libdmtx is open source software for reading and writing Data Matrix 2D bar-codes on Linux, Unix, OS X, Windows, and mobile devices. At its core libdmtx is a shared library, allowing C/C++ programs to use its capabilities without restrictions or overhead. The included utility programs, dmtxread and dmtxwrite, provide the official interface to libdmtx from the command line, and also serve as a good reference for programmers who wish to write their own programs that interact with libdmtx.