Allocate executable ram in c on linux

Question

I want to make a simple just-in-time compiler with c on Linux.

How can I allocate memory such that I can write out raw x86 code to it and execute it as any other function?

Answer 1

See mprotect(). Once you have filled a (n-)page-sized memory region (allocated with mmap()) with code, change its permissions to disallow writes and allow execution.

Answer 2

In addition to using mprotect correctly to provide first write and then execute permission, on some OS/hardware operations you may need to flush the I-cache. At this moment (mid-2010), all recent x86 processors have separate level 1 caches for instructions and data, and somebody has to make sure that if you write new instructions into memory (which will update the D-cache), you don't then try to execute stale bits from the I-cache. Exactly how to flush the I-cache from userspace will depend on both your hardware and the OS. My advice would be to read Intel's documentation on "self-modifying code" for their IA-32 multiprocessors. This should be enough to get you through.