Using the lower layers

In the examples given so far, RPC takes care of many details automatically. This section shows how to change the defaults by using lower layers of the RPC library. It is assumed that the reader is familiar with sockets and the system calls for dealing with them.

In general, you should avoid using the lower layers of RPC. If you want to perform any of the following tasks, however, you must use the lower layers:

• Send more than 8K bytes of data.

• Allocate and free memory while serializing or deserializing with XDR routines. See the section ``Using XDR to allocate memory''.

• Perform authentication on either client or server by supplying credentials or verifying them. See the section ``Using authentication''.

Sample server program

A number of assumptions are built into registerrpc:

• The UDP datagram protocol is being used.

• The user does not want to do anything unusual while deserializing, because the deserialization process happens automatically before the user's server routine is called.

   #include <stdio.h>
   #include <rpc/rpc.h>
   #include <rpcsvc/rusers.h>
   

   int nuser();
   

   main()
   {
           SVCXPRT transp;
           transp = svcudp_create(RPC_ANYSOCK);
           if (transp == NULL){
                   fprintf(stderr, "could not create an RPC server\n");
                   exit(1);
           }
           pmap_unset(RUSERSPROG, RUSERSVERS);
           if (!svc_register(transp, RUSERSPROG, RUSERSVERS, nuser,
               IPPROTO_UDP)) {
                   fprintf(stderr, "could not register RUSER service\n");
                   exit(1);
           }
           svc_run();  / never returns /
           fprintf(stderr, "should never reach this point\n");
   }
   nuser(rqstp, transp)
           struct svc_req rqstp;
           SVCXPRT transp;
   {
           unsigned long nusers;
           switch (rqstp->rq_proc) {
           case NULLPROC:
                   if (!svc_sendreply(transp, xdr_void, 0)) {
                           fprintf(stderr, "could not reply to RPC call\n");
                           exit(1);
                   }
                   return;
           case RUSERSPROC_NUM:
                   /
                     code here to compute the number of users
                     and put in variable nusers
                    /
                   if (!svc_sendreply(transp, xdr_u_long, &nusers) {
                           fprintf(stderr, "could not reply to RPC call\n");
                           exit(1);
                   }
                   return;
           default:
                   svcerr_noproc(transp);
                   return;
           }
   }

When the user specifies RPC_ANYSOCK for a socket or gives an unbound socket, the system determines port numbers in the following way: when a server starts up, it advertises to a port mapper demon on its local machine, which picks a port number for the RPC procedure if the socket specified to svcudp_create is not already bound. When the clntudp_create call is made with an unbound socket, the system queries the port mapper on the machine to which the call is being made and gets the appropriate port number. If the port mapper is not running or has no port corresponding to the RPC call, the RPC call fails. Users can make RPC calls to the port mapper themselves. The appropriate procedure numbers are in the include file <rpc/pmap_prot.h>.

After creating an SVCXPRT, the next step is to call pmap_unset so that if the nusers server crashed earlier, any previous trace of it is erased before restarting. More precisely, pmap_unset erases the entry for RUSERS from the port mapper's tables.

Finally, the program number for nusers is associated with the procedure nuser. The final argument to svc_register is normally the protocol being used, which in this case is IPPROTO_UDP. Unlike registerrpc, there are no XDR routines involved in the registration process. Also, registration is done on the program level, rather than the procedure level.

The user routine nuser must call and dispatch the appropriate XDR routines, based on the procedure number. Two things are handled by nuser that are handled automatically by registerrpc:

• Procedure NULLPROC (currently zero) returns with no arguments. This can be used as a simple test for detecting whether a remote program is running.

• There is a check for invalid procedure numbers. If one is detected, svcerr_noproc is called to handle the error.

   case RUSERSPROC_BOOL: {
           int bool;
           unsigned nuserquery;
   

           if (!svc_getargs(transp, xdr_u_int, &nuserquery)) {
                   svcerr_decode(transp);
                   return;
           }
           /
             code to set nusers = number of users
            /
           if (nuserquery == nusers)
                   bool = TRUE;
           else
                   bool = FALSE;
           if (!svc_sendreply(transp, xdr_bool, &bool)){
                    fprintf(stderr, "could not reply to RPC call\n");
                    exit(1);
           }
           return;
   }

Using XDR to allocate memory

XDR routines do memory allocation in addition to doing input and output. This is why the second parameter of xdr_array is a pointer to an array, rather than the array itself. If it is NULL, then xdr_array allocates space for the array and returns a pointer to it, putting the size of the array in the third argument. As an example, consider the following XDR routine xdr_chararr1, which deals with a fixed array of bytes with length SIZE:

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

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

   char chararr[SIZE];
   

   svc_getargs(transp, xdr_chararr1, chararr);

If you want XDR to do the allocation, you would have to rewrite this routine in the following way:

   xdr_chararr2(xdrsp, chararrp)
           XDR xdrsp;
           char chararrp;
   {
           int len;
   

           len = SIZE;
           return (xdr_bytes(xdrsp, chararrp, &len, SIZE));
   }

   char arrptr;
   

   arrptr = NULL;
   svc_getargs(transp, xdr_chararr2, &arrptr);
   /
     use the result here
    /
   svc_freeargs(xdrsp, xdr_chararr2, &arrptr);

   svc_getargs(transp, xdr_finalexample, &finalp);

   svc_freeargs(xdrsp, xdr_finalexample, &finalp);

To summarize, each XDR routine is responsible for serializing, deserializing, and allocating memory. When an XDR routine is called from callrpc, the serializing part is used. When called from svc_getargs, the deserializer is used. When called from svc_freeargs, the memory deallocator is used. When building simple examples like those in this section, a user does not have to worry about the three modes.

Sample client program

When you use callrpc, you have no control over the RPC delivery mechanism or the socket used to transport the data. To illustrate the layer of RPC that allows adjustment of these parameters, consider the following code to call the nusers service:

   #include <stdio.h>
   #include <rpc/rpc.h>
   #include <rpcsvc/rusers.h>
   #include <sys/socket.h>
   #include <sys/fs/nfs/time.h>
   #include <netdb.h>
   

   main(argc, argv)
           int argc;
           char argv;
   {
           struct hostent hp;
           struct timeval pertry_timeout, total_timeout;
           struct sockaddr_in server_addr;
           int addrlen, sock = RPC_ANYSOCK;
           register CLIENT client;
           enum clnt_stat clnt_stat;
           unsigned long nusers;
   

           if (argc < 2) {
                   fprintf(stderr, "usage: nusers hostname\n");
                   exit(-1);
           }
           if ((hp = gethostbyname(argv[1])) == NULL) {
                   fprintf(stderr, "cannot get addr for '%s'\n", argv[1]);
                   exit(-1);
           }

           pertry_timeout.tv_sec = 3;
           pertry_timeout.tv_usec = 0;
           addrlen = sizeof(struct sockaddr_in);
           bcopy(hp->h_addr, (caddr_t)&server_addr.sin_addr, hp->h_length);
           server_addr.sin_family = AF_INET;
           server_addr.sin_port =  0;
           if ((client = clntudp_create(&server_addr, RUSERSPROG,
               RUSERSVERS, pertry_timeout, &sock)) == NULL) {
                   perror("clntudp_create");
                   exit(-1);
           }
           total_timeout.tv_sec = 20;
           total_timeout.tv_usec = 0;
           clnt_stat = clnt_call(client, RUSERSPROC_NUM, xdr_void, 0,
               xdr_u_long, &nusers, total_timeout);
           if (clnt_stat != RPC_SUCCESS) {
                   clnt_perror(client, "rpc");
                   exit(-1);
           }
           clnt_destroy(client);
   }

• a CLIENT pointer

• the procedure number

• the XDR routine for serializing the argument

• a pointer to the argument

• the XDR routine for deserializing the return value

• a pointer to where the return value will be placed

• the time in seconds to wait for a reply

The parameters to clntudp_create are as follows:

• the server address

• the program number

• the version number

• a timeout value (between tries)

• a pointer to a socket

Note that the clnt_destroy call deallocates any space associated with the CLIENT handle, but it does not close the socket associated with it, which was passed as an argument to clntudp_create. The reason is that if there are multiple client handles using the same socket, then it is possible to close one handle without destroying the socket that other handles are using.

To make a stream connection, the call to clntudp_create is replaced with a call to clnttcp_create.

   clnttcp_create(&server_addr, prognum, versnum, &socket,
   	inputsize, outputsize);