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

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 2

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 3

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;
}