Why isn't my kernel extension writing to kernel.log

Question

I have a simple kernel extension:

kern_return_t HelloWorld_start (kmod_info_t *ki, void * d) {
    printf("Hello World\n");
    return KERN_SUCCESS;
}

kern_return_t HelloWorld_stop (kmod_info_t * kid, void * d) {
    printf("Goodbye World\n");
    return KERN_SUCCESS;
}

Which I'm compiling and loading via:

sudo kextload HelloWorld.kext

And it appears in the kextstat listing:

...
  129    0 0xffffff7f80fac000 0x4000     0x4000     com.apple.driver.AppleProfileThreadInfoAction (85.2) <123 6 4 3 1>
  130    0 0xffffff7f80fb0000 0x4000     0x4000     com.apple.driver.AppleProfileTimestampAction (85.2) <123 5 4 3 1>
  132    0 0xffffff7f807c6000 0x2000     0x2000     sk-r-d.HelloWorld (1) <4>

However, I have nothing in kernel.log (or system.log) - I should be seeing my printf() statements. Any idea why?

Answer 1

For some reason the console app doesn't seem to display printf messages from kexts, but the log console program does.

I've had success with the following magic words...

log stream --predicate 'eventMessage contains "YourMatchingPatternHere"'

This will produce a filtered stream showing only messages that match your predefined pattern.

Answer 2

sudo nvram boot-args="kext-dev-mode=1 debug=0x22"