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);
if (m_fdFram != -1)
close(m_fdFram);
}
puts("sleeping");
sleep(50);
return 0;
}
$ gcc test.c -o test
$ cat /proc/sys/fs/file-nr
10016 0 387704
$ ./test &
sleeping
$ cat /proc/sys/fs/file-nr
15008 0 387704
As can be seen, when I run the test, it consumes 5000 fd's, but only
releases them when the application closes, 50 seconds later.
My application is designed to run for many years without a restart,
spawning other tasks to do various system jobs, these fd's never seem to
get released, and eventually the system runs out of available fd's.
So my questions are:-
(a) Since all the mappings have been removed, so why don't the fd's get
released ?
(b) Can I somehow force some form of garbage collection to allow the
fd's to be freed up ?
Again, apologies if this is the wrong place to ask, but I'm out of ideas.
Thanks in advance
Mark JACKSON
[1] http://pubs.opengroup.org/onlinepubs/007908775/xsh/mmap.html
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