How to get const char * from external dll

I am integrating CMU's PocketSphinx with Unity by compiling my own C++ project in Visual Studio 2010 as a DLL, which I am calling from a C# script from within Unity Pro. I know that the dll works because I have made another project as an exe with the very same code, compiled it, and it works perfectly as a standalone program. I'm using the pocketsphinx_continuous project example, which gets microphone inputs and outputs text to the console. I have customized this code to be called from inside Unity and it is supposed to output back to my C# code as a string rather than to the console. I feel that I almost have this working, but the const char * is just not making it back as a string. I will end up getting access violation errors if I use this declaration:

private static extern string recognize_from_microphone();

so, I have tried to use this one:

private static extern IntPtr recognize_from_microphone();

and then, I use this line of code to try to print the output of that function:

print("you just said " + Marshal.PtrToStringAnsi(recognize_from_microphone()));

but, then I get only "you just said" in return. I can manage to get a memory address back if I do this: print("you just said " + recognize_from_microphone()); So, I know that something is getting returned.

Here is my C++ code (much of this was written originally in C as the example code from pocketsphinx):

 char* MakeStringCopy (const char* str) 
 {
       if (str == NULL) return NULL;
       char* res = (char*)malloc(strlen(str) + 1);
       strcpy(res, str);
       return res;
 }
 
 
 extern __declspec(dllexport) const char * recognize_from_microphone()
 {
     //this is a near complete duplication of the code from main()
     char const *cfg;
     config = cmd_ln_init(NULL, ps_args(), TRUE,
     "-hmm", MODELDIR "\\hmm\\en_US\\hub4wsj_sc_8k",
     "-lm", MODELDIR "\\lm\\en\\turtle.DMP",
     "-dict", MODELDIR "\\lm\\en\\turtle.dic",
     NULL);
 
     if (config == NULL)
     {
         return "config is null";
     }
 
     ps = ps_init(config);
     if (ps == NULL)
     {
         return "ps is null";
     }
 
     ad_rec_t *ad;
     int16 adbuf[4096];
     int32 k, ts, rem;
     char const *hyp;
     char const *uttid;
     cont_ad_t *cont;
     char word[256];
     char words[1024] = "";
     //char temp[] = "hypothesis";
     //hyp = temp;
 
     if ((ad = ad_open_dev(cmd_ln_str_r(config, "-adcdev"),
                           (int)cmd_ln_float32_r(config, "-samprate"))) == NULL)
         E_FATAL("Failed to open audio device\n");
 
     /* Initialize continuous listening module */
     if ((cont = cont_ad_init(ad, ad_read)) == NULL)
         E_FATAL("Failed to initialize voice activity detection\n");
     if (ad_start_rec(ad) < 0)
         E_FATAL("Failed to start recording\n");
     if (cont_ad_calib(cont) < 0)
         E_FATAL("Failed to calibrate voice activity detection\n");
 
     for (;;) {
         /* Indicate listening for next utterance */
         //printf("READY....\n");
         fflush(stdout);
         fflush(stderr);
         
         /* Wait data for next utterance */
         while ((k = cont_ad_read(cont, adbuf, 4096)) == 0)
             sleep_msec(100);
 
         if (k < 0)
             E_FATAL("Failed to read audio\n");
         
         /*
          * Non-zero amount of data received; start recognition of new utterance.
          * NULL argument to uttproc_begin_utt => automatic generation of utterance-id.
          */
         if (ps_start_utt(ps, NULL) < 0)
             E_FATAL("Failed to start utterance\n");
 
         ps_process_raw(ps, adbuf, k, FALSE, FALSE);
         //printf("Listening...\n");
         fflush(stdout);
         
         /* Note timestamp for this first block of data */
         ts = cont->read_ts;
         
         /* Decode utterance until end (marked by a "long" silence, >1sec) */
         for (;;) {
             
             /* Read non-silence audio data, if any, from continuous listening module */
             if ((k = cont_ad_read(cont, adbuf, 4096)) < 0)
                 E_FATAL("Failed to read audio\n");
             if (k == 0) {
                 /*
                  * No speech data available; check current timestamp with most recent
                  * speech to see if more than 1 sec elapsed.  If so, end of utterance.
                  */
                 if ((cont->read_ts - ts) > DEFAULT_SAMPLES_PER_SEC)
                     break;
             }
             else {
                 /* New speech data received; note current timestamp */
                 ts = cont->read_ts;
             }
             
             /*
              * Decode whatever data was read above.
              */
             rem = ps_process_raw(ps, adbuf, k, FALSE, FALSE);
 
             /* If no work to be done, sleep a bit */
             if ((rem == 0) && (k == 0))
                 sleep_msec(20);
         }
         
         /*
          * Utterance ended; flush any accumulated, unprocessed A/D data and stop
          * listening until current utterance completely decoded
          */
         ad_stop_rec(ad);
         while (ad_read(ad, adbuf, 4096) >= 0);
         cont_ad_reset(cont);
         fflush(stdout);
         /* Finish decoding, obtain and print result */
         ps_end_utt(ps);
         
         hyp = ps_get_hyp(ps, NULL, &uttid);
         fflush(stdout);
 
         /* Exit if the first word spoken was GOODBYE */
         //actually, for unity, exit if any word was spoken at all! this will avoid an infinite loop of doom!
         if (hyp) {
             /*sscanf(hyp, "%s", words);
             if (strcmp(word, "goodbye") == 0)*/
                 break;
         }
         else
             return "nothing returned";
         /* Resume A/D recording for next utterance */
         if (ad_start_rec(ad) < 0)
             E_FATAL("Failed to start recording\n");
     }
     cont_ad_close(cont);
     ad_close(ad);
     ps_free(ps);
     const char *temp = new char[1024];
     temp = MakeStringCopy(hyp);
     return temp;
 }

If change return temp; to return "some string here"; Then I see the text appear inside Unity. That's not helpful, though, because I don't need hardcoded text, I need the output of the speech recognition code, which ends up getting stored inside the hyp variable.

I've spent all day reading web postings on this, including the following: http://www.rbcafe.com/Softwares/Unity/Documentation/Manual/Plugins.html http://limbioliong.wordpress.com/2011/06/16/returning-strings-from-a-c-api/ http://www.mono-project.com/Interop_with_Native_Libraries http://answers.unity3d.com/questions/22553/marshaling-a-string-from-c.html?sort=oldest

Can anyone help me figure out what I'm doing wrong? Thanks!