在protobuf-c中打包重复的字节

Question

所以我试图序列化一个包含字节数组及其大小的结构：

struct Msg {
    unsigned char* payload;
    size_t payload_size;
}; 

以下代码使用名为msg的此结构的实例。

    DiagResp resp  = DIAG_RESP__INIT;

    ProtobufCBinaryData payload;
    payload.data = msg->payload;
    payload.len = msg->payload_size;

    resp.payload = &payload;
    resp.n_payload = msg->payload_size;

    resp.has_payload_size = 1;
    resp.payload_size = msg->payload_size;

    size_t packed_size = diag_resp__get_packed_size(&resp);
    printf("packed_size=%lu, msg->received_bytes=%lu\n", 
           packed_size, msg->received_bytes);
    *ret_payload = malloc(packed_size);
    diag_resp__pack(&resp, *ret_payload);

这来自.proto文件：

message DiagResp {
  ...
  optional int32 payload_size = 10;
  repeated bytes payload = 11;
}

printf来电打印出来：

packed_size=4256922, msg->received_bytes=4

...但如果我不尝试序列化payload字段，一切都按预期工作。那么如何序列化repeated bytes字段呢？ protobuf-c文档中没有介绍......