StarPilot/cereal

What is cereal?

cereal is the messaging system for openpilot. It uses msgq as a pub/sub backend, and Cap'n proto for serialization of the structs.

Messaging Spec

You'll find the message types in log.capnp. It uses Cap'n proto and defines one struct called Event.

All Events have a logMonoTime and a valid. Then a big union defines the packet type.

Best Practices

• All fields must describe quantities in SI units, unless otherwise specified in the field name.
• In the context of the message they are in, field names should be completely unambiguous.
• All values should be easy to plot and be human-readable with minimal parsing.

Maintaining backwards-compatibility

When making changes to the messaging spec you want to maintain backwards-compatibility, such that old logs can be parsed with a new version of cereal. Adding structs and adding members to structs is generally safe, most other things are not. Read more details here.

Custom forks

Forks of openpilot might want to add things to the messaging spec, however this could conflict with future changes made in mainline cereal/openpilot. Rebasing against mainline openpilot then means breaking backwards-compatibility with all old logs of your fork. So we added reserved events in custom.capnp that we will leave empty in mainline cereal/openpilot. If you only modify those, you can ensure your fork will remain backwards-compatible with all versions of mainline openpilot and your fork.

An example of compatible changes:

diff --git a/cereal/custom.capnp b/cereal/custom.capnp
index 3348e859e..3365c7b98 100644
--- a/cereal/custom.capnp
+++ b/cereal/custom.capnp
@@ -10,7 +10,11 @@ $Cxx.namespace("cereal");
 # DO rename the structs
 # DON'T change the identifier (e.g. @0x81c2f05a394cf4af)

-struct CustomReserved0 @0x81c2f05a394cf4af {
+struct SteeringInfo @0x81c2f05a394cf4af {
+  active @0 :Bool;
+  steeringAngleDeg @1 :Float32;
+  steeringRateDeg @2 :Float32;
+  steeringAccelDeg @3 :Float32;
 }

 struct CustomReserved1 @0xaedffd8f31e7b55d {
diff --git a/cereal/log.capnp b/cereal/log.capnp
index 1209f3fd9..b189f58b6 100644
--- a/cereal/log.capnp
+++ b/cereal/log.capnp
@@ -2558,14 +2558,14 @@ struct Event {

     # DO change the name of the field
     # DON'T change anything after the "@"
-    customReservedRawData0 @124 :Data;
+    rawCanData @124 :Data;
     customReservedRawData1 @125 :Data;
     customReservedRawData2 @126 :Data;

     # DO change the name of the field and struct
     # DON'T change the ID (e.g. @107)
     # DON'T change which struct it points to
-    customReserved0 @107 :Custom.CustomReserved0;
+    steeringInfo @107 :Custom.SteeringInfo;
     customReserved1 @108 :Custom.CustomReserved1;
     customReserved2 @109 :Custom.CustomReserved2;
     customReserved3 @110 :Custom.CustomReserved3;

---

Example

import cereal.messaging as messaging

# in subscriber
sm = messaging.SubMaster(['sensorEvents'])
while 1:
  sm.update()
  print(sm['sensorEvents'])

# in publisher
pm = messaging.PubMaster(['sensorEvents'])
dat = messaging.new_message('sensorEvents', size=1)
dat.sensorEvents[0] = {"gyro": {"v": [0.1, -0.1, 0.1]}}
pm.send('sensorEvents', dat)