tcpdump prints a description of the contents of packets on a network interface which match a given boolean expression. It can also be run with the -w flag, which causes it to save the packet data to a file for later analysis, and/or with the -r flag, which causes it to read from a saved packet file rather than to read packets from a network interface. In all cases, only packets which match the expression will be processed by tcpdump. tcpdump logs more than just TCP, IP, or ethernet packets, but has a whole suite of decoders, including ones for USB.
Felix is a programming language with a fully featured purely functional subsystem embedded in a a traditional Algol-like imperative programming system. The system features garbage collection, static typing, parametric polymorphism, overloading, regular expression matching, and a lot more. A scripting harness is used to invoke source files, but fully compiled binary shared libraries are generated, yielding both rapid prototyping and high performance. Seamless binding of C/C++ code makes interfacing existing libraries easy. Control inversion provides language-level cooperative multi-tasking.