Question about output message and object update for ceph class

[Dennis Chen <kernel.org.gnu@xxxxxxxxx>]

Hello,

I write a ceph client using rados lib to execute a funcution upon the object.

CLIENT SIDE CODE
===============
int main()
{
  ...
    strcpy(in, "from client");
    err = rados_exec(io, objname, "devctl", "devctl_op", in,
strlen(in), out, 128);
    if (err < 0) {
        fprintf(stderr, "rados_exec() failed: %s\n", strerror(-err));
        rados_ioctx_destroy(io);
        rados_shutdown(cluster);
        exit(1);
    }
    out[err] = '\0';
    printf("err = %d, exec result out = %s, in = %s\n", err, out, in);
  ...
}

CLASS CODE IN OSD SIDE
======================
static int devctl_op(cls_method_context_t hctx, bufferlist *in, bufferlist *out)
{
  ...

  i = cls_cxx_stat(hctx, &size, NULL);
  if (i < 0)
    return i;

  bufferlist read_bl, write_bl;
  i = cls_cxx_read(hctx, 0, size, &read_bl);
  if (i < 0) {
    CLS_ERR("cls_cxx_read failed");
    return i;
  }


  // we generate our reply
  out->append("Hello, ");
  if (in->length() == 0)
    out->append("world");
  else
    out->append(*in);
  out->append("!");

#if 1
  const char *tstr = "from devctl func";
  write_bl.append(tstr);
  i = cls_cxx_write(hctx, size, write_bl.length(), &write_bl);
  if (i < 0) {
    CLS_ERR("cls_cxx_write failed: %s", strerror(-i));
    return i;
  }
#endif

  // this return value will be returned back to the librados caller
  return 0;
}

I found that if I update the content of the object when calling
cls_cxx_write(), then the 'out' will be null in the client side,
otherwise the out will be "Hello, from client!".

Does anybody here can give some hints?

-- 
Den
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