Bad attempt at using @fieldParentPtr

https://kagi.com/assistant/a8f0a9ac-a11a-472e-af38-98d7e877e293

src/message.zig

@@ -36,9 +36,30 @@ pub const ZeroCopyMessage = packed struct {
         dest: @Vector(4, u8),
         payload: void,
 
-        pub fn getPayload(self: *align(1) Relay) []u8 {
-            const len: *u16 = @ptrFromInt(@intFromPtr(self) - @sizeOf(u16));
-            return @as([*]u8, @ptrCast(&self.payload))[0 .. len.* - @sizeOf(Relay)];
+        pub fn getPayload(self: *align(@alignOf(ZeroCopyMessage)) Relay) []u8 {
+            // Cast the 'self' pointer (which points to the Relay header,
+            // located at the same memory as the parent's 'bytes' field)
+            // to a pointer to void, as required by @fieldParentPtr for a void field.
+            // Preserve the known alignment.
+            const self_as_void_ptr: *align(@alignOf(ZeroCopyMessage)) void = @ptrCast(self);
+
+            // Cast the resulting *void pointer to the parent type *ZeroCopyMessage.
+            // This cast performs the necessary alignment check.
+            const parent: *ZeroCopyMessage = @alignCast(@fieldParentPtr("bytes", self_as_void_ptr));
+
+            // The 'length' field in the parent ZeroCopyMessage contains
+            // the size of the header (Relay) + payload length.
+            const total_len = parent.length;
+
+            // Payload length = total_len - size of the Relay header
+            const payload_len = total_len - @sizeOf(Relay);
+
+            // The payload starts immediately after the fixed fields of the Relay struct.
+            // The address of the 'payload' field represents this starting point.
+            const payload_start_ptr: [*]u8 = @ptrCast(&self.payload);
+
+            // Return a slice from the payload start address with the calculated length.
+            return payload_start_ptr[0..payload_len];
         }
     };
     const Connection = packed struct {
@@ -94,17 +115,17 @@ pub const ZeroCopyMessage = packed struct {
         allocator.free(self.asBytes());
     }
 
-    fn getRelay(self: *Self) *align(1) Relay {
+    fn getRelay(self: *Self) *align(@alignOf(Self)) Relay {
         return std.mem.bytesAsValue(Relay, &self.bytes);
     }
-    fn getConnection(self: *Self) *align(1) Connection {
+    fn getConnection(self: *Self) *align(@alignOf(Self)) Connection {
         return std.mem.bytesAsValue(Connection, &self.bytes);
     }
 
     pub fn getSaprusTypePayload(self: *Self) Error!(union(PacketType) {
-        relay: *align(1) Relay,
+        relay: *align(@alignOf(Self)) Relay,
         file_transfer: void,
-        connection: *align(1) Connection,
+        connection: *align(@alignOf(Self)) Connection,
     }) {
         return switch (self.type) {
             .relay => .{ .relay = self.getRelay() },
@@ -172,6 +193,7 @@ test "testing variable length zero copy struct" {
     // Create a view of the byte slice as a ZeroCopyMessage
     const zcm: *ZeroCopyMessage = try .init(gpa, .relay, payload.len);
     defer zcm.deinit(gpa);
+    std.debug.print("outer: {*}\n", .{zcm});
 
     {
         // Set the message values