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ONC+ RPC Developer's Guide

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Updated: November 2020

MT User Mode

In MT User mode, the RPC library does not create any threads. This mode works, in principle, like the single-threaded, or default mode. The only difference is that it passes copies of data structures, such as the transport service handle to the service-dispatch routine to be MT safe.

The RPC server developer takes the responsibility for creating and managing threads through the thread library. In the dispatch routine, the service developer can assign the task of procedure execution to newly created or existing threads. The thr_create() API is used to create threads having various attributes. All thread library interfaces are defined in /usr/include/thread.h and /usr/include/pthread.h.

This mode provides flexibility to the service developer. Threads can now have different stack sizes based on service requirements. Threads can be bound. Different procedures can be executed by threads with different characteristics. The service developer might choose to run some services single threaded. The service developer might choose to do special thread-specific signal processing.

As in the Auto mode, you use the rpc_control() library call to turn on User mode. Note that the rpc_control() operations shown in Figure 15, Table 15, rpc_control Library Routines, except for RPC_SVC_MTMODE_GET(), apply only to MT Auto mode. If used in MT User mode or the single-threaded default mode, the results of the operations can be undefined.

Freeing Library Resources in User Mode

In the MT User mode, service procedures must invoke svc_done() before returning. svc_done() frees resources allocated to service a client request directed to the specified service transport handle. This function is invoked after a client request has been serviced, or after an error or abnormal condition that prevents a reply from being sent. After svc_done() is invoked, the service procedure should not reference the service transport handle. The following example shows a server in MT User mode.

Note - svc_done() must only be called within MT User mode. For more information about this call, see the rpc_svc_calls(3C) man page.
Example 74  MT User Mode: rpc_test.h
#define	SVC2_PROG 0x30000002
	#define	SVC2_VERS 1
	#define SVC2_PROC_ADD 1)
	#define SVC2_PROC_MULT 2

	struct intpair {
		u_short	a;
		u_short	b;

	typedef struct intpair intpair;

	struct svc2_add_args {
		int argument;
		SVCXPRT *transp;

	struct svc2_mult_args {
		intpair mult_argument;
		SVCXPRT *transp;

	extern bool_t xdr_intpair();

	#define NTHREADS_CONST 500

The following code example is the client for MT User mode.

Example 75  Client for MT User Mode
#define	 _REENTRANT
#include <stdio.h>

#include <rpc/rpc.h>
#include <sys/uio.h>

#include <netconfig.h>
#include <netdb.h>

#include <rpc/nettype.h>
#include <thread.h>
#include "rpc_test.h"
void *doclient();
struct thread_info {
	thread_t client_id;
	int client_status;
struct thread_info save_thread[NTHREADS_CONST];
main(argc, argv)
	int argc;
	char *argv[];
	int index, ret;
	int thread_status;
	thread_t departedid, client_id;
	char *hosts;
	if (argc < 3) {
		printf("Usage: do_operation [n] host\n");
		printf("\twhere n is the number of threads\n");
	} else
		if (argc == 3) {
			hosts = argv[1];  /* live_host */
		} else {
			NTHREADS = atoi(argv[1]);
			hosts = argv[2];
	for (index = 0; index < NTHREADS; index++){
		if (ret = thr_create(NULL, NULL, doclient,
		(void *)  hosts, THR_BOUND, &client_id)){
			printf("thr_create failed: return value %d", ret);
			printf(" for %dth thread\n", index);
		save_thread[index].client_id = client_id;
	for (index = 0; index < NTHREADS; index++){

		if (thr_join(save_thread[index].client_id, &departedid,	
		(void *)
			printf("thr_join failed for thread %d\n",
		save_thread[index].client_status = thread_status;
	void *doclient(host)
	char *host;
	struct timeval tout;
	enum clnt_stat test;
	int result = 0;
	u_short mult_result = 0;
	int add_arg;
	int EXP_RSLT;
	intpair pair;
	CLIENT *clnt;
	if ((clnt = clnt_create(host, SVC2_PROG, SVC2_VERS, "udp"
==NULL) {
		clnt_pcreateerror("clnt_create error: ");
		thr_exit((void *) -1);
	tout.tv_sec = 25;
	tout.tv_usec = 0;
	memset((char *) &result, 0, sizeof (result));

	memset((char *) &mult_result, 0, sizeof (mult_result));
	if (thr_self() % 2){
		EXP_RSLT = thr_self() + 1;
		add_arg = thr_self();
		test = clnt_call(clnt, SVC2_PROC_ADD, (xdrproc_t) xdr_int,

		(caddr_t) &add_arg, (xdrproc_t) xdr_int, (caddr_t) &result,
	} else {
		pair.a = (u_short) thr_self();
		pair.b = (u_short) 1;
		EXP_RSLT = pair.a * pair.b;
		test = clnt_call(clnt, SVC2_PROC_MULT, (xdrproc_t)
		(caddr_t) &pair, (xdrproc_t) xdr_u_short,

		(caddr_t) &mult_result, tout);
		result = mult_result;
	if (test != RPC_SUCCESS) {
		printf("THREAD: %d clnt_call hav
		thr_exit((void *) -1);
	thr_exit((void *) 0);

MT performance is enhanced if the function svc_getargs() is called by every procedure other than NULLPROC, even if there are no arguments. xdr_void may be used in this case. This result is true for both the MT Auto and MT User modes. For more information about this call, see the rpc_svc_calls(3C) man page.

Note - You must link in the thread library when writing RPC multithreaded-safe applications. The thread library must be the last named library on the link line. Specify the –lthread option in the compile command.