Design differences from UN*X and POSIX systems

No fork(), no exec()

In traditional UN*X, new processes are created by duplicating the resources of the current process via a call to fork(). Forking a process will, among other things, clone the current thread of that process. After fork(), both threads typically test whether they are the parent or the child instance. Very often, the child thread makes a call to exec() to load a new program. Calling exec() replaces the contents of the address space of the entire process with the new content of the loaded program. As can be seen, especially without heavy optimizations such as copy-on-write allocation of memory pages and non-duplicating variants of fork() such as vfork(), the procedure of spawning a new program is quite wasteful.

In HelenOS, there is no fork() or exec() or any variant thereof. Programs are simply spawned by calling task_spawn(), which creates a separate new task for the loaded program and allows the caller to continue to run. Likewise, new threads and thread-like entities are created by calling thread_create() and fibril_create().

Running in background

When you run a command in the shell, the starts the command and waits for it to terminate / return exit status. In UN*X a process that wants to return control to the parent shell and continue running in the background (typically a daemon) does a fork(), the parent immediately exits (thus returning exit status) and the child continues running. In HelenOS there is no fork(). Instead it is possible for a task to return status to the caller without exiting. A HelenOS server usually initializes and registers itself with some naming service, then it calls task_retval() which returns status to the caller (effectively returning command line to the user) without terminating the task. After that the server usually enters the main message loop.

No signals

To Be Written.

Lexical dot-dot resolution

To Be Written.