Internal Library Development

Unikraft libraries are no different than external ones, except for the fact that

  1. they are part of the main Unikraft repo and live under the lib/name/ directories.
  2. they do not use any external source files, i.e., all source code is within that directory.
  3. they must not have dependencies on external libraries, i.e., the Unikraft repo must be able to be built on its own. Remember that for such builds, libnolibc has to be sufficient libc replacement to compile, link, and execute internal libraries. This means that nolibc has to be extended from time to time.

Other than that the process of creating and adding a Unikraft lib is the same as for external libraries.

Unikraft APIs

One important thing to point out regarding Unikraft internal libraries is that for each “category” of library (e.g., memory allocators, schedulers, device buses, network drivers, etc.) Unikraft defines (or will define) an API that each library under that category must comply with. This is so that it’s possible to easily plug and play different libraries of a certain type (e.g., using a co-operative scheduler or a pre-emptive one).

An API consists of a header file defining the actual API as well as an implementation of some generic/compatibility functions, if any, that are common to all libraries under a specific category. This functionality is all wrapped in a Unikraft library and placed under lib/uk[category] (e.g., lib/ukalloc for the memory allocator API, lib/uknetdev for the network driver API). The directory structure for an API generally looks as follows:

├── [category].c
├── include
│   └── uc
│       └── [category].h

The and Makefile.uc files are fairly straightforward, you can refer to an existing API in the repo to see what they look like. The header file contains the API itself and follows a generic structure that all APIs should follow; please refer to existing API header files if you’d like to see what these look like (e.g., libukalloc, libuksched).

To implement a library that complies with an API, we begin by creating a similar directory structure as above, using uk[category][name] for the library name (e.g., ukschedcoop for a co-operative scheduler). In the file, state that your library depends on the library API. For instance, for a new scheduler you would write:

      bool "ukschemysched: my scheduler"
      default n
      select LIBUKALLOC
      select LIBUKSCHED

Like other Unikraft libraries, you’ll need a simple

$(eval $(call addlib_s,libucschedmysched,$(LIBUKSCHEDMYSCHED)))

The header file does little else than include the API header file and define an init function:

#include <uk/sched.h>
#include <uk/alloc.h>
struct uk_sched *uc_schedmysched_init(struct uk_alloc *a);

Finally the C file implements this init function and any other functions required by the API. The init function uses the uk_sched_init macro defined in the API header file to register its functions, e.g.,:

struct uk_sched *uc_schedmysched_init(struct uk_alloc *a)
    struct uk_sched *a;
    uk_sched_init(a, uc_schedmysched_schedule, ..., uc_schedmysched_get_idle);
    return a;

And that’s it, you should now be able to build your new library just like any other in Unikraft (i.e., you’ll need to create an application, to select the library in the menu, and to type make).