Finding how much data is available for read from a SOCK_SEQPACKET

Hello,

I have a working program that receives blocks of data from a
SOCK_SEQPACKET using a blocking read(). So far I use a
fixed-size buffer of the maximum size.

I would like to query for the size available, malloc a block
of the appropriate size, then read. Is that feasible?

Note: I think that I can not put the size at the head of the
packet since 
" SOCK_SEQPACKET
   Provides a sequenced, reliable, two-way connection-based
   data transmission path for datagrams of fixed maximum length;
   a consumer is required to read an entire packet with each
   read system call."

TIA,
  Francois Grieu