phyunsj/xdr-data-serialization

XDR over ØMQ example (for my own reference)

Data Serialization

"The process of translating data structures or object state into a format that can be stored (for example, in a file or memory buffer, or transmitted across a network connection link) and reconstructed later in the same or another computer environment." ...from Wiki

XDR

"eXternal Data Representation (XDR) is a standard data serialization format, for uses such as computer network protocols. It allows data to be transferred between different kinds of computer systems. Converting from the local representation to XDR is called encoding. Converting from XDR to the local representation is called decoding. XDR is implemented as a software library of functions which is portable between different operating systems and is also independent of the transport layer." ...from Wiki

📌 XDR uses a base unit of 4 bytes, serialized in big-endian order; smaller data types still occupy four bytes each after encoding. Variable-length types such as string and opaque are padded to a total divisible by four bytes.

XDR Stream Access

Files	In-Memory	Record (TCP/IP)
xdrstdio_create	xdrmem_create	xdrrec_create

Must specify the below operations :

• XDR_ENCODE : serialise (encode) in-memory structures onto the wire.
• XDR_DECODE : deserialise (decode) from the wire into in-memory structures.
• XDR_FREE : free an in-memory structure.

For example,

xdrstdio_create(&xdr, stdout, XDR_ENCODE) ;

Example : XDR over ØMQ

Combined ONC/RPC rls & ØMQ Request-Reply Sample.

$ make
rm -f dir.c
rpcgen -c -o dir.c dir.x
rm -f dir.h
rpcgen -h -o dir.h dir.x
gcc -g -Wall -Wno-unused -Werror -Wformat   -c -o rls.o rls.c
gcc -g -Wall -Wno-unused -Werror -Wformat   -c -o dir.o dir.c
gcc -g -Wall -Wno-unused -Werror -Wformat -lzmq rls.o dir.o -o rls
gcc -g -Wall -Wno-unused -Werror -Wformat   -c -o rls_svc.o rls_svc.c
gcc -g -Wall -Wno-unused -Werror -Wformat -lzmq rls_svc.o dir.o -o rls_svc

$ ./rls_svc &

$ ./rls .
Connecting to rls server
.
..
rls.o
rls
rls_svc.o
Makefile
rls_svc
dir.c
dir.x
rls.c
rls_svc.c
dir.h
dir.o

📖 Code Snippet : Decode zmq response from zmq_recv().

read_reply (char *response, int response_size)
{
  XDR xdr;
  readdir_res result = { 0 };

  xdrmem_create (&xdr, response, response_size , XDR_DECODE);
  if (!xdr_readdir_res (&xdr, &result)) {
      fprintf (stderr, "read_reply: could not decode\n");
      exit (1);
  }

  print_reply( &result );
	  
  xdr_free ((xdrproc_t) xdr_readdir_res, (char *) &result);
  xdr_destroy (&xdr);
}

Related Posts

• RPC 4506 : XDR External Data Representation Standard
• XDR : Technical Notes from Cisco
• XDR : Technical Notes from Oracle
• GNU libc:XDR parser
• Custom XDR Stream Implementation
• XDR allocate, free and destroy