What do these constants mean?

[SIFE]

While I am reading procstat(1) source code I find these constants (KERN_PROC, CTL_KERN, KERN_PROC_PID etc...) used many times (especially as a part from sysctl argument), so what do these constants mean, and for what are they used?

 

[SirDice]

I think they are used to identify the structure or type of data that's being returned from a sysctl call. They're all defined in /usr/include/sys/sysctl.h.