Is there a way to mark a chunk of allocated memory readonly?

On most hardware architectures you can only change protection attributes on entire memory pages; you can’t mark a fragment of a page read-only.

The relevant APIs are:

• mprotect() on Unix;
• VirtualProtect() on Windows.

You’ll need to ensure that the memory page doesn’t contain anything that you don’t want to make read-only. To do this, you’ll either have to overallocate with malloc(), or use a different allocation API, such as mmap(), posix_memalign() or VirtualAlloc().