Cross-platform software

In computing, cross-platform software (also multi-platform software or platform-independent software) is computer software that is implemented on multiple computing platforms. Cross-platform software may be divided into two types; one requires individual building or compilation for each platform that it supports, and the other one can be directly run on any platform without special preparation, e.g., software written in an interpreted language or pre-compiled portable bytecode for which the interpreters or run-time packages are common or standard components of all platforms.
For example, a cross-platform application may run on Microsoft Windows, Linux, and macOS. Cross-platform programs may run on as many as all existing platforms, or on as few as two platforms. Cross-platform frameworks (such as Codename One, Kivy, Qt, Flutter, NativeScript, Xamarin, Phonegap, Ionic, and React Native) exist to aid cross-platform development.
Platforms.
"Platform" can refer to the type of processor (CPU) or other hardware on which a given operating system or application runs, the type of operating system on a computer or the combination of the type of hardware and the type of operating system running on it. An example of a common platform is Microsoft Windows running on the x86 architecture. Other well-known desktop computer platforms include Linux/Unix and macOS - both of which are themselves cross-platform. There are, however, many devices such as smartphones that are also effectively computer platforms but less commonly thought about in that way. Application software can be written to depend on the features of a particular platform—either the hardware, operating system, or virtual machine it runs on. The Java platform is a virtual machine platform which runs on many operating systems and hardware types, and is a common platform for software to be written for.
Hardware platforms.
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 operating systems as well, 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 platforms.
Software platforms can either be an operating system or programming environment, though more commonly it is a combination of both. A notable exception to this is Java, which uses an operating system independent virtual machine for its compiled code, known in the world of Java as bytecode. Examples of software platforms are:
Java platform.
As previously noted, the Java platform is an exception to the general rule that an operating system is a software platform. The Java language typically compiles to a virtual machine: a virtual CPU which runs all of the code that is written for the language. This enables the same executable binary to run on all systems that implement a Java Virtual Machine (JVM). Java programs can be executed natively using a Java processor. This isn't common and is mostly used for 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 setup allows 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. JNI can also be used to enable access to operating system specific functions.
Currently, Java Standard Edition programs can run on Microsoft Windows, macOS, several Unix-like operating systems, and several more non-UNIX-like operating systems like embedded systems. 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.
Implementations.
For a piece of software to be considered cross-platform, it must be able to function on more than one computer architecture or operating system. Developing such a program can be a time-consuming task because different operating systems have different application programming interfaces (API). For example, Linux uses a different API for application software than Windows does.
Software written for a particular operating system does not automatically work on all architectures that operating system supports. One example as of August 2006 was OpenOffice.org, which did not natively run on the AMD64 or Intel 64 lines of processors implementing the x86-64 standards for computers; this has since been changed, and the OpenOffice.org suite of software is “mostly” ported to these 64-bit systems. This also means that just because a program is written in a popular programming language such as C or C++, it does not mean it will run on all operating systems that support that programming language—or even on the same operating system on a different architecture.
Web applications.
Web applications are typically described as cross-platform because, ideally, they are accessible from any of various web browsers within different operating systems. Such applications generally employ a client–server system architecture, and vary widely in complexity and functionality. This wide variability significantly complicates the goal of cross-platform capability, which is routinely at odds with the goal of advanced functionality.
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. These types of applications were 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 functionalities.
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 advanced applications routinely depend on additional features found only in the more recent versions of popular web browsers. These dependencies include Ajax, JavaScript, Dynamic HTML, SVG, and other components of rich web applications. Older versions of popular web browsers tend to lack support for certain features.
Design strategies.
Because of the competing interests of cross-platform compatibility and advanced functionality, numerous alternative web application design strategies have emerged. Such strategies include:
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. This differs from other cross-platform techniques, which attempt to provide equivalent functionality, not just adequate functionality, across platforms.
Multiple codebase applications maintain distinct codebases for different (hardware and OS) platforms, with equivalent functionality. This obviously requires a duplication of effort in maintaining 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 client browsers or operating systems, 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. 
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 strategies.
One complicated aspect of cross-platform web application design is the need for software testing. In addition to the complications mentioned previously, there is the additional restriction that some web browsers prohibit installation of different versions of the same browser on the same operating system. Although, there are several development approaches that companies use to target multiple platforms, all of them result in software that requires substantial manual effort for testing and maintenance across the supported platforms. Techniques such as full virtualization are sometimes used as a workaround for this problem.
Using tools such as the Page Object Model, cross platform tests can be scripted in such a way that one test case is usable for multiple versions of an app. So long as the different versions have similar user interfaces, both versions can be tested at one time, 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.
Traditionally in modern computing, application software has been distributed to end-users as binary file, especially executable files. Executables only support the operating system and computer architecture that they were built for—which means that making a single cross-platform executable would be something of a massive task, and is generally replaced by offering a selection of executables for the platforms supported.
For software that is distributed as a binary executable, such as software written in C or C++, the programmer must "build the software" for each different operating system and computer architecture, i.e. must use 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 from the same source code.
The use of different toolsets to perform different builds may not be sufficient to achieve a variety of working executables for different platforms. In this case, the software engineer must "port" it, i.e. amend the code to be suitable to a new computer architecture or operating system. For example, a program 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 by conditional compilation (see above). Note that, while
porting must be accompanied by cross-platform building, the reverse is not the case.
As an alternative to porting, "cross-platform virtualization" allows applications compiled for one CPU and operating system to run on a system with a different CPU and/or operating system, without modification to 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 operating system.
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 provided by the language. That is, 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; there is also more than one implementation of Python that will run the same scripts (e.g., IronPython for .NET Framework). The same goes for many of the open-source programming languages that are available and are 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 issues, however, such as the type of new line character that sits between the lines. Generally, however, little or no work has to be done to make a script written for one system, run on another.
Some quite popular cross-platform scripting or interpreted 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, specialized computers dedicated to the task of playing games. Examples of cross-platform video 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 (PCs), and mobile devices.
The characteristics of a particular system may lengthen the time taken to implement a video game across multiple platforms. So, a video game may initially be released on a few platforms and then later released on remaining platforms. Typically, this situation occurs when a new gaming system is released, because video game developers need to acquaint themselves with the hardware and software associated with the new console.
Some games may not become cross-platform because of licensing agreements between developers and video game console manufacturers that limit development of a game 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, Disney may in exchange be required to release the game solely on Sony's console for a short time or indefinitely—effectively prohibiting a cross-platform release for the duration.
Cross-platform play.
Several developers have implemented means 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.
Cross-platform programming.
Cross-platform programming is the practice of actively writing software that will work on more than one platform.
Approaches to cross-platform programming.
There are different ways of approaching the problem of writing a cross-platform application program. One such approach is simply to create multiple versions of the same program in different "source trees"—in other words, the Microsoft Windows version of a program might have one set of source code files and the Macintosh version might have another, while a FOSS *nix system might have another. While this is a straightforward approach to the problem, it has the potential to be considerably more expensive in development cost, development time, or both, especially for corporate entities. The idea behind this is to create more than two different programs that have the ability to behave similarly to each other. It is also possible that this means of developing a cross-platform application will result in more problems with bug tracking and fixing, because the two different "source trees" would have different programmers, and thus different defects in each version.
Another approach that is used is to depend on pre-existing software that hides the differences between the platforms—called abstraction of the platform—such that the program itself is unaware of the platform it is running on. It could be said that such programs are "platform agnostic". Programs that run on the Java Virtual Machine (JVM) are built in this fashion.
Some applications mix various methods of cross-platform programming to create the final application. An example of this is the Firefox web browser, which uses abstraction to build some of the lower-level components, separate source subtrees for implementing platform-specific features (like the GUI), and the implementation of more than one scripting language to help facilitate ease of 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, as well.
Cross-platform programming toolkits and environments.
There are a number of tools which are available to help facilitate the process of cross-platform programming:
Challenges to cross-platform development.
There are certain issues associated with cross-platform development. Some of these include: