Commpage

Commpage is a special memory structure that is always located at the same address in all processes. On macOS, this mapping is provided by the kernel. In case of Darling, this functionality is supplemented by the kernel module.

Purpose

• CPU information (number of cores, CPU capabilities etc.)
• Current precise date and time (this information is not filled in by Darling, causing a fall back to a system call).
• PFZ - preemption-free zone. Contains a piece of code which when run prevents the process from being preempted. This is used for a lock-free implementation of certain primitives (e.g. OSAtomicFifoEnqueue). (Not available under Darling.)

It is somewhat related to vDSO on Linux, except that vDSO behaves like a real library, while commpage is just a chunk of data.

The commpage is not documented anywhere, meaning it's not an API intended to be used by 3rd party software. It is however used in source code provided on opensource.apple.com. Darling provides a commpage for compatibility reasons.

Location

The address differs between 32-bit and 64-bit systems.

• 32-bit systems: 0xffff0000
• 64-bit systems: 0x7fffffe00000

Note that the 32-bit address is outside of permissible user space area on 32-bit Linux kernels. This is why Darling runs only under 64-bit kernels, which don't have this limitation.