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From: nj@magnolia.Berkeley.EDU (Narciso Jaramillo)
Newsgroups: comp.sys.amiga.programmer
Subject: Re: Message Ports
Message-ID: <NJ.91May25224115@magnolia.Berkeley.EDU>
Date: 26 May 91 05:41:15 GMT
References: <BQAcp*wk@asgard.pub.uu.oz.au>
Sender: nobody@ucbvax.BERKELEY.EDU
Distribution: comp
Organization: Postcarcinogenic Bliss, Inc.
Lines: 82
In-reply-to: bart@asgard.pub.uu.oz.au's message of 25 May 91 15:09:09 GMT

In article <BQAcp*wk@asgard.pub.uu.oz.au> bart@asgard.pub.uu.oz.au (John Butcher) writes:

> Ive been trying to do message ports, but I havent had lots of success, my 
> ports wont get properly recognized by name.  Below small test program I 
> wrote :

> main( argc, argv )
> int argc;
> char *argv[];
> {
>   struct MsgPort *mp;

>   mp = FindPort( argv[1] );
>   if ( mp == NULL )                            /* No port with that name */
>   {
>     mp = CreatePort( argv[1], 0 );                    /* so create one ! */
>     printf("Created \"%s\" at 0x%0x\n", argv[1], mp );
>   }
>   else                                           /* found the given port */
>   {
>     printf("Found \"%s\" at 0x%0x\n", argv[1], mp );
>     DeletePort( mp );                                    /* so remove it */
>   }
> }

>   2.RAMB0:> port crud        
>   Created "crud" at 0x208d28        Yay, it created ok !!
>   2.RAMB0:> port anticrud     
>   Found "anticrud" at 0x208d28      Whats this, I thought I called you crud !

What's probably happening is that CreatePort doesn't copy the string
you pass to it.  When you create "crud", mp->mp_Node.ln_Name gets
argv[1], which currently points to "crud."  Your program then exits.
When you invoke the program again, argv[1] happens to be in the same
place, and so mp->mp_node.ln_Name points to whatever name you put on
the command line in the second invocation.  This makes the name of the
message port magically change to the name you give it the second time
around (i.e. the new argv[1]).  So the program thinks the port name is
the same as the one you passed, and it finds it.

The only way around this that I can think of is to AllocMem space for
a new string when you create a port, but don't free it when you exit;
free it only after you delete the port the next time you invoke the
program with that port name.  (If you AllocMem some memory in one
program, can you FreeMem it in another?  I assume so, as long as it's
MEMF_PUBLIC.)

In other words:

--
void main(int argc, char *argv[]) {

  struct MsgPort *mp;
  char *new;

  mp = FindPort(argv[1]);
  if (mp == NULL) {
    new = AllocMem(strlen(argv[1])+1, MEMF_PUBLIC); /* need 1 byte for \0 */
    if (!new) {
      die_horribly();
    }
    strcpy(new, argv[1]);
    mp = CreatePort(new, 0);
    printf("Created \"%s\" at 0x%0x\n", new, mp);
  } else {
    printf("Found \"%s\" at 0x%0x\n", argv[1], mp);
    Forbid(); /* just in case--may not be necessary */
    FreeMem(mp->mp_Node.ln_Name, strlen(mp->mp_Node.ln_Name)+1);
    DeletePort(mp);
    Permit();
  }

}    
--

That solution is kind of gross, but then again I don't think ports
were meant to hang around after a program exited.  Do you really need
a port that lasts after your program's termination, or was this just
for the test program?


nj