- Qt 6.8
- Qt Core
- C++ Classes
- QFileSystemWatcher
- Qt 6.8.3 Reference Documentation

# QFileSystemWatcher Class

The QFileSystemWatcher class provides an interface for monitoring files and directories for modifications. More...

Header: | `#include <QFileSystemWatcher>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS Core)` `target_link_libraries(mytarget PRIVATE Qt6::Core)`
qmake: | `QT += core`
Inherits: | QObject

- List of all members, including inherited members
- QFileSystemWatcher is part of Input/Output and Networking.

Note: All functions in this class are reentrant.

## Public Functions

| QFileSystemWatcher(QObject *parent = nullptr)
| QFileSystemWatcher(const QStringList &paths, QObject *parent = nullptr)
virtual | ~QFileSystemWatcher()
bool | addPath(const QString &path)
QStringList | addPaths(const QStringList &paths)
QStringList | directories() const
QStringList | files() const
bool | removePath(const QString &path)
QStringList | removePaths(const QStringList &paths)

## Signals

void | directoryChanged(const QString &path)
void | fileChanged(const QString &path)

## Detailed Description

QFileSystemWatcher monitors the file system for changes to files and directories by watching a list of specified paths.

Call addPath() to watch a particular file or directory. Multiple paths can be added using the addPaths() function. Existing paths can be removed by using the removePath() and removePaths() functions.

QFileSystemWatcher examines each path added to it. Files that have been added to the QFileSystemWatcher can be accessed using the files() function, and directories using the directories() function.

The fileChanged() signal is emitted when a file has been modified, renamed or removed from disk. Similarly, the directoryChanged() signal is emitted when a directory or its contents is modified or removed. Note that QFileSystemWatcher stops monitoring files once they have been renamed or removed from disk, and directories once they have been removed from disk.

- Notes:
  - On systems running a Linux kernel without inotify support, file systems that contain watched paths cannot be unmounted.
  - The act of monitoring files and directories for modifications consumes system resources. This implies there is a limit to the number of files and directories your process can monitor simultaneously. On all BSD variants, for example, an open file descriptor is required for each monitored file. Some system limits the number of open file descriptors to 256 by default. This means that addPath() and addPaths() will fail if your process tries to add more than 256 files or directories to the file system monitor. Also note that your process may have other file descriptors open in addition to the ones for files being monitored, and these other open descriptors also count in the total. macOS uses a different backend and does not suffer from this issue.

See also QFile and QDir.

## Member Function Documentation

### QFileSystemWatcher::QFileSystemWatcher(QObject *parent = nullptr)

Constructs a new file system watcher object with the given parent.

### QFileSystemWatcher::QFileSystemWatcher(const QStringList &paths, QObject *parent = nullptr)

Constructs a new file system watcher object with the given parent which monitors the specified paths list.

### `[virtual noexcept]` QFileSystemWatcher::~QFileSystemWatcher()

Destroys the file system watcher.

### bool QFileSystemWatcher::addPath(const QString &path)

Adds path to the file system watcher if path exists. The path is not added if it does not exist, or if it is already being monitored by the file system watcher.

If path specifies a directory, the directoryChanged() signal will be emitted when path is modified or removed from disk; otherwise the fileChanged() signal is emitted when path is modified, renamed or removed.

If the watch was successful, true is returned.

Reasons for a watch failure are generally system-dependent, but may include the resource not existing, access failures, or the total watch count limit, if the platform has one.

Note: There may be a system dependent limit to the number of files and directories that can be monitored simultaneously. If this limit is been reached, path will not be monitored, and false is returned.

See also addPaths() and removePath().

### QStringList QFileSystemWatcher::addPaths(const QStringList &paths)

Adds each path in paths to the file system watcher. Paths are not added if they not exist, or if they are already being monitored by the file system watcher.

If a path specifies a directory, the directoryChanged() signal will be emitted when the path is modified or removed from disk; otherwise the fileChanged() signal is emitted when the path is modified, renamed, or removed.

The return value is a list of paths that could not be watched.

Reasons for a watch failure are generally system-dependent, but may include the resource not existing, access failures, or the total watch count limit, if the platform has one.

Note: There may be a system dependent limit to the number of files and directories that can be monitored simultaneously. If this limit has been reached, the excess paths will not be monitored, and they will be added to the returned QStringList.

See also addPath() and removePaths().

### QStringList QFileSystemWatcher::directories() const

Returns a list of paths to directories that are being watched.

See also files().

### `[private signal]` void QFileSystemWatcher::directoryChanged(const QString &path)

This signal is emitted when the directory at a specified path is modified (e.g., when a file is added or deleted) or removed from disk. Note that if there are several changes during a short period of time, some of the changes might not emit this signal. However, the last change in the sequence of changes will always generate this signal.

Note: This is a private signal. It can be used in signal connections but cannot be emitted by the user.

See also fileChanged().

### `[private signal]` void QFileSystemWatcher::fileChanged(const QString &path)

This signal is emitted when the file at the specified path is modified, renamed or removed from disk.

Note: As a safety measure, many applications save an open file by writing a new file and then deleting the old one. In your slot function, you can check `watcher.files().contains(path)`. If it returns `false`, check whether the file still exists and then call `addPath()` to continue watching it.

Note: This is a private signal. It can be used in signal connections but cannot be emitted by the user.

See also directoryChanged().

### QStringList QFileSystemWatcher::files() const

Returns a list of paths to files that are being watched.

See also directories().

### bool QFileSystemWatcher::removePath(const QString &path)

Removes the specified path from the file system watcher.

If the watch is successfully removed, true is returned.

Reasons for watch removal failing are generally system-dependent, but may be due to the path having already been deleted, for example.

See also removePaths() and addPath().

### QStringList QFileSystemWatcher::removePaths(const QStringList &paths)

Removes the specified paths from the file system watcher.

The return value is a list of paths which were not able to be unwatched successfully.

Reasons for watch removal failing are generally system-dependent, but may be due to the path having already been deleted, for example.

See also removePath() and addPaths().