Introducing seL4 2.0.0

Version 2.0.0 introduces more consistent terminology to manual and code, as well as API and performance improvements, including to seL4’s formally verified ARMv6 version.

We clarified the notion of signals and notifications vs IPC, and added a new feature to the seL4 API that allows user programs to receive notifications while waiting for IPC messages. You can now also poll notification objects. Both of these changes are aimed at simplifying user-level interaction with interrupts.

In addition to the new features, seL4 has become even faster than it was before, esp. in scheduling threads.

The new features and performance improvements are available on all supported platforms, and are formally verified to full sel4 standards. We have also switched our release process to semantic versioning, so it’s easy to tell which seL4 releases are binary-compatible, source-compatible, or will require updates to user-level code.

Terminology Changes

IPC vs Notifications

The old terminology of “synchronous IPC” vs “asynchronous IPC” was confusing and misleading. We changed this to reflect the fact that IPC in seL4 is always synchronous and is simply referred to as “IPC”, which is enabled by “endpoint” objects. The receive operation is now more appropriately called “receive”.

What used to be called incorrectly “asynchronous IPC” is now called Notifications, enabled by “notification” objects. Notifications are not a form of message passing, but arrays of binary semaphores, and the new terminology describes this better. The operations on them are consequently called “signal” and “wait”

New Features

Notification binding

A notification can be bound to a thread and is referred to as the thread’s bound notification object. This is a 1:1 relationship. Whenever a thread waits for an IPC on an endpoint, it will receive any signals sent to its bound notification object, with the signal flags converted into a single-word message.

Threads can also explicitly wait for signals by waiting on the notification object itself.

For more details, see the 2.0.0 manual

Implementation improvements

  • introduces the bitfield scheduler: faster scheduler (Was linear in the

number of runnable threads, now log n)

  • improved benchmarking macros: can now specify multiple benchmarking

tracepoints at once added CONFIG_RELEASE_PRINTF in addition to CONFIG_DEBUG and CONFIG_RELEASE, which enables printf in a release build

API Changes

  • seL4_Recv replaces seL4_Wait on endpoints
  • seL4_Wait is now only used on notification objects
  • Async endpoint -> notification object
  • sync endpoint -> endpoint
  • seL4_Recv on an endpoint may now return signals sent to a thread’s

bound notification object.

API Additions

  • seL4_NotificationObject replaces deprecated seL4_AsyncEndpointObject
  • seL4_NotificationBits size in bits of a notification object
  • seL4_IRQHandler_SetNotification replaces deprecated seL4_IRQHandler_SetEndpoint
  • seL4_Recv replaces seL4_Wait for endpoints
  • seL4_Wait used on notifications
  • seL4_NBRecv non-blocking (polling) receive on an endpoint, which fails

if there is no message waiting. Opposite of NBSend (which silently fails if there is no receiver waiting)

  • seL4_Poll collects any signals from a notification objects, returns zero if there are none
  • seL4_Signal replaces deprecated seL4_Notify
  • seL4_TCB_BindNotification bind a notification to a tcb
  • seL4_TCB_UnbindNotification unbind a notification from a tcb


  • seL4_AsyncEndpointObject
  • seL4_Notify
  • seL4_IRQHandler_SetEndpoint

Note on Syscall names

Logically there are only two operations on capabilities, send and receive, which can take opcodes specifying sup-operations. For example, the Notification operations Signal and Wait are mapped to Send and Receive, respectively. For efficiency reasons, many kernel objects use separate kernel entry points for their operations. You must not rely on either, as this can change at any time. Instead, always use the object-specific wrapper functions.

User-level repositories

To simply repository management we have merged many of our user level library repositories. To see the changes please compare 1.0.4.xml and 2.0.x.xml in

Upgrade notes

Calls to seL4_Wait no longer return a seL4_MessageInfo_t as seL4_Wait is intended to be used on notification objects. Calls to the prior version of seL4_Wait need to be replaced with seL4_Recv.

If you don’t want to upgrade yet - don’t worry. Both the sel4test-manifest and verification manifest repositories have manifests titled 1.0.4.xml which point to the tips of the previously released repositories before today. All library repositories have a branch called ‘1.0.x-compatible’ which are compatible with the 1.0.4 kernel.

Note that to use the new merged repositories, you must upgrade to 2.0.0.

Full changelog

Use git log 1.0.4..2.0.0 in

More details

See the 2.0.0 manual included in the release for detailed descriptions of the new features. Or ask on this mailing list!