C program to monitor and notify changes in a directory / file using inotify

Sometime we need to monitor some directory or file for some changes in it and based on that take necessary action. This program shows how we can do the same using C program and inotify API’s.

 $ vim notify.c 

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/inotify.h>
#include <limits.h>
#include <unistd.h>
  
#define MAX_EVENTS 1024 /*Max. number of events to process at one go*/
#define LEN_NAME 1024 /*Assuming length of the filename won't exceed 16 bytes*/
#define EVENT_SIZE  ( sizeof (struct inotify_event) ) /*size of one event*/
#define BUF_LEN     ( MAX_EVENTS * ( EVENT_SIZE + LEN_NAME )) /*buffer to store the data of events*/
 
void get_event (int fd) {
    char buffer[BUF_LEN];
    int length, i = 0;
 
    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) {
                if (event->mask & IN_ISDIR)
                    printf( "The directory %s was Created.\n", event->name );       
                else
                    printf( "The file %s was Created with WD %d\n", event->name, event->wd );       
            }
            
            if ( event->mask & IN_MODIFY) {
                if (event->mask & IN_ISDIR)
                    printf( "The directory %s was modified.\n", event->name );       
                else
                    printf( "The file %s was modified with WD %d\n", event->name, event->wd );       
            }
            
            if ( event->mask & IN_DELETE) {
                if (event->mask & IN_ISDIR)
                    printf( "The directory %s was deleted.\n", event->name );       
                else
                    printf( "The file %s was deleted with WD %d\n", event->name, event->wd );       
            }  
            i += EVENT_SIZE + event->len;
        }
    }
}
 
int main( int argc, char **argv ) {
    int wd, fd;
  
    fd = inotify_init();
    if ( fd < 0 ) {
        perror( "Couldn't initialize inotify");
    }
  
    wd = inotify_add_watch(fd, argv[1], IN_CREATE | IN_MODIFY | IN_DELETE); 
    if (wd == -1) {
        printf("Couldn't add watch to %s\n",argv[1]);
    } else {
        printf("Watching:: %s\n",argv[1]);
    }
  
    /* do it forever*/
    while(1) {
        get_event(fd); 
    } 
 
    /* Clean up*/
    inotify_rm_watch( fd, wd );
    close( fd );
    
    return 0;
}

 $ gcc -o notify notify.c 

Now open another terminal and lets create a test directory as,

 $ mkdir helloworld_dir 

In the previous terminal, start notify program as,

$ ./notify helloworld_dir/
Watching:: helloworld_dir/ 

Now, go to another terminal and create a test file,

 $ cd helloworld_dir 

$ touch hello.txt

In this case, the first terminal will print like below,

$ ./notify helloworld_dir/
Watching:: helloworld_dir/
The file hello.txt was Created with WD 1

Now, in another window try to edit the hello.txt as,

 $ echo "modify hello.txt to add some lines " > hello.txt 

Then the first terminal should print a message as,

 The file hello.txt was modified with WD 1 

Reference : https://opensourceforu.com/2011/04/getting-started-with-inotify/