inotify file in C

Question

I am trying to run an example of inotify in C..but it's not working. I want to monitor modifications to a file (the file is tmp.cfg), but it doesn't work..I don't know if i'm running it correctly, because I understand how to monitor directories, but not a single file Here´s the example:

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/inotify.h>
#include <unistd.h>

#define EVENT_SIZE  ( sizeof (struct inotify_event) )
#define BUF_LEN     ( 1024 * ( EVENT_SIZE + 16 ) )

int main( int argc, char **argv )
{
  int length, i = 0;
  int fd;
  int wd;
  char buffer[BUF_LEN];

  fd = inotify_init();

  if ( fd < 0 ) {
    perror( "inotify_init" );
  }

  wd = inotify_add_watch( fd, "/home/name/tmp.cfg",
                         IN_MODIFY | IN_CREATE | IN_DELETE );
  length = read( fd, buffer, BUF_LEN );

  if ( length < 0 ) {
    perror( "read" );
  }

  while ( i < length ) {
    struct inotify_event *event = ( struct inotify_event * ) &buffer[ i ];
      if ( event->mask & IN_CREATE ) {
          printf( "The file %s was created.\n", event->name );
      }
      else if ( event->mask & IN_DELETE ) {
          printf( "The file %s was deleted.\n", event->name );
      }
      else if ( event->mask & IN_MODIFY ) {
          printf( "The file %s was modified.\n", event->name );
      }
    i += EVENT_SIZE + event->len;
  }

  ( void ) inotify_rm_watch( fd, wd );
  ( void ) close( fd );

  return 0;
}

Once i run it, if i write something on the file and then save it, nothing happens. i've tryed debugging it..the problem seems to be the if ( event->mask & IN_MODIFY ), as it doesn't recognize it as a modification

Answer 1

You have 2 issues. First, as far as I can tell, inotify does not really work on files - it needs directory name to watch.

Second, you missed if (event->len) { inside while loop.

This code works for me for creating, deleting and modifying files in current directory:

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/inotify.h>
#include <unistd.h>

#define EVENT_SIZE  (sizeof(struct inotify_event))
#define BUF_LEN     (1024 * (EVENT_SIZE + 16))

int main(int argc, char **argv) {
    int length, i = 0;
    int fd;
    int wd;
    char buffer[BUF_LEN];

    fd = inotify_init();

    if (fd < 0) {
        perror("inotify_init");
    }

    wd = inotify_add_watch(fd, ".",
        IN_MODIFY | IN_CREATE | IN_DELETE);
    length = read(fd, buffer, BUF_LEN);

    if (length < 0) {
        perror("read");
    }

    while (i < length) {
        struct inotify_event *event =
            (struct inotify_event *) &buffer[i];
        if (event->len) {
            if (event->mask & IN_CREATE) {
                printf("The file %s was created.\n", event->name);
            } else if (event->mask & IN_DELETE) {
                printf("The file %s was deleted.\n", event->name);
            } else if (event->mask & IN_MODIFY) {
                printf("The file %s was modified.\n", event->name);
            }
        }
        i += EVENT_SIZE + event->len;
    }

    (void) inotify_rm_watch(fd, wd);
    (void) close(fd);

    return 0;
}