Why does this code work on iOS 32 bit but not on 64 bit?

Question

The code below receives MIDI data from a musical instrument connected to the iOS device. It works fine on a 32 bit iOS device. On 64 bit, the callback function is also called for every event, but the data received in 'pktlist' is invalid. What is wrong?

The data I receive in MidiReadProc is always the same on a 64 bit device, and it is clearly wrong as the length would normally never be 0:

pktlist^.numPackets = 1
lPacket.MIDItimestamp=$E4FE000100000961
lPacket.length=$0000
lPacket.data[0]=$00

The Callback function:

procedure MidiReadProc(pktlist: MIDIPacketListRef; refCon, connRefCon: Pointer); cdecl;
var
  lPacket: MIDIPacket;
  lPacketRef: MIDIPacketRef;
  j: Integer;
  lPtr: ^Byte;
begin
  lPacketRef := MIDIPacketRef(@(pktlist^.Packet[0]));
  for j := 0 to pktlist^.numPackets-1 do
  begin
    lPacket := lPacketRef^;
    if (lPacket.length > 0) and (lPacket.data[0] <> $F0) then
      //handle data here

    //translation of the MIDIPacketNext Macro:
    lPtr := @lPacketRef^.data[lPacketRef^.length];
    lPacketRef := MIDIPacketRef((UInt64(lPtr) + 3) and (not 3));
  end;
end;

In case it could be related to the header translation, here is the translation:

Extract from CoreMIDI.h:

typedef UInt64 MIDITimeStamp;

#pragma pack(push, 4)
struct MIDIPacket
{
    MIDITimeStamp       timeStamp;
    UInt16              length;
    Byte                data[256];
};
typedef struct MIDIPacket           MIDIPacket;

struct MIDIPacketList
{
    UInt32              numPackets; 
    MIDIPacket          packet[1];
};
typedef struct MIDIPacketList MIDIPacketList;
#pragma pack(pop)

typedef void
(*MIDIReadProc)(const MIDIPacketList *pktlist, void *readProcRefCon, void *srcConnRefCon);


#if TARGET_CPU_ARM || TARGET_CPU_ARM64
// MIDIPacket must be 4-byte aligned
#define MIDIPacketNext(pkt) ((MIDIPacket *)(((uintptr_t)(&(pkt)->data[(pkt)->length]) + 3) & ~3))

Extract from CoreMIDI.pas (the translation of CoreMIDI.h is made by Pavel Jiri Strnad and is available here):

MIDITimeStamp = UInt64;

MIDIPacket = record
   timeStamp: MIDITimeStamp;
   length: UInt16;
   data: array [0..255] of Byte;
end;
MIDIPacketRef = ^MIDIPacket;

MIDIPacketList = record
   numPackets: UInt32;
   packet: array [0..0] of MIDIPacket;
end;
MIDIPacketListRef = ^MIDIPacketList;

MIDIReadProc = procedure (pktlist: MIDIPacketListRef; readProcRefCon: pointer; srcConnRefCon: pointer); cdecl;

Update:

As suggested by David in the comments, here are field offsets of the records:

MIDIPacketList offsets:

64 bit Align 8: numPackets=0 packet=8 <- this was the one causing problems
64 bit Align 1: numPackets=0 packet=4
32 bit Align 8: numPackets=0 packet=4
32 bit Align 1: numPackets=0 packet=4

MIDIPacket offsets:

64 bit Align 8: timeStamp=0 length=8 data=10
64 bit Align 1: timeStamp=0 length=8 data=10
32 bit Align 8: timeStamp=0 length=8 data=10
32 bit Align 1: timeStamp=0 length=8 data=10

Answer 1

As suggested in the comments, I looked further into the CoreMIDI.h header file. Though I have never worked with any C languages, I did spot this line: #pragma pack(push, 4) above the definition of MIDIPacket (I have now added it to extract in the question), which shows clearly what should be done.

Setting {$Align 1} for the whole file was not the correct solution. Instead only the two records (MIDIPacket and MIDIPacketList) should have 4 byte alignments instead of 8 byte alignment.