matio is an C library for reading and writing MATLAB MAT files.
One cannot write structs or nested cell arrays to files when using ZLIB compression. The suggested, unofficial hacks to getting around the problem do not generally work, either. The API is cumbersome but still usable. Much of the source code is all but unreadable, which makes bug fixing a daunting task at best. Overall, I would give this library a 7.5 out of 10.