KLEE is a symbolic virtual machine built on top of the LLVM compiler infrastructure. Currently, there are two primary components:
The core symbolic virtual machine engine; this is responsible for executing LLVM bitcode modules with support for symbolic values. This is comprised of the code in lib/.
A POSIX/Linux emulation layer oriented towards supporting uClibc, with additional support for making parts of the operating system environment symbolic.
Additionally, there is a simple library for replaying computed inputs on native code (for closed programs). There is also a more complicated infrastructure for replaying the inputs generated for the POSIX/Linux emulation layer, which handles running native programs in an environment that matches a computed test input, including setting up files, pipes, environment variables, and passing command line arguments.
For further information, see the webpage.