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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.
For example:
#include <stdio.h> #include <stdlib.h> #include <sys/time.h> #include <sys/types.h> #include <sys/inotify.h> #include <unistd.h> int main(int argc, char *argv[]) { int fd; int wd; int retval; struct timeval tv; fd = inotify_init(); /* checking modification of a file - writing into */ wd = inotify_add_watch(fd, "./myConfig", IN_MODIFY); if (wd < 0) { printf("inotify cannot be used\n"); /* switch back to previous checking */ } fd_set rfds; FD_ZERO(&rfds); FD_SET(fd, &rfds); tv.tv_sec = 5; tv.tv_usec = 0; retval = select(fd + 1, &rfds, NULL, NULL, &tv); if (retval == -1) perror("select()"); else if (retval) { printf("file was modified\n"); } else printf("timeout\n"); return EXIT_SUCCESS; }
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, see the inotify(7) man page.