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Subject: Re: POSIX.Memory_Mapping.Map_Memory
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From: Mark Lorenzen <mark.lorenzen@ofir.dk>
Date: 30 Jan 2005 16:39:48 +0100
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Date: 2005-01-30T16:39:48+01:00
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"Adrian Hoe" <byhoe@greenlime.com> writes:

> Mark Lorenzen wrote:
> >
> > In the way you are using Map_Memeory, it is probably because the
> value
> > of Offset (i.e. 0) is not page aligned.
> >
> > On many implementations, you can only map a multiple of the page size
> > and the memory has to be page aligned.
> >
> 
> 
> 
> I wrote a short C program to test mmap. It works and seems like no
> problem doing mmap. The C code is as below:
> 
> ---------------------------------------------------------------------
> #include <stdio.h>
> #include <fcntl.h>
> #include "linux/ioctl.h"
> #include "sys/ioctl.h"
> #include "sys/mman.h"
> 
> int main ()
> {
> int fd;
> int bufsize;
> int result;
> char *buf;
> 
> fd = open ("/dev/video0", O_RDWR);
> 
> if (fd < 0) {
> printf ("Error open device.\n");
> exit (1);
> }
> 
> bufsize = 320 * 240 * 3;
> buf=mmap(0, bufsize, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
> 
> if (!buf) {

Note that mmap wil NEVER return 0, so the check should be "if (buf ==
MAP_FAILED)".

> printf ("mmap error.\n");
> exit (1);
> }
> else {
> printf ("mmap successful.\n");
> }
> 
> result = munmap (buf, bufsize);
> printf ("unmap successful.\n");
> close (fd);
> printf ("device closed.\n");
> 
> return (0);
> }
> ---------------------------------------------------------------------
> 
> Is this gnat/florist error or just as what you mentioned about page
> alignment?

I corrected the above mentioned problem and successfully opened and
mapped a file. Strange.

> 
> I am using gcc/gnat 3.3.3 and florist 3.15p on SuSE Linux 9.1 Pro on
> AMD Athlon. I have no problem with both gcc/gnat and florist. My other
> programs using both for RS-232 interface have no problems at all.
> Now, I'm in despair with curiosity! :-)

Please try to run the program using strace and see how mmap is
actually called.

> --
> Adrian Hoe

- Mark Lorenzen