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<P><B><FONT SIZE=+3>CORBA编程</FONT></B><P>
<P><B><FONT SIZE=+2>IDL的语言映射</FONT></B><P>
要想用某种语言实现CORBA IDL定义的对象，需要在IDL和该语言间进行映射，称为language mapping。这也是CORBA标准的一部分。目前已制定了映射标准的语言包括：C，C++，Java，Smalltalk，COBOL等。C因为不是OO的语言，所以在CORBA应用中较少使用，但因为GNOME基本是用C实现的，所以这里使用C的映射。
<P>GNOME中CORBA ORB的实现称为ORBit。最开始GNOME的实现者并未打算实现一个自己的ORB，因为已经有大量免费的实现。他们选择了MICO,但很快发现MICO不能满足GNOME开发的需要，于是Elliot Lee和Dick Porter 决定自己从头写一个新的符合要求的ORB，这就是ORBit。ORBit是按照CORBA2.2标准写的，特别是它的hooks使得与GNOME的应用可以很好的交互。
<P>ORBit是用C写的，非常小而且非常快，使得它可以应用于很多以前认为不适于CORBA技术的场合。它的主要特点：
<P><UL><LI>基于C的实现；</LI><BR>
<LI>执行迅速，节省时间；</LI><BR>
<LI>空间使用效率很高；</LI><BR>
<LI>解决实际问题，而不仅仅是一个艺术品；</LI><BR>
<LI>它是自由软件。</LI><BR></UL>
<P>目前ORBit支持的语言包括：perl, C++, TCL, Python, Ada, Eiffel。

<P><B><FONT SIZE=+2>举例说明</FONT></B><P>
<P>编写ORBit下程序常见问题：
<P><UL><LI>LD_LIBRARY_PATH未定义；</LI><BR>
<LI>IOR后面跟了一个换行符；</LI><BR>
<LI>忘了激活POAManager；</LI><BR></UL>
<P><B><FONT SIZE=+1>IDL文件</FONT></B><P>
<PRE>
interface Echo {
	void echoString(in string input);
};
</PRE>

<P><B><FONT SIZE=+1>Client程序</FONT></B><P>
共分三步：1)初始化ORB， 2)获得对象； 3)使用对象。
<PRE>
#include "stdio.h"
#include "orb/orbit.h"

/*
 * This header file was generated from the idl
 */

#include "echo.h"

/*
 * This is our Echo Object
 */

Echo echo_client;

int
main (int argc, char *argv[])
{
    CORBA_Environment ev;
    CORBA_ORB orb;

    FILE * ifp;
    char * ior;
    char filebuffer[1024];

    /*
     * Standard initalisation of the orb. Notice that
     * ORB_init 'eats' stuff off the command line
     */

    CORBA_exception_init(&ev);
    orb = CORBA_ORB_init(&argc, argv, "orbit-local-orb", &ev);

    /*
     * Get the IOR (object reference). It should be written out
     * by the echo-server into the file echo.ior. So - if you
     * are running the server in the same place as the client,
     * this should be fine!
     */

    ifp = fopen("echo.ior","r");
    if( ifp == NULL ) {
      g_error("No echo.ior file!");
      exit(-1);
    }

    fgets(filebuffer,1024,ifp);
    ior = g_strdup(filebuffer);

    fclose(ifp);
    /*
     * Actually get the object. So easy!
     */

    echo_client = CORBA_ORB_string_to_object(orb, ior, &ev);
    if (!echo_client) {
	printf("Cannot bind to %s\n", ior);
	return 1;
    }

    /*
     * Ok. Now we use the echo object...
     */

    printf("Type messages to the server\n. as the only thing on the line stops\n");
    while( fgets(filebuffer,1024,stdin) ) {
      if( filebuffer[0] == '.' && filebuffer[1] == '\n' ) 
	break;

      /* chop the newline off */
      filebuffer[strlen(filebuffer)-1] = '\0';
      
      /* using the echoString method in the Echo object               */
      /* this is defined in the echo.h header, compiled from echo.idl */

      Echo_echoString(echo_client,filebuffer,&ev);

      /* catch any exceptions (eg, network is down) */

      if(ev._major != CORBA_NO_EXCEPTION) {
	printf("we got exception %d from echoString!\n", ev._major);
	return 1;
      }
    }
      
    /* Clean up */
    CORBA_Object_release(echo_client, &ev);
    CORBA_Object_release((CORBA_Object)orb, &ev);

    return 0;
}


</PRE>

<P><B><FONT SIZE=+1>Server程序</FONT></B><P>
<PRE>
#include "stdio.h"
#include "stdlib.h"
#include "string.h"
#include "signal.h"
#include "orb/orbit.h"
#include "echo.h"

/* This is so we can get out a valid IOR later... */
Echo echo_client = CORBA_OBJECT_NIL;

/* declaration of the meat of the process*/
static void
do_echoString(PortableServer_Servant servant,
	      CORBA_char *astring,
	      CORBA_Environment *ev);

/*
 * I have **no** idea what this bit does
 */
PortableServer_ServantBase__epv base_epv = {
  NULL,
  NULL,
  NULL
};
POA_Echo__epv echo_epv = { NULL, do_echoString };
POA_Echo__vepv poa_echo_vepv = { &base_epv, &echo_epv };
POA_Echo poa_echo_servant = { NULL, &poa_echo_vepv };

int
main (int argc, char *argv[])
{
    PortableServer_ObjectId objid = {0, sizeof("myEchoString"), "myEchoString"};
    PortableServer_POA poa;

    CORBA_Environment ev;
    char *retval;
    CORBA_ORB orb;
    FILE * ofp;

    signal(SIGINT, exit);
    signal(SIGTERM, exit);

    CORBA_exception_init(&ev);
    orb = CORBA_ORB_init(&argc, argv, "orbit-local-orb", &ev);

    POA_Echo__init(&poa_echo_servant, &ev);

    poa = (PortableServer_POA)CORBA_ORB_resolve_initial_references(orb, "RootPOA", &ev);
    PortableServer_POAManager_activate(PortableServer_POA__get_the_POAManager(poa, &ev), &ev);
    PortableServer_POA_activate_object_with_id(poa,
					       &objid, &poa_echo_servant, &ev);

    echo_client = PortableServer_POA_servant_to_reference(poa,
							  &poa_echo_servant,
							  &ev);
    if (!echo_client) {
	printf("Cannot get objref\n");
	return 1;
    }

    retval = CORBA_ORB_object_to_string(orb, echo_client, &ev);

    ofp = fopen("echo.ior","w");

    fprintf(ofp,"%s", retval);
    fclose(ofp);

    CORBA_free(retval);

    fprintf(stdout,"Written the file echo.ior with the IOR of this server.\n Now waiting for requests...\n");
    fflush(stdout);
    CORBA_ORB_run(orb, &ev);

    return 0;
}

static void
do_echoString(PortableServer_Servant servant,
	      CORBA_char *astring,
	      CORBA_Environment *ev)
{
  g_message("[server] %s", astring);
  return;
}
</PRE>

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