:sig
pid_t task(String key, std::function<void()> exec_func)

:params
key : string uniquely identifying the task
exec_func : function to execute
return value : the process ID of the started (or still running) task

:desc
task() starts the 'exec_func' in a new process and returns that process' ID. If a process with the same 'key' is already running, task will not start a new process but instead just return the PID of the process that is already running.

:see
>task