using select with directory descriptor

bujecas · 06-19-2006, 10:55 AM

Hello,

I'm trying to write a program that is aware of directory contents changes. I'm using a select in the directory descriptor obtained by dirfd function. My code is something like this:

Code:

int main()
{
  fd_set rset;
  struct timeval tm;
  int fd, ret;
  DIR *dfd;

  dfd = opendir("directory");

  fd = dirfd(dfd);

  while(1) {
    tm.tv_sec = 1;
    tm.tv_usec = 0;
   
    FD_ZERO(&rset);
    FD_SET(fd, &rset);
    ret = select(fd+1, &rset, NULL, NULL, &tm);

    if(ret>0)
      printf("Directory contents changed");
  }
  return 0;  
}

My idea is: when I add a file to the directory, the select will detect and do something but with this code the fd is always true even without adding any file to the directory.

Any ideas? Thanks.

jonaskoelker · 06-19-2006, 04:02 PM

First off: why doesn't your solution work? That's actually quite simple: what select does is see if there's any input available. But since you have the directory on the disk, there's always input available.

What you want to test instead is whether or not the date of last change is altered.
Demo:

Code:

$ ls -ld .
drwxr-xr-x 84 jonas jonas 7400 2006-05-18 13:43 .
$ mkdir non-existent
$ ls -ld .
drwxr-xr-x 84 jonas jonas 7400 2006-06-19 22:55 .

If you want to do that from C, look at stat(2).

firstfire · 06-20-2006, 10:47 AM

Hi.
Think about `dnotify' and `inotify'. dnotify stands for 'directory notify' -- if directory contents changed, you will get a signal. Look at linux-2.N.M/Documentation/dnotify.txt (kernel sources).
inotify -- another file change notification system. It's newer than dnotify and "therefore" -- better

Unfortunately this stuff don't allow you to determine which process is changing filesystem. I try to solve such problem with `ptrace' -- debugging system call. In another points of view /[d|i]notify/ is the best solution I think.

Good luck.