Shared Memory

Testing shared memory on linux and macos with Rust and C and Assembly.

Atomics

x86_64

• memory accesses that are aligned to their access size are guaranteed to be atomic
• atomics implemented using lock cmpxchg and lock xchg for unaligned accesses

arm32 (arm)

• ldrexd/strexd faults if the base register is not 8 byte aligned
• ldrex/strex (? alignment restrictions)
• atomics implemented using load/store exclusives

arm64 (aarch64)

• ldxr/stxr (? alignment restrictions)
• atomics implemented using load/store exclusives

x86_64-linux

• cargo run to setup shared memory
• compile and run writer.c to write to shared memory
• for some strange reason nasm does not export the global symbols, so the atomics are written for the as assembler instead of nasm

arm-linux

• cargo run to setup shared memory
• compile and run writer.c to write to shared memory

aarch64-linux

• cargo run to setup shared memory
• compile and run writer.c to write to shared memory

aarch64-macos

• cargo run to setup shared memory
• compile and run writer.c to write to shared memory
  • strangely running writer.c and attempting to access shared memory fails with Permission Denied, but works fine when run using sudo
• symbols exported from assembly must have _ prefixed to them, a quirk of macos
• when building a static library use the ar provided by the system, otherwise linking will fail with the unhelpful error building for macOS-arm64 but attempting to link with file built for macOS-arm64

Creating a static library

1. compile the assembler files using as
2. generate a static library using ar
3. library file names must always start with lib

example assembler file tmp.s

as tmp.s -o tmp.o
ar -rcs libtmp.a tmp.o