The clipboard is a data buffer used for short-term data storage and/or data transfer between documents or applications used by cut, copy and paste operations and provided by the operating system. It is usually implemented as an anonymous, temporary data buffer, sometimes called the paste buffer, that can be accessed from most or all programs within the environment via defined programming interfaces. A typical application accesses clipboard functionality by mapping user input such as keybindings, menu selections, etc. to these interfaces. The semantics of the clipboard facility varies from one operating system to another, and can also vary between versions of the same system. They can sometimes be changed by different programs or by user preference.
When an element is copied or cut, the clipboard holds every available format of it, since at this point, it is not known which format is needed when the content is pasted. The core functionality of the clipboard provided by the operating system can be extended by applications and clipboard managers.
Windows, Linux and macOS support a single clipboard transaction. Each cut or copy overwrites the previous contents. Normally, paste operations copy the contents, leaving the contents available in the clipboard for further pasting operations. Clipboard data is stored in the computer's RAM.
Drag and drop enables users to drag and drop information from one control to another similar to the functionality of cut, copy and paste from the users view, but it doesn't affect the clipboard.
Clipboards as buffers for small text snippets were first used by Pentti Kanerva when he used it to store deleted texts in order to restore them. Since one could delete a text in one place and restore it in another, the term "delete" wasn't what one would expect in this case. Larry Tesler renamed this in 1973 as cut, copy, and paste and coined the term "clipboard" for this buffer, since these techniques need a clipboard for temporary saving the copied or cut data.
Applications communicate through the clipboard by providing either serialized representations of an object, or a promise(for larger objects). In some circumstances the transfer of certain common data formats may be achieved opaquely through the use of an abstract factory, for example Mac OS X uses the a class called NSImage to provide access to image data stored on the clipboard, though the actual format of the image data backing the object is hidden. The sending and receiving application negotiate the formats which can be transferred in between them, oftentimes with the active GUI widget responsible for providing acceptable type transformations. The pasteboard allows for transfer of common items such as URLs, colors, images, strings, attributed strings(Rich text), and sounds. The operating system and GUI toolkit may provide some common conversions, for example converting from rich text to plain text and vice versa. Various type identifiers for data transfer are supported by modern operating systems, which may automatically provide acceptable mappings between type systems, such as between MIME and Uniform Type Identifier.
There have been exploits where web pages grab clipboard data. In early 2013 researchers exposed risks stemming from Android-based password managers and documented how passwords in 21 of the most popular of these apps could be accessed by any other app on an Android device including those with extremely low-level privileges. Joe Siegrist notes that this is an "OS-level issue that impacts everything running on Android".
Clipboard management and extensions
Clipboard manager extensions add functionality to the integrated clipboard functions of an operating system. They are applications that enable the user to manipulate the clipboard. On platforms such as linux where that use multiple incompatible GUI toolkits, clipboard managers are often used to transfer data between applications using different such frameworks.
When a clipboard manager provides multiple cut and paste transactions, the clipboard is treated as a stack or scrap book, with new cuts and copies being placed on a list of recent transactions. The standard paste operation copies the most recent transaction, while specialized pastes provide access to the other stored transactions. These managers generally also provide a window that displays the transaction history and allows the user to select earlier copies, edit them, change their format and even search amongst them.
Since most operating systems (e.g. Windows, macOS, Linux, X11, Android, iOS) do not save the clipboard contents to any persistent storage – when a user logs out or reboots the system the clipboard contents are deleted – an added functionality is to save the clipboard persistently. Another example is making the local clipboard work with online applications by saving the clipboard data to the online location upon a copy or cut event, making this data available to online applications for pasting. Clipboard managers can also serve as tools to overcome the limitation of software not supporting copying and pasting (for example, while logging into remote Windows server, one cannot copy and paste their user name and password).
Operating system-specific clipboards
The clipboard in Microsoft Windows holds one item in multiple available formats.
Every item has at least one clipboard format, but can have different types of format of the same data. The three different types of possible formats are:
- standard formats (e.g. CF_BITMAP, or CF_UNICODETEXT),
- registered formats (e.g. CF_HTML)
- private formats for internal use
# to paste the content of a directory to the clipboard Set-Clipboard -Path "C:\directory\" # to get the content of the clipboard Get-Clipboard
Data can be stored to the clipboard via command line:
$ # to paste the content of a folder to the clipboard: $ dir | clip
The clipboard in macOS holds one item in multiple available formats.
The contents of the clipboard can be viewed by selecting the Show Clipboard menu item from the Finder's Edit menu. The raw data and the stored formats can be seen using the ClipboardViewer.
$ # to copy data into the clipboard: $ echo 'hello world' | pbcopy $ # to paste from the clipboard: $ pbpaste hello world
X Window System
The usage and handling of various selections is not standardized. However most modern toolkits and desktop environments, such as GNOME or KDE, follow a widely accepted convention, outlined in the freedesktop.org specification. One selection, CLIPBOARD, is used for traditional clipboard semantics, with shortcuts identical to Windows. Another selection, PRIMARY, is an X11-specific mechanism. Data is "copied" immediately upon highlighting and pasted with the third (middle) mouse button. This copied data is usually separated from the CLIPBOARD selection and does not change its contents. SECONDARY was planned as an alternative to PRIMARY but is only used inconsistently.
There are two command line tools (xsel and xlclip) which can access the clipboard:
$ # to paste standard output to the clipboard using xclip $ echo text | xclip -in -selection clipboard $ # to paste standard output to the clipboard using xsel $ echo text | xsel --clipboard
The main difference to OS X and Windows is, that no data is actually stored in the CLIPBOARD-clipboard but only the reference to the copied or cut data. The application claims the owernship of the CLIPBOARD selection and communicates its ownership to the X Server. When pasting this data, the data and its available formats are requested from the application that owns the CLIPBOARD selection.
Android provides a clipboard framework which can hold up to one clip object and is accessible system-wide. While simple text data is stored directly in the clipboard complex data is stored by reference. The format of the clip object has one of three forms:
- text strings
- URI objects
The data cointained in the clipboard can't be accessed via the GUI but only from the system and applications.
If one wants to interact with the clipboard as part of an application one should use the class ClipboardManager which fascilates copying and pasting data structures and streams.
The clipboard is called "pasteboard" in iOS similar to OS X. Apps on this operating system can create additional pasteboards, called instances of the UIPasteboard class, which can be public or private. One instance can hold a single item or multiple items in different formats.
The data contained in the pasteboard can't be accessed via the GUI but only from the system and applications.
Applications can access the clipboard or its data via APIs.
In Qt a wrapper for every supported platform (e.g. Windows or OS X) exists. It provides access to window system clipboards with the use of the class QClipboard. This class facilitates access to common data types by functions. The data type of the element stored in the clipboard is indicated via MIME and MIME data can also be put in the clipboard with help of functions from this class.
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