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ONC+ RPC Developer's Guide

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Updated: November 2020

Memory Allocation With XDR

XDR routines normally serialize and deserialize data. XDR routines often automatically allocate memory and free automatically allocated memory. The convention is to use a NULL pointer to an array or structure to indicate that an XDR function must allocate memory when deserializing. The next example, xdr_chararr1(), processes a fixed array of bytes with length SIZE and cannot allocate memory if needed:

xdr_chararr1(xdrsp, chararr)
   XDR *xdrsp;
   char chararr[];
   char *p;
   int len;

   p = chararr;
   len = SIZE;
   return (xdr_bytes(xdrsp, &p, &len, SIZE));

If space has already been allocated in chararr, it can be called from a server as follows.

char chararr[SIZE];
svc_getargs(transp, xdr_chararr1, chararr);

    Any structure through which data is passed to XDR or RPC routines must be allocated so that its base address is at an architecture-dependent boundary. An XDR routine that does the allocation must be written so that it can:

  • Allocate memory when a caller requests

  • Return the pointer to any memory it allocates

In the following example, the second argument is a NULL pointer, meaning that memory should be allocated to hold the data being deserialized.

xdr_chararr2(xdrsp, chararrp)
   XDR *xdrsp;
   char **chararrp;

   int len;

   len = SIZE;
   return (xdr_bytes(xdrsp, charrarrp, &len, SIZE));

The corresponding RPC call is:

char *arrptr;
arrptr = NULL;
svc_getargs(transp, xdr_chararr2, &arrptr);
 * Use the result here
svc_freeargs(transp, xdr_chararr2, &arrptr);

After use, free the character array through svc_freeargs(). svc_freeargs() does nothing if passed a NULL pointer as its second argument.

    To summarize:

  • An XDR routine normally serializes, deserializes, and frees memory.

  • svc_getargs() calls the XDR routine to deserialize.

  • svc_freeargs() calls the XDR routine to free memory.