Path: utzoo!attcan!uunet!cs.utexas.edu!sun-barr!newstop!sun!ingersoll!sxn
From: sxn%ingersoll@Sun.COM (Stephen X. Nahm)
Newsgroups: comp.protocols.nfs
Subject: Re: Is there a convention for the use of xdr_destroy() and xdr_free()?
Keywords: xdr_destroy xdr_free
Message-ID: <128366@sun.Eng.Sun.COM>
Date: 25 Nov 89 03:48:49 GMT
References: <21476@ppgbms.UUCP>
Sender: news@sun.Eng.Sun.COM
Distribution: na
Lines: 74

paul@ppgbms (Paul Evan Matz) writes:

>I assume this is the right group to ask this question, since NFS is built
>using rpc and xdr (and if not the right group, please direct me to the
>proper one).

It's as good a place as any...

>I gather that xdr_free() frees any memory malloced during an XDR_DECODE
>operation (for strings, etc.),

Right: xdr_free() frees any temporarily allocated memory space used
by the XDR encoding/decoding routine when processing an object (usually
strings and pointers).  After calling xdr_free(), you shouldn't reference the
object anymore.

Here's the man page entry for xdr_free() (from SunOS 4.0 or RPCSRC 4.0):

     void
     xdr_free(proc, objp)
     xdrproc_t proc;
     char *objp;

          Generic freeing routine. The first argument is the  XDR
          routine for the object being freed. The second argument
          is a pointer to the object itself.  Note:  the  pointer
          passed to this routine is not freed, but what it points
          to is freed (recursively).

>                        and that xdr_destroy() is the inverse of
>the xdrxxx_create() calls.  What exactly is destroyed?  Clearly not the
>"XDR xdrs;" auto variable.  Perhaps just the data area pointed to by
>x_private?

It depends on the XDR stream.  Here's the destroy routine from the
xdrmem stream type (memory stream are used when sending data to the net):

    static void
    xdrmem_destroy(/*xdrs*/)
          /*XDR *xdrs;*/
    {
    }

That is, it does nothing.  (It's static because it is called via a dispatch
table, not directly.)  Here's the xdrstdio version of destroy (STDIO streams
are used to send data to files):

  /*
   * Destroy a stdio xdr stream.
   * Cleans up the xdr stream handle xdrs previously set up by xdrstdio_create.
   */
   static void
   xdrstdio_destroy(xdrs)
         register XDR *xdrs;
   {
         (void)fflush((FILE *)xdrs->x_private);
         /* xx should we close the file ?? */
   };

That is, it flushes the STDIO stream.  The implementor could have indeed
closed the STDIO stream, since the intention of xdr_destroy() is that
the XDR stream will no longer be used.  Here's the man page entry:

     void
     xdr_destroy(xdrs)
     XDR *xdrs;

          A macro that invokes  the  destroy  routine  associated
          with   the   XDR  stream,  xdrs.   Destruction  usually
          involves freeing  private  data  structures  associated
          with   the   stream.    Using   xdrs   after   invoking
          xdr_destroy() is undefined.

Steve Nahm                              sxn@sun.COM or sun!sxn