• attr - Commands for manipulating filesystem extended attributes. GNU GPLv2.1 or later.
• bfd - A library for manipulating binary object files. Part of GNU binutils. GNU GPLv3 or later.
• ccv - C-based/Cached/Core Computer Vision library; modern computer vision. 3-clause BSD.
• cf4ocl - The C Framework for OpenCL; a cross-platform object-oriented framework for developing and benchmarking OpenCL. projects. GNU LGPLv3 only (library), GNU GPLv3 or later (other code).
• CommonMark - An implementation of the CommonMark spec. Variety of licenses, all free.
• CException - An implementation of exceptions. Expat.
• D-Bus - A simple way for applications to talk to one another. AFLv2.1 or GPLv2.1 or later.
• dlx - An implementation of Knuth's Algorithm X, with example solvers. GNU GPLv3 or later.
• docopt.c - An 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 GPLv2.1 only.
• 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. GNU LGPLv2.1 only.
• Genann - A simple ANN in C89, without additional dependencies. zlib.
• gjrand - A library of random-number generation routines. GNU GPLv2.1 only or GNU GPLv3 only (user's choice).
• GNU FreeIPMI - An in-band and out-of-band IPMI implementation. GNU GPLv3 only.
• GNU gperf - A perfect hash function generator, given a list of strings. Outputs C code. GNU GPLv3 or later.
• GNU Libffcall - A collection of libraries for building foreign function interfaces. GNU GPLv3 or later.
• gperftools - A collection of utilities for measuring and improving performance. 3-clause BSD.
• hammer - Parser combinators for binary formats. GNU GPLv2.1 only.
• Hans Boehm GC - Garbage collection for C? Don't mind if I do! Various licenses, all free.
• Hoedown - A fully-standards-compliant, extension-supporting, UTF-8 aware, fast Markdown parser. Expat.
• huffandpuff - A minimal Huffman encoder and decoder. Public domain.
• iniparser - A parser for .ini files. Expat.
• jemalloc - A malloc implementation that emphasizes avoidance of fragmentation and scalable concurrency support. FreeBSD.
• jwHash - A fast hashtable implementation. Apache2.0.
• kdtree - A simple library for working with KD-trees. 3-clause BSD.
• Kitsune - An efficient, general-purpose framework for dynamic software updating. GNU LGPLv3 or later.
• libavl - A library containing a range of self-balancing binary trees. GNU GPLv3 or later.
• libbson - A BSON utility library. Apache2.0.
• libCello - A library introducing higher-level programming to C. 3-clause BSD.
• libcox - A library which permits cross-platform system calls and standard utilities across different operating systems. FreeBSD.
• libcsv - A simple, streaming CSV parser. GNU LGPLv2.1 or later.
• libffi - A portable foreign-function interface library. Expat.
• libgit2 - Pure C implementation of Git. GNU GPL2 only, with a linking exception.
• 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. GNU LGPLv2.1 or later (library), GNU GPLv2.1 or later (tools).
• libnfc - A platform-independent Near-Field Communication library. GNU LGPLv3 only.
• libPhenom - An eventing framework for building high-scalability and high-performance systems. Apache2.0.
• libucl - A universal configuration library parser. FreeBSD.
• libuv - Cross-platform asynchronous I/O. Expat.
• libYAML - A YAML 1.1 parser and emitter. Expat.
• 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. FreeBSD.
• mpc - A parser combinator library. FreeBSD.
• ncurses - Coloured terminal UI library. GNU GPLv3 or later.
• nope.c - An ultra-light software platform for scalable server-side and networking applications (think node.js for C programmers). GNU GPLv2.1 only.
• pbc - A protocol buffers library. Expat.
• rabbitmq-c - A client library for RabbitMQ. Expat.
• Ragel - A DSL for state machines that compiles to C. GNU GPLv2.1 only.
• sort - A collection of sorting routines, designed to run at compile-time with a user-defined type. Expat.
• termbox - A library for writing text-based interfaces. Expat.
• tinyexpr - A tiny recursive-descent parser, compiler and evaluation engine for simple mathematical expressions. 3-clause BSD.
• uthash - A hash table implementation, allowing existing structures to be stored in a hash table easily. 1-clause BSD.
• vector.h - A header library for typed lists. Expat.
• Viola - A simplification of libCello. Expat.
• XLSX I/O - A cross-platform library for reading and writing .xlsx files. Expat.
• ZeroMQ - A distributed messaging protocol implementation. GNU LGPLv3 with static linking exception only.
• zproto - A protocol framework for ZeroMQ. Expat.

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. Expat.
• libroxml - Another XML library. GNU LGPLv2.1 with static linking exception only.
• libxml2 - A standards-compliant, portable XML parser. Expat.
• mini-xml - A small XML reading and writing library. No dependencies aside from C standard library. GNU LGPLv2.1 with static linking exception only.