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From: Terry Lambert <terry@lambert.org>
Message-Id: <199612170201.TAA07421@phaeton.artisoft.com>
Subject: Re: Almost have mmap() figured, I think...
To: scrappy@hub.org (Marc G. Fournier)
Date: Mon, 16 Dec 1996 19:01:54 -0700 (MST)
Cc: hackers@freebsd.org
In-Reply-To: <Pine.NEB.3.95.961216200508.283F-100000@hub.org> from "Marc G. Fournier" at Dec 16, 96 08:20:55 pm
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> 	 Altho I really hate to ask, I think I almost have this licked, but 
> there is something I'm still missing, so the code isn't *quite* working the 
> way it should.
> 
> 	Essentially, to simplify things while I "understand it better", 
> I've created n mmap()'d regions, each representing a frame.  There are
> n corresponding mmap()'d regions, each of which correspond to the
> *size* of the frame.  (I know, inelegant, but I can make it more
> elegant *after* I know better that which I'm doing...)

[ ... ]

> 	The regions are being created as:
> 
> ======
>   while(ii < FRAMES) {
>     sprintf(framestr, "/tmp/frame-%02d", ii);
>  
>     /* Setup Video Stream Buffer */
>     if((fd = open(framestr, O_RDWR|O_CREAT, 0666)) < 0) {
>       fprintf(stderr, "cannot open %s\n", framestr);
>       exit(-1);
>     }
>     ftruncate(fd, FRAMESIZE);
>     if((frame[ii] =
>         mmap(0, FRAMESIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0))
>         == (caddr_t) -1) {
>       fprintf(stderr, "cannot mmap frame buffer #%02d\n", ii);
>       exit(-1);
>     }
>     close(fd);
>   }

Alternately:

start = mmap( 0, FRAMESIZE * FRAMES, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0));
for( i = 0; i < FRAMES; i++)
	frame[i] = start + i * FRAMESIZE;

And use one file... 8-).

Also: Are you mapping in both the client and the server?  Or just the server?


> 	The problem comes up when I try and print out the frame itself,
> using 'printf("%s\n", frame[ii]);'...the value printed out each time
> is exactly the same: <FF><D8><FF><E0>

What is the data in the frame?  Maybe that's right?  printf("%s", ...)
will stop printing if it hit a <FF><D8><FF><E0><00>...

> 	So, I'm making the assumption that either the way I'm calling, or
> using, readn() is incorrect for use with mmap()'d regions...would I be
> better off to read in the socket, then memcpy() the data, or strncpy()?  Or
> is what I'm missing even more simple then that?

No; reading directly is a win.


					Terry Lambert
					terry@lambert.org
---
Any opinions in this posting are my own and not those of my present
or previous employers.