Fix MIDIPacketNext to not trigger Rust's UB check (#18)

This should fix #17.

It also simplifies MIDIEventPacketNext, which doesn't need the platform-specific distinction, as the size is 4-byte aligned anyways (see also https://github.com/phracker/MacOSX-SDKs/blob/041600eda65c6a668f66cb7d56b7d1da3e8bcc93/MacOSX11.3.sdk/System/Library/Frameworks/CoreMIDI.framework/Versions/A/Headers/MIDIServices.h#L2387).

src/lib.rs

@@ -17,28 +17,32 @@ include!("generated.rs");
 pub unsafe fn MIDIPacketNext(pkt: *const MIDIPacket) -> *const MIDIPacket {
  // Get pointer to potentially unaligned data without triggering undefined behavior
  // addr_of does not require creating an intermediate reference to unaligned data.
+ // See also the definition of `MIDIPacketNext` in the official SDK MIDIServices.h
  let ptr = ptr::addr_of!((*pkt).data) as *const u8;
- let offset = (*pkt).length as isize;
+ let ptr_length = ptr::addr_of!((*pkt).length) as *const u16;
  if cfg!(any(target_arch = "arm", target_arch = "aarch64")) {
- // MIDIPacket must be 4-byte aligned on ARM
+ // MIDIPacket must be 4-byte aligned on ARM, so we need to calculate an aligned offset.
+ // We do not need `read_unaligned` for the length, because the length will never
+ // be unaligned, and `read_unaligned` would lead to less efficient machine code.
+ let offset = ptr_length.read() as isize;
  ((ptr.offset(offset + 3) as usize) & !(3usize)) as *const MIDIPacket
  } else {
+ // MIDIPacket is unaligned on non-ARM, so reading the length requires `read_unaligned`
+ // to not trigger Rust's UB check (although unaligned reads are harmless on Intel
+ // and `read_unaligned` will generate the same machine code as `read`).
+ let offset = ptr_length.read_unaligned() as isize;
  ptr.offset(offset) as *const MIDIPacket
  }
 }
 
 #[inline]
 pub unsafe fn MIDIEventPacketNext(pkt: *const MIDIEventPacket) -> *const MIDIEventPacket {
- // Get pointer to potentially unaligned data without triggering undefined behavior
- // addr_of does not require creating an intermediate reference to unaligned data.
+ // Each EventPacket's size is a multiple of 4 bytes, so no special care
+ // needs to be taken when reading the data (except the timeStamp, which is not 8-byte aligned).
+ // See also the definition of `MIDIEventPacketNext` in the official SDK MIDIServices.h
  let ptr = ptr::addr_of!((*pkt).words) as *const u8;
  let offset = (((*pkt).wordCount as usize) * mem::size_of::<u32>()) as isize;
- if cfg!(any(target_arch = "arm", target_arch = "aarch64")) {
- // MIDIEventPacket must be 4-byte aligned on ARM
- ((ptr.offset(offset + 3) as usize) & !(3usize)) as *const MIDIEventPacket
- } else {
- ptr.offset(offset) as *const MIDIEventPacket
- }
+ ptr.offset(offset) as *const MIDIEventPacket
 }
 
 #[allow(dead_code)]
@@ -96,7 +100,8 @@ mod tests {
  );
 
  let second_packet = MIDIPacketNext(first_packet);
- let len = (*second_packet).length as usize;
+ let ptr_length = ptr::addr_of!((*second_packet).length) as *const u16;
+ let len = ptr_length.read_unaligned() as usize;
  assert_eq!(
  &(*second_packet).data[0..len],
  &[0x90, 0x41, 0x7f]