A.2. Wake-ups
Many applications scan configuration files for changes. In many cases, the scan is performed at a fixed interval, for example, every minute. This can be a problem, because it forces a disk to wake up from spindowns. The best solution is to find a good interval, a good checking mechanism, or to check for changes with inotify and react to events. Inotify can check variety of changes on a file or a directory.
Example A.1. Using inotify
int fd;
fd = inotify_init();
int wd;
/* checking modification of a file - writing into */
wd = inotify_add_watch(fd, "./myConfig", IN_MODIFY);
if (wd < 0) {
inotify_cant_be_used();
switching_back_to_previous_checking();
}
...
fd_set rdfs;
struct timeval tv;
int retval;
FD_ZERO(&rdfs);
FD_SET(0, &rdfs);
tv.tv_sec = 5;
value = select(1, &rdfs, NULL, NULL, &tv);
if (value == -1)
perror(select);
else {
do_some_stuff();
}
...
The advantage of this approach is the variety of checks that you can perform.
The main limitation is that only a limited number of watches are available on a system. The number can be obtained from
/proc/sys/fs/inotify/max_user_watches and although it can be changed, this is not recommended. Furthermore, in case inotify fails, the code has to fall back to a different check method, which usually means many occurrences of #if #define in the source code.
For more information on inotify, refer to the inotify(7) man page.
