mmap()'d file contents disappear

TANAKA Yoshitomo (serow@silkwood.rim.or.jp)
Fri, 26 Apr 1996 02:28:36 +0900

I am confused by mmap()'d file contents disappearance.
First, I noticed that content of /etc/psdevtab were filled by 0x00.
(/etc/psdevtab is created by porgrams in procps-0.99a(w, ps,..))
Removing /etc/psdevtab and re-create it by 'w'/'ps'. It goes good.
Few hours after, /etc/psdevtab filled by 0x00 again....
Time stamps({a,m,c}_time) were not changed from when that was created.

After some trial and error, I tested following.
'mmtest1' is munmap()'ed, 'mmtest2' is not.
Immediately after excuting this,
># od -h mmtest1
>0000000 0101 0101 0101 0101 0101 0101 0101 0101
>*
>0030000
># od -h mmtest2
>0000000 0202 0202 0202 0202 0202 0202 0202 0202
>*
>0030000

But after few hours(or a day, or after something done...)

># od -h mmtest1
>0000000 0101 0101 0101 0101 0101 0101 0101 0101
>*
>0030000
># od -h mmtest2
>0000000 0000 0000 0000 0000 0000 0000 0000 0000
>*
>0030000

I think I found such behavior at updateing kernel to 1.3.94.
Same at 1.3.95.
Procps does't munmap().
I guess disappearing /etc/psdevtab and mmtest2 are same reason.

--
/*
 * first of all, create two test files.
 *   dd if=/dev/zero of=mmtest1 bs=12288 count=1
 *   dd if=/dev/zero of=mmtest2 bs=12288 count=1
 */
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/mman.h>


#define LEN	12288
#define FNAM1	"/tmp/mmtest1"
#define FNAM2	"/tmp/mmtest2"


main()
{
    caddr_t p;
    int fd;
    int i, j;


    if ((fd = open(FNAM1, O_RDWR)) < 0) {
        perror("open");
        exit(1);
    }
    p = mmap(0, LEN, PROT_READ|PROT_WRITE, MAP_FILE|MAP_SHARED, fd, 0);
    if (p == (caddr_t) -1) {
        perror("mmap");
        exit(1);
    }
    close(fd);
    memset((void *) p, 1, LEN);
    munmap(p, LEN);


    if ((fd = open(FNAM2, O_RDWR)) < 0) {
        perror("open");
        exit(1);
    }
    p = mmap(0, LEN, PROT_READ|PROT_WRITE, MAP_FILE|MAP_SHARED, fd, 0);
    if (p == (caddr_t) -1) {
        perror("mmap");
        exit(1);
    }
    close(fd);
    memset((void *) p, 2, LEN);
}


----_--__---_-_-_-__--_-__-__---_-_----_--_-_---_---_----
_/     TANAKA Yoshitomo       _//
/  Suginami-ku Tokyo, Japan  _// serow@silkwood.rim.or.jp