What is the proper way to use inotify?

Question

I want to use the inotify mechanism on Linux. I want my application to know when a file aaa was changed. Can you please provide me with a sample how to do that?

Answer 1

• Documentation ( from Monitor file system activity with inotify )

The inotify C API

inotify provides three system calls to build file system monitors of all kinds:

• inotify_init() creates an instance of the inotify subsystem in the kernel and returns a file descriptor on success and -1 on failure. Like other system calls, if inotify_init() fails, check errno for diagnostics.
• inotify_add_watch(), as its name implies, adds a watch. Each watch must provide a pathname and a list of pertinent events, where each event is specified by a constant, such as IN_MODIFY. To monitor more than one event, simply use the logical or — the pipe (|) operator in C—between each event. If inotify_add_watch() succeeds, the call returns a unique identifier for the registered watch; otherwise, it returns -1. Use the identifier to alter or remove the associated watch.
• inotify_rm_watch() removes a watch.

The read() and close() system calls are also needed. Given the descriptor yielded by inotify_init(), call read() to wait for alerts. Assuming a typical file descriptor, the application blocks pending the receipt of events, which are expressed as data in the stream. The common close() on the file descriptor yielded from inotify_init() deletes and frees all active watches as well as all memory associated with the inotify instance. (The typical reference count caveat applies here, too. All file descriptors associated with an instance must be closed before the memory consumed by the watches and by inotify is freed.)

• An example ( from Kernel Korner - Intro to inotify )

#include "inotify.h"  
#include "inotify-syscalls.h"  
int wd;   
wd = inotify_add_watch (fd,   
            "/home/rlove/Desktop", IN_MODIFY | IN_CREATE | IN_DELETE);
if (wd < 0)
      perror ("inotify_add_watch");

This example adds a watch on the directory /home/rlove/Desktop for any modifications, file creations or file deletions.

Answer 2

Below is a snippet of how you can use inotify to watch "aaa". Note that I haven't tested this, I haven't even compiled it! You will need to add error checking to it.

Instead of using a blocking read you can also use poll/select on inotfd.

const char *filename = "aaa";
int inotfd = inotify_init();

int watch_desc = inotify_add_watch(inotfd, filename, IN_MODIFY);

size_t bufsiz = sizeof(struct inotify_event) + PATH_MAX + 1;
struct inotify_event* event = malloc(bufsiz);

/* wait for an event to occur */
read(inotfd, event, bufsiz);

/* process event struct here */

Answer 3

If all you need is a commandline application, there is one called inotifywait that watches files using inotify

from terminal 1

# touch cheese
# while inotifywait -e modify cheese; do 
>   echo someone touched my cheese
> done

from terminal 2

echo lol >> cheese

here is what is seen on terminal 1

Setting up watches.
Watches established.
cheese MODIFY 
someone touched my cheese
Setting up watches.
Watches established.

Update: use with caution and see the comments.

Answer 4

Since the initial question seems to mention Qt as a tag as noted in several comments here, search engines may have lead you here.

If somebody want to know how to do it with Qt, see http://doc.qt.io/qt-5/qfilesystemwatcher.html for the Qt-version. On Linux it uses a subset of Inotify, if it is available, see explanation on the Qt page for details.

Basically the needed code looks like this:

in mainwindow.h add :

QFileSystemWatcher * watcher;
private slots:
    void directoryChanged(const QString & path);
    void fileChanged(const QString & path);

and for mainwindow.cpp:

#include <QFileInfo>
#include <QFileSystemWatcher>

watcher = new QFileSystemWatcher(this);
connect(watcher, SIGNAL(fileChanged(const QString &)), this, SLOT(fileChanged(const QString &)));
connect(watcher, SIGNAL(directoryChanged(const QString &)), this, SLOT(directoryChanged(const QString &)));
watcher->addPath("/tmp/"); // watch directory
watcher->addPath("/tmp/a.file");  // watch file

also add the slots in mainwindow.cpp which are called if a file/directory-change is noticed:

void MainWindow::directoryChanged(const QString & path) {
     qDebug() << path;
}
void MainWindow::fileChanged(const QString & path) {
     qDebug() << path;
}