Compel is sub-project of CRIU. It's an utility to execute code in foreign process address space. The code should be compiled with compel flags and packed, then it can be executed in other task's context. The parasite code executes in an environment w/o glibc, thus you cannot call the usual stdio/stdlib/etc. functions. Instead, compel provides a set of so called plugins for your convenience. Plugins get linked to parasite binary on the pack stage (see below).