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From: pdg@ihdev.UUCP
Newsgroups: comp.unix.questions
Subject: Re: System V.2 IPC (msgop - message passing) ?
Message-ID: <1319@ihdev.ATT.COM>
Date: Sat, 11-Apr-87 17:07:26 EST
Article-I.D.: ihdev.1319
Posted: Sat Apr 11 17:07:26 1987
Date-Received: Wed, 15-Apr-87 01:29:47 EST
References: <419@artecon.artecon.UUCP>
Reply-To: pdg@ihdev.UUCP (Joe Isuzu)
Organization: American Nasal Amputation Centre
Lines: 48
Keywords: Include file definitions

In article <419@artecon.artecon.UUCP> tony@artecon.artecon.UUCP (Anthony D. Parkhurst) writes:

>Question about System V message passing (IPC):
> [ manual page excerpt editied out ]
>However, looking at the include file when this is defined <sys/msg.h>,
>it defines the structure like this:
>
>struct msgbuf {
>	int	mtype;
>	char	mtext[1];	/* note the '1' */
>	}
>
>What is going on here, I can't redefine this pointer, and I certainly
>can't pass descent messages in *1* byte.

They don't expect you to.  Just because it is one byte long in the
msgbuf definition, doesn't mean you must only pass 1 byte at a time.
You just allocate more space at the end of the structure, and
reference the space as mtext[index].  The standard SysV compiler does
not do any array-bounds checking, so this is OK.  Take the following
example (an excerpt from the client side of my SysV version of the GNU
client/server code)

  struct msgbuf *msgp = (struct msgbuf *)malloc(sizeof(struct msgbuf)+ 256);

  ......

  (void)strcpy(msgp->mtext,out);
  msgp->mtype = 1;
  if(msgsnd(msgqid,msgp,sizeof(struct msgbuf)+strlen(out)+1,1)==-1) 
    {
      perror("msg send ");
      exit(1);
    }

  .....

So anyway, I just allocate a message 256b + sizeof (msgbuf) long, then
use it again and again, by copying what I want to send into the
message, and sending it, specifying the size of what I want to send,
not the size of what I alloced.  This works well, and is a pretty
standard usage.


-- 

Paul Guthrie
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