Monthly Archives: March 2022

Quieting CVDisplayLink Logging

In recent months I’ve noticed an accumulation of garbage log messages when I’m debugging an app in Xcode. Line after line along the lines of:

[] [0x124858a00] CVDisplayLinkStart
[] [0x124858a20] CVDisplayLink::start
[] [0x600003c3cee0] CVXTime::reset

Today I went digging to find the source of these log messages, and in the process I discovered a workaround that disables them. Just pass “-cv_note 0” as a parameter to the Run task in the Xcode scheme. Alternatively, you could disable them across all of your apps by setting this global default:

defaults write cv_note 0

The only downside to disabling the messages globally is that you will have to remember you disabled it if you ever decide you want to see massive quantities of Core Video debugging logs!

I discovered this user default by spelunking the system frameworks code from which they originate. After setting a breakpoint early in my app’s launch, I set a breakpoint on a variety of logging related methods until I discovered that these messages are all originating from a function called cv_note() in the CoreVideo framework. Within that function, there is a call to another function called “cv_note_init_logging_once”, and within that is a check for a user default value along with this tasty morsel:

Screenshot of Xcode disassembly showing a log message: Thanks for setting defaults write cv_note -int %d

And thank you, Apple, for providing such a mechanism, even if it’s unfortunately configured to log by default. I filed FB9960090: “CoreVideo logging should default to 0”, requesting that Apple change the behavior of this initialization function so that if a user has not set the cv_note value to anything particular, it defaults to 0.