P9_HDRSZ
0, 0, P9_HDRSZ + 4,
clnt->msize - (P9_HDRSZ + 4),
m->rc.capacity = P9_HDRSZ; /* start by reading header */
m->rc.size = P9_HDRSZ;
char tmp_buf[P9_HDRSZ];
rc.capacity = P9_HDRSZ;
rc.size = P9_HDRSZ;