Libdill is a C library that makes writing structured concurrent programs easy.

Concurrency allows multiple functions to run independent of one another.

Structured concurrency means that lifetimes of concurrent functions are cleanly
nested. If coroutine foo launches coroutine bar, then bar must finish before foo
finishes.

Please note that concurrency isn't the same as parallelism. libdill provides
concurrency, but not parallelism.