Convert from Zig struct to C struct

include/zaprus.h

@@ -25,6 +25,7 @@ struct SaprusMessage {
       struct {
 	char dest[4];
       };
+      size_t payload_len;
       char *payload;
     } relay;
     struct {
@@ -36,6 +37,7 @@ struct SaprusMessage {
 	char _reserved;
 	char options;
       };
+      size_t payload_len;
       char *payload;
     } connection;
   };

src/c_api.zig

@@ -4,11 +4,38 @@ const c = @cImport({
 
 fn zigToCMessage(msg: ?*zaprus.Message) ?*c.SaprusMessage {
     if (msg) |m| {
-        return switch (m.*) {
-            .relay => |_| @constCast(&c.SaprusMessage{}),
-            .connection => |_| @constCast(&c.SaprusMessage{}),
-            else => |_| null,
+        var res = c.SaprusMessage{
+            .packet_type = @intFromEnum(m.*),
         };
+        switch (m.*) {
+            .relay => |r| {
+                res.unnamed_0 = .{ .relay = .{
+                    .unnamed_0 = .{
+                        .dest = r.header.dest,
+                    },
+                    .payload_len = r.payload.len,
+                    .payload = (allocator.alloc(u8, r.payload.len) catch return null).ptr,
+                } };
+            },
+            .connection => |con| {
+                res.unnamed_0 = .{
+                    .connection = .{
+                        .unnamed_0 = .{
+                            .src_port = con.header.src_port,
+                            .dest_port = con.header.dest_port,
+                            .seq_num = con.header.seq_num,
+                            .msg_id = con.header.msg_id,
+                            ._reserved = con.header.reserved,
+                            .options = @bitCast(con.header.options),
+                        },
+                        .payload_len = con.payload.len,
+                        .payload = (allocator.alloc(u8, con.payload.len) catch return null).ptr,
+                    },
+                };
+            },
+            else => return null,
+        }
+        return &res;
     } else return null;
 }