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Computer software implemented on multiple computing platforms

In computing, cross platform software (also called multi platform software, platform agnostic software, or platform independent software) is computer software that is designed to work in several computing platforms. Some cross platform software requires a separate build for each platform, but some can be directly run on any platform without special preparation, being written in an interpreted language or compiled to portable bytecode for which the interpreters or run time packages are common or standard components of all supported platforms. For example, a cross platform application may run on Linux, macOS and Microsoft Windows. Cross platform software may run on many platforms, or as few as two. Some frameworks for cross platform development are Codename One, ArkUI X, Kivy, Qt, GTK, Flutter, NativeScript, Xamarin, Apache Cordova, Ionic, and React Native.

The Java language is typically compiled to run on a VM that is part of the Java platform. The Java virtual machine (Java VM, JVM) is a CPU implemented in software, which runs all Java code. This enables the same code to run on all systems that implement a JVM. Java software can be executed by a hardware based Java processor. This is used mostly in embedded systems. Java code running in the JVM has access to OS related services, like disk input/output (I/O) and network access, if the appropriate privileges are granted. The JVM makes the system calls on behalf of the Java application. This lets users to decide the appropriate protection level, depending on an access control list (ACL). For example, disk and network access is usually enabled for desktop applications, but not for browser based applets. The Java Native Interface (JNI) can also be used to access OS specific functions, with a loss of portability. Currently, Java Standard Edition software can run on Microsoft Windows, macOS, several Unix like OSs, and several real time operating systems for embedded devices. For mobile applications, browser plugins are used for Windows and Mac based devices, and Android has built in support for Java. There are also subsets of Java, such as Java Card or Java Platform, Micro Edition, designed for resource constrained devices.

For software to be considered cross platform, it must function on more than one computer architecture or OS. Developing such software can be a time consuming task because different OSs have different application programming interfaces (API). Software written for one OS may not automatically work on all architectures that OS supports. Just because software is written in a popular programming language such as C or C++, it does not mean it will run on all OSs that support that language—or even on different versions of the same OS.

Web applications are typically described as cross platform because, ideally, they are accessible from any web browser: the browser is the platform. Web applications generally employ a client–server model, but vary widely in complexity and functionality. It can be hard to reconcile the desire for features with the need for compatibility. Basic web applications perform all or most processing from a stateless server, and pass the result to the client web browser. All user interaction with the application consists of simple exchanges of data requests and server responses. This type of application was the norm in the early phases of World Wide Web application development. Such applications follow a simple transaction model, identical to that of serving static web pages. Today, they are still relatively common, especially where cross platform compatibility and simplicity are deemed more critical than advanced functionality. Prominent examples of advanced web applications include the Web interface to Gmail and Google Maps. Such applications routinely depend on additional features found only in the more recent versions of popular web browsers. These features include Ajax, JavaScript, Dynamic HTML, SVG, and other components of rich web applications.

Because of the competing interests of compatibility and functionality, numerous design strategies have emerged. Many software systems use a layered architecture where platform dependent code is restricted to the upper and lowermost layers.

Graceful degradation attempts to provide the same or similar functionality to all users and platforms, while diminishing that functionality to a least common denominator for more limited client browsers. For example, a user attempting to use a limited feature browser to access Gmail may notice that Gmail switches to basic mode, with reduced functionality but still of use.

Some software is maintained in distinct codebases for different (hardware and OS) platforms, with equivalent functionality. This requires more effort to maintain the code, but can be worthwhile where the amount of platform specific code is high.

This strategy relies on having one codebase that may be compiled to multiple platform specific formats. One technique is conditional compilation. With this technique, code that is common to all platforms is not repeated. Blocks of code that are only relevant to certain platforms are made conditional, so that they are only interpreted or compiled when needed. Another technique is separation of functionality, which disables functionality not supported by browsers or OSs, while still delivering a complete application to the user. (See also: Separation of concerns.) This technique is used in web development where interpreted code (as in scripting languages) can query the platform it is running on to execute different blocks conditionally.

Third party libraries attempt to simplify cross platform capability by hiding the complexities of client differentiation behind a single, unified API, at the expense of vendor lock in.

Responsive web design (RWD) is a Web design approach aimed at crafting the visual layout of sites to provide an optimal viewing experience—easy reading and navigation with a minimum of resizing, panning, and scrolling—across a wide range of devices, from mobile phones to desktop computer monitors. Little or no platform specific code is used with this technique.

Cross platform applications need much more integration testing. Some web browsers prohibit installation of different versions on the same machine. There are several approaches used to target multiple platforms, but all of them result in software that requires substantial manual effort for testing and maintenance. Techniques such as full virtualization are sometimes used as a workaround for this problem. Tools such as the Page Object Model allow cross platform tests to be scripted so that one test case covers multiple versions of an app. If different versions have similar user interfaces, all can be tested with one test case.

Web applications are becoming increasingly popular but many computer users still use traditional application software which does not rely on a client/web server architecture. The distinction between traditional and web applications is not always clear. Features, installation methods and architectures for web and traditional applications overlap and blur the distinction. Nevertheless, this simplifying distinction is a common and useful generalization.

Cross platform or multi platform is a term that can also apply to video games released on a range of video game consoles. Examples of cross platform games include: Miner 2049er, Tomb Raider: Legend, FIFA series, NHL series and Minecraft. Each has been released across a variety of gaming platforms, such as the Wii, PlayStation 3, Xbox 360, personal computers, and mobile devices. Some platforms are harder to write for than others, requiring more time to develop the video game to the same standard. To offset this, a video game may be released on a few platforms first, then later on others. Typically, this happens when a new gaming system is released, because video game developers need to acquaint themselves with its hardware and software. Some games may not be cross platform because of licensing agreements between developers and video game console manufacturers that limit development to one particular console. As an example, Disney could create a game with the intention of release on the latest Nintendo and Sony game consoles. Should Disney license the game with Sony first, it may be required to release the game solely on Sony's console for a short time or indefinitely.

Several developers have implemented ways to play games online while using different platforms. Psyonix, Epic Games, Microsoft, and Valve all possess technology that allows Xbox 360 and PlayStation 3 gamers to play with PC gamers, leaving the decision of which platform to use to consumers. The first game to allow this level of interactivity between PC and console games was Quake 3. Games that feature cross platform online play include Rocket League, Final Fantasy XIV, Street Fighter V, Killer Instinct, Paragon and Fable Fortune, and Minecraft with its Better Together update on Windows 10, VR editions, Pocket Edition and Xbox One.

Cross platform programming is the practice of deliberately writing software to work on more than one platform.

There are different ways to write a cross platform application. One approach is to create multiple versions of the same software in different source trees—in other words, the Microsoft Windows version of an application might have one set of source code files and the Macintosh version another, while a FOSS *nix system might have a third. While this is straightforward, compared to developing for only one platform it can cost much more to pay a larger team or release products more slowly. It can also result in more bugs to be tracked and fixed. Another approach is to use software that hides the differences between the platforms. This abstraction layer insulates the application from the platform. Such applications are platform agnostic. Applications that run on the JVM are built this way. Some applications mix various methods of cross platform programming to create the final application. An example is the Firefox web browser, which uses abstraction to build some of the lower level components, with separate source subtrees for implementing platform specific features (like the GUI), and the implementation of more than one scripting language to ease software portability. Firefox implements XUL, CSS and JavaScript for extending the browser, in addition to classic Netscape style browser plugins. Much of the browser itself is written in XUL, CSS, and JavaScript.

There are many challenges when developing cross platform software. Testing cross platform applications may be considerably more complicated, since different platforms can exhibit slightly different behaviors or subtle bugs. This problem has led some developers to deride cross platform development as "write once, debug everywhere", a take on Sun Microsystems' "write once, run anywhere" marketing slogan. Developers are often restricted to using the lowest common denominator subset of features which are available on all platforms. This may hinder the application's performance or prohibit developers from using the most advanced features of each platform. Different platforms often have different user interface conventions, which cross platform applications do not always accommodate. For example, applications developed for macOS and GNOME are supposed to place the most important button on the right hand side of a window or dialog, whereas Microsoft Windows and KDE have the opposite convention. Though many of these differences are subtle, a cross platform application which does not conform to these conventions may feel clunky or alien to the user. When working quickly, such opposing conventions may even result in data loss, such as in a dialog box confirming whether to save or discard changes. Scripting languages and VM bytecode must be translated into native executable code each time they are used, imposing a performance penalty. This penalty can be alleviated using techniques like just in time compilation; but some computational overhead may be unavoidable. Different platforms require the use of native package formats such as RPM and MSI. Multi platform installers such as InstallAnywhere address this need. Cross platform execution environments may suffer cross platform security flaws, creating a fertile environment for cross platform malware.