diff --git a/trace-msg.c b/trace-msg.c
index 2e4a9bfb1d64..2e61b297151f 100644
--- a/trace-msg.c
+++ b/trace-msg.c
@@ -52,12 +52,15 @@ typedef __be32 be32;
 #define MSG_META_MAX_LEN		(MSG_MAX_LEN - MIN_META_SIZE)
 
 
-#define MIN_TINIT_SIZE	offsetof(struct tracecmd_msg, tinit.opt)
+#define MIN_TINIT_SIZE	(sizeof(struct tracecmd_msg_header) + \
+			 sizeof(struct tracecmd_msg_tinit))
 
 /* Not really the minimum, but I couldn't think of a better name */
-#define MIN_RINIT_SIZE offsetof(struct tracecmd_msg, rinit.port_array)
+#define MIN_RINIT_SIZE	(sizeof(struct tracecmd_msg_header) + \
+			 sizeof(struct tracecmd_msg_rinit))
 
-#define MIN_META_SIZE 	offsetof(struct tracecmd_msg, meta.buf)
+#define MIN_META_SIZE	(sizeof(struct tracecmd_msg_header) + \
+			 sizeof(struct tracecmd_msg_meta))
 
 /* for both client and server */
 bool use_tcp;
@@ -83,17 +86,14 @@ struct tracecmd_msg_tinit {
 	be32 cpus;
 	be32 page_size;
 	be32 opt_num;
-	struct tracecmd_msg_opt *opt;
 } __attribute__((packed));
 
 struct tracecmd_msg_rinit {
 	be32 cpus;
-	be32 *port_array;
 } __attribute__((packed));
 
 struct tracecmd_msg_meta {
 	be32 size;
-	void *buf;
 } __attribute__((packed));
 
 enum tracecmd_msg_cmd {
@@ -116,6 +116,11 @@ struct tracecmd_msg {
 		struct tracecmd_msg_rinit	rinit;
 		struct tracecmd_msg_meta	meta;
 	};
+	union {
+		struct tracecmd_msg_opt		*opt;
+		be32				*port_array;
+		void				*buf;
+	};
 } __attribute__((packed));
 
 struct tracecmd_msg *errmsg;
@@ -138,13 +143,13 @@ static ssize_t msg_do_write_check(int fd, struct tracecmd_msg *msg)
 
 	switch (ntohl(msg->hdr.cmd)) {
 	case MSG_TINIT:
-		ret = msg_write(fd, msg, MIN_TINIT_SIZE, msg->tinit.opt);
+		ret = msg_write(fd, msg, MIN_TINIT_SIZE, msg->opt);
 		break;
 	case MSG_RINIT:
-		ret = msg_write(fd, msg, MIN_RINIT_SIZE, msg->rinit.port_array);
+		ret = msg_write(fd, msg, MIN_RINIT_SIZE, msg->port_array);
 		break;
 	case MSG_SENDMETA:
-		ret = msg_write(fd, msg, MIN_META_SIZE, msg->meta.buf);
+		ret = msg_write(fd, msg, MIN_META_SIZE, msg->buf);
 		break;
 	default:
 		ret = __do_write_check(fd, msg, ntohl(msg->hdr.size));
@@ -170,7 +175,7 @@ static int make_tinit(struct tracecmd_msg *msg)
 			return -ENOMEM;
 		opt->size = htonl(sizeof(*opt));
 		opt->opt_cmd = htonl(MSGOPT_USETCP);
-		msg->tinit.opt = opt;
+		msg->opt = opt;
 		size += sizeof(*opt);
 	}
 
@@ -192,13 +197,13 @@ static int make_rinit(struct tracecmd_msg *msg)
 
 	msg->rinit.cpus = htonl(cpu_count);
 
-	msg->rinit.port_array = malloc(sizeof(*port_array) * cpu_count);
-	if (!msg->rinit.port_array)
+	msg->port_array = malloc(sizeof(*port_array) * cpu_count);
+	if (!msg->port_array)
 		return -ENOMEM;
 
 	size += sizeof(*port_array) * cpu_count;
 
-	ptr = msg->rinit.port_array;
+	ptr = msg->port_array;
 
 	for (i = 0; i < cpu_count; i++) {
 		/* + rrqports->cpus or rrqports->port_array[i] */
@@ -244,13 +249,13 @@ static void msg_free(struct tracecmd_msg *msg)
 {
 	switch (ntohl(msg->hdr.cmd)) {
 	case MSG_TINIT:
-		free(msg->tinit.opt);
+		free(msg->opt);
 		break;
 	case MSG_RINIT:
-		free(msg->rinit.port_array);
+		free(msg->port_array);
 		break;
 	case MSG_SENDMETA:
-		free(msg->meta.buf);
+		free(msg->buf);
 		break;
 	}
 }
@@ -315,7 +320,7 @@ static int tracecmd_msg_read_extra(int fd, struct tracecmd_msg *msg, int *n)
 
 	switch (ntohl(msg->hdr.cmd)) {
 	case MSG_TINIT:
-		msg->tinit.opt = NULL;
+		msg->opt = NULL;
 
 		rsize = MIN_TINIT_SIZE - *n;
 
@@ -325,19 +330,19 @@ static int tracecmd_msg_read_extra(int fd, struct tracecmd_msg *msg, int *n)
 
 		if (size > *n) {
 			size -= *n;
-			msg->tinit.opt = malloc(size);
-			if (!msg->tinit.opt)
+			msg->opt = malloc(size);
+			if (!msg->opt)
 				return -ENOMEM;
 			*n = 0;
-			return msg_read(fd, msg->tinit.opt, size, n);
+			return msg_read(fd, msg->opt, size, n);
 		}
 		return 0;
 	case MSG_RINIT:
 		return msg_read_extra(fd, msg, n, size, MIN_RINIT_SIZE,
-				      (void **)&msg->rinit.port_array);
+				      (void **)&msg->port_array);
 	case MSG_SENDMETA:
 		return msg_read_extra(fd, msg, n, size, MIN_META_SIZE,
-				      (void **)&msg->meta.buf);
+				      (void **)&msg->buf);
 	}
 
 	return msg_read(fd, msg, size - MSG_HDR_LEN, n);
@@ -435,7 +440,7 @@ int tracecmd_msg_send_init_data(int fd)
 	cpus = ntohl(recv_msg.rinit.cpus);
 	client_ports = malloc_or_die(sizeof(int) * cpus);
 	for (i = 0; i < cpus; i++)
-		client_ports[i] = ntohl(recv_msg.rinit.port_array[i]);
+		client_ports[i] = ntohl(recv_msg.port_array[i]);
 
 	/* Next, send meta data */
 	send_metadata = true;
@@ -508,7 +513,7 @@ int tracecmd_msg_initial_setting(int fd, int *cpus, int *pagesize)
 			ret = -EINVAL;
 			goto error;
 		}
-		opt = (void *)msg.tinit.opt + offset;
+		opt = (void *)msg.opt + offset;
 		offset += ntohl(opt->size);
 		size += ntohl(opt->size);
 		if (ntohl(msg.hdr.size) < size) {
@@ -581,8 +586,8 @@ int tracecmd_msg_metadata_send(int fd, const char *buf, int size)
 	if (ret < 0)
 		return ret;
 
-	msg.meta.buf = malloc(MSG_META_MAX_LEN);
-	if (!msg.meta.buf)
+	msg.buf = malloc(MSG_META_MAX_LEN);
+	if (!msg.buf)
 		return -ENOMEM;
 
 	msg.meta.size = htonl(MSG_META_MAX_LEN);
@@ -591,13 +596,13 @@ int tracecmd_msg_metadata_send(int fd, const char *buf, int size)
 	n = size;
 	do {
 		if (n > MSG_META_MAX_LEN) {
-			memcpy(msg.meta.buf, buf+count, MSG_META_MAX_LEN);
+			memcpy(msg.buf, buf+count, MSG_META_MAX_LEN);
 			n -= MSG_META_MAX_LEN;
 			count += MSG_META_MAX_LEN;
 		} else {
 			msg.hdr.size = htonl(MIN_META_SIZE + n);
 			msg.meta.size = htonl(n);
-			memcpy(msg.meta.buf, buf+count, n);
+			memcpy(msg.buf, buf+count, n);
 			n = 0;
 		}
 		ret = msg_do_write_check(fd, &msg);
@@ -655,7 +660,7 @@ int tracecmd_msg_collect_metadata(int ifd, int ofd)
 		t = n;
 		s = 0;
 		do {
-			s = write(ofd, msg.meta.buf+s, t);
+			s = write(ofd, msg.buf+s, t);
 			if (s < 0) {
 				if (errno == EINTR)
 					continue;
