Best C libraries

[jjjaime]

I've come back to C after a Java period. The great advantage of Java is the number of open libraries available for almost any purpose, so you can focus on your target. Now, I'm experimenting with libev (http://software.schmorp.de/pkg/libev.html) and http_parser (https://github.com/ry/http-parser).

But, which libraries (apart from standard ones) are you using? For example, to manage:
- Threads
- Sockets
- HTTP
- JSON
- XML
- containers (e.g. maps)