The open source GeoIP C API includes a free GeoLite Country database. The code is licensed under the LGPL. This API should work on most UNIX and GNU/Linux platforms. A free GeoLite City database is also available. A FreeBSD Port is available from FreeBSD.org. Also work on Windows platform.

CLAM is a full-fledged software framework for research and application development in the Audio and Music domain. It offers a conceptual model as well as tools for the analysis, synthesis and transformation of audio signals. If you are interested in learning more about CLAM you should visit the documentation section in this same website and read any of the articles or presentations listed there.

Find C/C++ libraries that can be used in robot control programs at DMOZ open directory and in Google.

LiDIA is a C++ library for computational number theory which provides a collection of highly optimized implementations of various multiprecision data types and time-intensive algorithms. LiDIA is developed by the LiDIA Group at the Darmstadt University of Technology. LiDIA is free for non-commercial purposes (license) and comes with the full source code. The current release of LiDIA contains the following components:

• Arithmetic Interfaces to cln, gmp, libI, piologie.
• Basic arithmetic over Z, Q, R, C, interval arithmetic, GF(2n), GF(pn).
• Factorization: Integer Factorization (Trial Division, Elliptic Curve Method, Self-Initializing Multipolynomial Quadratic Sieve with Lanczos algorithm), Factorization of Polynomials over finite fields (V. Shoup’s algorithms), Factoring ideals of algebraic number fields.
• Lattice Basis Reduction: various versions of LLL/MLLL (Schnorr-Euchner, Benne de Weger).
• Linear Algebra over Z: basic operations, normal forms of matrices (G. Havas algorithms)
  Number Fields: Quadratic Number Fields including a new implementation of Buchmann’s sub exponential algorithm for computing classgroups using mpqs techniques, Higher-Degree Number Fields arithmetic and maximal order.
• Polynomials: template classes for univariate polynomials with special algorithms for different domains.
• Elliptic Curves: elliptic curves over the rational and over finite fields (includes code written by J.Cremona and N. Smart). There is also a package for counting points on elliptic curves as well as a package for generating cryptographically strong curves.
• Primality Proofing: Besides the standard probabilistic primality tests in the base package there is a new elliptic curve based primality proofer in the GEC package.
• Other Generic Data Types: vectors, matrices, power series, and hash tables implemented as templates.

GD library is an open source code C library for the dynamic creation of images by programmers. GD is written in C, and “wrappers” are available for Perl, PHP and other languages. GD creates PNG, JPEG and GIF images, among other formats. GD is commonly used to generate charts, graphics, thumbnails, and most anything else, on the fly. While not restricted to use on the web, the most common applications of GD involve web site development.

…FFTW is a C subroutine library for computing the discrete Fourier transform (DFT) in one or more dimensions, of arbitrary input size, and of both real and complex data (as well as of even/odd data, i.e. the discrete cosine/sine transforms or DCT/DST). They believe that FFTW, which is free software, should become the FFT library of choice for most applications.
The benchmarks, performed on a variety of platforms, show that FFTW’s performance is typically superior to that of other publicly available FFT software, and is even competitive with vendor-tuned codes. In contrast to vendor-tuned codes, however, FFTW’s performance is portable: the same program will perform well on most architecture without modification. Hence the name, “FFTW,” which stands for the somewhat whimsical title of “Fastest Fourier Transform in the West”. The FFTW package was developed at MIT by Matteo Frigo and Steven G. Johnson…