Re: Advice on mmap() "feature"

[richard -rw- weinberger]

On Sun, Sep 16, 2012 at 9:21 PM, Mark Jackson <mpfj-list@xxxxxxxxxx> wrote:
> Apologies if this is the wrong place to post this query.  Please feel free
> to redirect me to the correct place.
>
> I have come across a weird (but documented [1]) "feature" of mmap(), which
> is:-
>
> "The mmap()function adds an extra reference to the file associated with the
> file descriptor fildeswhich is not removed by a subsequent close() on that
> file descriptor. This reference is removed when there are no more mappings
> to the file."
>
> In my embedded application, this is resulting in the consumption of
> available file descriptors.
>
> As an example I wrote the following test running on Ubuntu 11.04 64bit.
>
> #include <fcntl.h>
> #include <errno.h>
> #include <stdio.h>
> #include <sys/mman.h>
>
> #define FRAM_SIZE            (64 * 1024)
> #define FRAM_BASE_ADDRESS    0x00000000
>
> int main(int argc, char **argv)
> {
>     int i;
>     for (i = 0; i < 5000; i++)
>     {
>         // open the fram device
>         int m_fdFram = open("/dev/mem", O_RDWR | O_SYNC);
>         if (m_fdFram < 0)
>         {
>             printf("could not open /dev/mem, exiting with errno %d\n",
> errno);
>             return 0;
>         }
>
>         // map the device to memory
>         char *m_pFram = (char *)mmap(    0,
>                                         FRAM_SIZE * sizeof(unsigned short),
>                                         PROT_READ | PROT_WRITE,
>                                         MAP_SHARED,
>                                         m_fdFram,
>                                         FRAM_BASE_ADDRESS);
>         if ((int)m_pFram == -1)
>         {
>             close(m_fdFram);
>             m_fdFram = -1;
>             printf("could not mmap, exiting with errno %d\n", errno);
>             return 0;
>         }
>
>         // now remove the mapping
>         if (m_pFram)
>         munmap(m_pFram, FRAM_SIZE);

You unmap less memory than you map...

-- 
Thanks,
//richard
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