Cross-platform software

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 Microsoft Windows, Linux, and macOS. Cross-platform software may run on many platforms, or as few as two. Some frameworks for cross-platform development are Codename One, Kivy, Qt, Flutter, NativeScript, Xamarin, Phonegap, Ionic, and React Native.
Platforms.
"Platform" can refer to the type of processor (CPU) or other hardware on which an operating system (OS) or application runs, the type of OS, or a combination of the two. An example of a common platform is the Microsoft Windows OS running on the x86 architecture. Other well-known desktop platforms are Linux/Unix and macOS - both of which are themselves cross-platform. There are, however, many devices such as smartphones that are also platforms. Applications can be written to depend on the features of a particular platform—either the hardware, OS, or virtual machine (VM) it runs on. For example, the Java platform is a common VM platform which runs on many OSs and hardware types. 
Hardware.
A hardware platform can refer to an instruction set architecture. For example: x86 architecture and its variants such as IA-32 and x86-64. These machines often run one version of Microsoft Windows, though they can run other OSs including Linux, OpenBSD, NetBSD, macOS and FreeBSD.
The 32-bit ARM architectures (and newer 64-bit version) is common on smartphones and tablet computers, which run Android, iOS and other mobile operating systems.
Software.
A software platform can be either an OS or programming environment, though more commonly it is a combination of both. An exception is Java, which uses an OS-independent VM to execute Java bytecode. Examples of software platforms are:
Java.
The Java language is typically compiled to run on a VM that is part of the Java platform. The 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 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 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.
Implementation.
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). For example, Linux uses a different API from Windows.
Software written for one OS may not automatically work on all architectures that OS supports. One example is OpenOffice.org, which in 2006 did not natively run on AMD64 or Intel 64 processors implementing the x86-64 standards; by 2012 it was "mostly" ported to these systems. 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.
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, A9.com, Google Maps website, and the Live Search service (now Bing) from Microsoft. 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. Older versions often lack these.
Design.
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.
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.
Multiple codebases.
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.
Single codebase.
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.
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.
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.
Testing.
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.
Traditional applications.
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.
Binary software.
Traditional application software has been distributed as binary files, especially executable files. Executables only support platform they were built for—which means that a single cross-platform executable could be very bloated with code that never executes on a particular platform. Instead, generally there is a selection of executables, each built for one platform.
For software that is distributed as a binary executable, such as that written in C or C++, there must be a software build for each platform, using a toolset that translates—transcompiles—a single codebase into multiple binary executables. For example, Firefox, an open-source web browser, is available on Windows, macOS (both PowerPC and x86 through what Apple Inc. calls a Universal binary), Linux, and BSD on multiple computer architectures. The four platforms (in this case, Windows, macOS, Linux, and BSD) are separate executable distributions, although they come largely from the same source code. In rare cases, executable code built for several platforms is combined into a single executable file called a fat binary.
The use of different toolsets may not be enough to build a working executables for different platforms. In this case, programmers must "port" the source code to the new platform. For example, an application such as Firefox, which already runs on Windows on the x86 family, can be modified and re-built to run on Linux on the x86 (and potentially other architectures) as well. The multiple versions of the code may be stored as separate codebases, or merged into one codebase.
An alternative to porting is "cross-platform virtualization", where applications compiled for one platform can run on another without modification of the source code or binaries. As an example, Apple's Rosetta, which is built into Intel-based Macintosh computers, runs applications compiled for the previous generation of Macs that used PowerPC CPUs. Another example is IBM PowerVM Lx86, which allows Linux/x86 applications to run unmodified on the Linux/Power OS.
Example of cross-platform binary software:
Scripts and interpreted languages.
A script can be considered to be cross-platform if its interpreter is available on multiple platforms and the script only uses the facilities built into the language. For example, a script written in Python for a Unix-like system will likely run with little or no modification on Windows, because Python also runs on Windows; indeed there are many implementations (e.g. IronPython for .NET Framework). The same goes for many of the open-source scripting languages.
Unlike binary executable files, the same script can be used on all computers that have software to interpret the script. This is because the script is generally stored in plain text in a text file. There may be some trivial issues, such as the representation of a new line character.
Some popular cross-platform scripting languages are:
Video games.
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", "", "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. 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.
Cross-platform play.
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", "", "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.
Programming.
Cross-platform programming is the practice of deliberately writing software to work on more than one platform.
Approaches.
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.
Toolkits and environments.
There are many tools available to help the process of cross-platform programming:
Challenges.
There are many challenges when developing cross-platform software.