While writing my code I thought of having a common implementation for all POSIX OSes as opposed to separate implementation for each OS. One of the POSIX API I use is posix_fallocate() and while testing I found it not being supported by MacOS.
Had I known this earlier I would have not used this API or have had separate implementations for each OS.
So my question is - what is an easy way to find support for a particular posix call in different OSes? Do people always have to search documentation for each target OS?
Thanks.
Looking at the documentation is a good start, but it often won't tell you when a particular function has been implemented, which is also important. For obscure platforms, it may be difficult to tell which older versions are still relevant, which makes it even harder to decide whether a dependency on a particular POSIX feature is acceptable.
The other question is whether a feature is implemented, but with substandard quality. posix_fallocate is an interesting corner case in this regard. The glibc implementation uses emulation if the file system lacks support for an actual low-level fallocate operation (such as NFS until recently):
https://www.gnu.org/software/libc/manual/html_node/Storage-Allocation.html
Depending on what your application does, this behavior might not be acceptable. Just checking header files and documentation might not reveal this (the Note part in the documentation above was added only recently, for example).
In the end, there isn't a good substitute for building and testing as early as possible on all relevant targets, but I understand that this is increasingly difficult for non-Linux targets.
I have seen a lot of C/C++ Libraries and Gui Toolkits. Among them there are some like GTK+ , Qt , Swing etc. which claim to be platform-independent. While some , like WxWidgets, SWT etc. which claim to be cross-platform. At first I thought it to be just a change in wording, but the terms have been used with such consistency that I have started to wonder. What is the difference?
Cross-platform only implies that you support multiple platforms. It usually means Linux, Mac, and Windows. Platform-independent implies that you support any platform that your language supports- i.e., you depend on no behaviour that is not specified in the language specification. However that's just my personal opinion and most uses just take both of them to mean "multiple platforms", usually "Windows and ...".
For what its worth, this is what Wikipedia has to say on the matter:
In order for software to be considered cross-platform, it must be able to function on more than one computer architecture or operating system.
Software that is platform independent does not rely on any special features of any single platform, or, if it does, handles those special features such that it can deal with multiple platforms
"Platform-independent" would usually be a program written in a high-level language that doesn't need recompiling or adjusting for another platform and can just run "as is" (Java, most Perl scripts, etc.) as long as the particular platform implements the language runtime. This is possible when there are layers below the program that deal with different platforms (the VM or the interpreter).
"Cross-platform" implies that the program can be compiled/adjusted for multiple platforms, but not for any platform. Think of #ifdef's in C and C++ code.
Cross-platform means your program can work on different platforms example Windows, Linux, and Mac.
Platform Independent means your code is Write once Run Anywhere i.e. you don't need to change your code to run on different platforms
All platform independent programs are cross-platform but vice versa not true
We have a business logic that works with the file systems on OS that we want to implement on both Linux and Windows platforms. The language we have selected is Python for Linux and C# for Windows. GUI is not a priority for now. We were looking for ways to abstract the business logic in a way that we dont have to repeat the business logic (ofcourse I understand since it is related to file system, some code will differ from platform to platform).
Any ideas on how to implement it? Is C/C++ the only option. We dont want to use Java.
Thanks,
Pranz
yea, pick a common language for the logic first. Punting down in to C/C++ pretty much eliminates any of the real values to development that the Python and C# languages provide. Done write, MOST of your logic will be "Business Logic" with the rest glue to external services (i.e. databases, etc.).
So, you should pick a portable environment from the get go. Dropping down to C/C++ and linking it in is a viable alternative, but most likely not worth the time.
Mono is an option you'll probably want to look into.
Quote from the site for easy explanation:
Mono is a software platform designed to allow developers to easily create cross platform applications. Sponsored by Novell, Mono is an open source implementation of Microsoft's .NET Framework based on the ECMA standards for C# and the Common Language Runtime. A growing family of solutions and an active and enthusiastic contributing community is helping position Mono to become the leading choice for development of Linux applications.
Either use Mono or Python.
Mono allows you to run C# .NET code on both platforms. Python can be executed on both platforms already.
Qt has cross-platform libraries for all sorts of things, including UI and file system. It does, however, use C++.
I just caught one of Google's commercials for Chrome where at the end they mention that it runs on Linux, Mac, and PC. So I started wondering how they are able to develop a program that can run on multiple platforms like that? I have experience with Java, and .Net but only on a windows machine. Java is by design portable, but I wouldn't think Google is using Java for something like Chrome where performance is such a concern.
I understand that each version is going to have some platform specific code, such as for the UI. But there must also be some central code that is reused across each platform. What language is this written in?
Here is an entire article about the development of Chrome. It's mostly written in C++.
Chrome is written in C++, so they will have a significant amount of platform-specific code for each OS. They most likely maintain a separate branch for each OS.
Any reasonably standardized and popular language will do, because the goal is to compile it on all platforms, not create one binary that will work on Windows, MacOSX, Linux, and z/OS. C and C++ are popular choices, because they'll work readily with pretty much anything. Java is a good choice, because it runs on pretty much everything. For applications with low performance requirements, Perl and Python are good.
The important point is to separate out what is platform-dependent from what isn't, since (except in the case of Java or scripting languages) it will probably be necessary to rewrite platform-specific stuff for each platform, and not necessarily in the same language. MacOSX is best programmed in Objective-C, and that's true for no other popular platform.
The programs are supposed to be portable in source level, not binary level. So you only need to compile it for different platforms, not necessary to make one universal binary. In fact, most languages are supported in all modern platforms including UNIX/Windows/Mac, so you can choose from almost all modern lanagues, which C/C++ is prefered by many people. BTW, C++ is the language of Chrome.
Scripting languages like Python/Perl are also good choices. One more thing, Java can be faster than you think - see Eclipse. Even without JNI Technology, Java is still good enough for most applications like JDownloader.
Google had to build different distributions for each OS (ie compiled for each platform as Francis's answer explains) - in fact the Mac OS version only recently became available - the Windows version has been around much longer.
Google Apps are 'thin applications' - the grunt-work is done on their servers.
The Apps are portable in the sense that the front-end is put together using HTML, CSS and Javascript - which are standard (in theory at least) across all browsers.
Google put a lot of effort into building Chrome's Javascript engine to be performant - to ensure any client-side logic is run quickly.
I see here that there are a load of languages aside from Java that run on the JVM. I'm a bit confused about the whole concept of other languages running in the JVM. So:
What is the advantage in having other languages for the JVM?
What is required (in high level terms) to write a language/compiler for the JVM?
How do you write/compile/run code in a language (other than Java) in the JVM?
EDIT: There were 3 follow up questions (originally comments) that were answered in the accepted answer. They are reprinted here for legibility:
How would an app written in, say, JPython, interact with a Java app?
Also, Can that JPython application use any of the JDK functions/objects??
What if it was Jaskell code, would the fact that it is a functional language not make it incompatible with the JDK?
To address your three questions separately:
What is the advantage in having other languages for the JVM?
There are two factors here. (1) Why have a language other than Java for the JVM, and (2) why have another language run on the JVM, instead of a different runtime?
Other languages can satisfy other needs. For example, Java has no built-in support for closures, a feature that is often very useful.
A language that runs on the JVM is bytecode compatible with any other language that runs on the JVM, meaning that code written in one language can interact with a library written in another language.
What is required (in high level terms) to write a language/compiler for the JVM?
The JVM reads bytecode (.class) files to obtain the instructions it needs to perform. Thus any language that is to be run on the JVM needs to be compiled to bytecode adhering to the Sun specification. This process is similar to compiling to native code, except that instead of compiling to instructions understood by the CPU, the code is compiled to instructions that are interpreted by the JVM.
How do you write/compile/run code in a language (other than Java) in the JVM?
Very much in the same way you write/compile/run code in Java. To get your feet wet, I'd recommend looking at Scala, which runs flawlessly on the JVM.
Answering your follow up questions:
How would an app written in, say, JPython, interact with a Java app?
This depends on the implementation's choice of bridging the language gap. In your example, Jython project has a straightforward means of doing this (see here):
from java.net import URL
u = URL('http://jython.org')
Also, can that JPython application use any of the JDK functions/objects?
Yes, see above.
What if it was Jaskell code, would the fact that it is a functional language not make it incompatible with the JDK?
No. Scala (link above) for example implements functional features while maintaining compatibility with Java. For example:
object Timer {
def oncePerSecond(callback: () => unit) {
while (true) { callback(); Thread sleep 1000 }
}
def timeFlies() {
println("time flies like an arrow...")
}
def main(args: Array[String]) {
oncePerSecond(timeFlies)
}
}
You need other languages on the JVM for the same reason you need multiple programming languages in general: Different languages are better as solving different problems ... static typing vs. dynamic typing, strict vs. lazy ... Declarative, Imperative, Object Oriented ... etc.
In general, writing a "compiler" for another language to run on the JVM (or on the .Net CLR) is essentially a matter of compiling that language into java bytecode (or in the case of .Net, IL) instead of to assembly/machine language.
That said, a lot of the extra languages that are being written for JVM aren't compiled, but rather interpreted scripting languages...
Turning this on its head, consider you want to design a new language and you want it to run in a managed runtime with a JIT and GC. Then consider that you could:
(a) write you own managed runtime (VM) and tackle all sorts of technically difficult issues that will doubtless lead to many bugs, bad performance, improper threading and a great deal of portability effort
or
(b) compile your language into bytecode that can run on the Java VM which is already quite mature, fast and supported on a number of platforms (sometimes with more than one choice of vendor impementation).
Given that the JavaVM bytecode is not tied so closely to the Java language as to unduly restrict the type of language you can implement, it has been a popular target environment for languages that want to run in a VM.
Java is a fairly verbose programming language that is getting outdated very quickly with all of the new fancy languages/frameworks coming out in the past 5 years. To support all the fancy syntax that people want in a language AND preserve backwards compatibility it makes more sense to add more languages to the runtime.
Another benefit is it lets you run some web frameworks written in Ruby ala JRuby (aka Rails), or Grails(Groovy on Railys essentially), etc. on a proven hosting platform that likely already is in production at many companies, rather than having to using that not nearly as tried and tested Ruby hosting environments.
To compile the other languages you are just converting to Java byte code.
I would answer, “because Java sucks” but then again, perhaps that's too obvious … ;-)
The advantage to having other languages for the JVM is quite the same as the advantage to having other languages for computer in general: while all turing-complete languages can technically accomplish the same tasks, some languages make some tasks easier than others while other languages make other tasks easier. Since the JVM is something we already have the ability to run on all (well, nearly all) computers, and a lot of computers, in fact already have it, we can get the "write once, run anywhere" benefit, but without requiring that one uses Java.
Writing a language/compiler for the JVM isn't really different from writing one for a real machine. The real difference is that you have to compile to the JVM's bytecode instead of to the machine's executable code, but that's really a minor difference in the grand scheme of things.
Writing code for a language other than Java in the JVM really isn't different from writing Java except, of course, that you'll be using a different language. You'll compile using the compiler that somebody writes for it (again, not much different from a C compiler, fundamentally, and pretty much not different at all from a Java compiler), and you'll end up being able to run it just like you would compiled Java code since once it's in bytecode, the JVM can't tell what language it came from.
Different languages are tailored to different tasks. While certain problem domains fit the Java language perfectly, some are much easier to express in alternative languages. Also, for a user accustomed to Ruby, Python, etc, the ability to generate Java bytecode and take advantage of the JDK classes and JIT compiler has obvious benefits.
Answering just your second question:
The JVM is just an abstract machine and execution model. So targetting it with a compiler is just the same as any other machine and execution model that a compiler might target, be it implemented in hardware (x86, CELL, etc) or software (parrot, .NET). The JVM is fairly simple, so its actually a fairly easy target for compilers. Also, implementations tend to have pretty good JIT compilers (to deal with the lousy code that javac produces), so you can get good performance without having to worry about a lot of optimizations.
A couple of caveats apply. First, the JVM directly embodies java's module and inheritance system, so trying to do anything else (multiple inheritance, multiple dispatch) is likely to be tricky and require convoluted code. Second, JVMs are optimized to deal with the kind of bytecode that javac produces. Producing bytecode that is very different from this is likely to get into odd corners of the JIT compiler/JVM which will likely be inefficient at best (at worst, they can crash the JVM or at least give spurious VirtualMachineError exceptions).
What the JVM can do is defined by the JVM's bytecode (what you find in .class files) rather than the source language. So changing the high level source code language isn't going to have a substantial impact on the available functionality.
As for what is required to write a compiler for the JVM, all you really need to do is generate correct bytecode / .class files. How you write/compile code with an alternate compiler sort of depends on the compiler in question, but once the compiler outputs .class files, running them is no different than running the .class files generated by javac.
The advantage for these other languages is that they get relatively easy access to lots of java libraries.
The advantage for Java people varies depending on language -- each has a story tell Java coders about what they do better. Some will stress how they can be used to add dynamic scripting to JVM-based apps, others will just talk about how their language is easier to use, has a better syntax, or so forth.
What's required are the same things to write any other language compiler: parsing to an AST, then transforming that to instructions for the target architecture (byte code) and storing it in the right format (.class files).
From the users' perspective, you just write code and run the compiler binaries, and out comes .class files you can mix in with those your java compiler produces.
The .NET languages are more for show than actual usefulness. Each language has been so butchered, that they're all C# with a new face.
There are a variety of reasons to provide alternative languages for the Java VM:
The JVM is multiplatform. Any language ported to the JVM gets that as a free bonus.
There is quite a bit of legacy code out there. Antiquated engines like ColdFusion perform better while offering customers the ability to slowly phase their applications from the legacy solution to the modern solution.
Certain forms of scripting are better suited to rapid development. JavaFX, for example, is designed with rapid Graphical development in mind. In this way it competes with engines like DarkBasic. (Processing is another player in this space.)
Scripting environments can offer control. For example, an application may wish to expose a VBA-like environment to the user without exposing the underlying Java APIs. Using an engine like Rhino can provide an environment that supports quick and dirty coding in a carefully controlled sandbox.
Interpreted scripts mean that there's no need to recompile anything. No need to recompile translates into a more dynamic environment. e.g. Despite OpenOffice's use of Java as a "scripting language", Java sucks for that use. The user has to go through all kinds of recompile/reload gyrations that are unnecessary in a dynamic scripting environment like Javascript.
Which brings me to another point. Scripting engines can be more easily stopped and reloaded without stopping and reloading the entire JVM. This increases the utility of the scripting language as the environment can be reset at any time.
It's much easier for a compiler writer to generate JVM or CLR byte-codes. They are a much cleaner and higher level abstraction than any machine language. Because of this, it is much more feasible to experiment with creating new languages than ever before, because all you have to do is target one of these VM architectures and you will have a set of tools and libraries already available for your language. They let language designers focus more on the language than all the necessary support infrastructure.
Because the JSR process is rendering Java more and more dead: http://www.infoq.com/news/2009/01/java7-updated
It's a shame that even essential and long known additions like Closures are not added just because the members cannot agree on an implementation.
Java has accumulated a massive user base over seven major versions (from 1.0 to 1.6). Its capability to evolve is limited by the need to preserve backwards compatibility for the uncountable millions of lines of Java code running in production.
This is a problem because Java needs to evolve to:
compete with newer programming languages that have learned from Java's successes and failures.
incorporate new advances in programming language design.
allow users to take full advantage of advances in hardware - e.g. multi-core processors.
fix some cutting edge ideas that introduced unexpected problems (e.g. checked exceptions, generics).
The requirement for backwards compatibility is a barrier to staying competitive.
If you compare Java to C#, Java has the advantage in mature, production ready libraries and frameworks, and a disadvantage in terms of language features and rate of increase in market share. This is what you would expect from comparing two successful languages that are one generation apart.
Any new language has the same advantage and disadvantage that C# has compared to Java to an extreme degree. One way of maximizing the advantage in terms of language features, and minimizing the disadvantage in terms of mature libraries and frameworks is to build the language for an existing virtual machine and make it interoperable with code written for that virtual machine. This is the reason behind the modest success of Groovy and Clojure; and the excitement around Scala. Without the JVM these languages could only ever have occupied a tiny niche in a very specialized market segment, whereas with the JVM they occupy a significant niche in the mainstream.
They do it to keep up with .Net. .Net allows C#, VB, J# (formerly), F#, Python, Ruby (coming soon), and c++. I'm probably missing some. Probably the big one in there is Python, for the scripting people.
To an extent it is probably an 'Arms Race' against the .NET CLR.
But I think there are also genuine reasons for introducing new languages to the JVM, particularly when they will be run 'in parallel', you can use the right language for the right job, a scripting language like Groovy may be exactly what you need for your page presentation, whereas regular old Java is better for your business logic.
I'm going to leave someone more qualified to talk about what is required to write a new language/compiler.
As for how to writing code, you do it in notepad/vi as usual! (or use a development tool that supports the language if you want to do it the easy way.) Compiling will require a special compiler for the language that will interpret and compile it into bytecode.
Since java also produces bytecode technically you don't need to do anything special to run it.
The reason is that the JVM platform offers a lot of advantages.
Giant number of libraries
Broader degree of platform
implementations
Mature frameworks
Legacy code that's
already part of your infrastructure
The languages Sun is trying to support with their Scripting spec (e.g. Python, Ruby) are up and comers largely due to their perceived productivity enhancements. Running Jython allows you to, in theory, be more productive, and leverage the capabilities of Python to solve a problem more suited to Python, but still be able to integrate, on a runtime level, with your existing codebase. The classic implementations of Python and Ruby effect the same ability for C libraries.
Additionally, it's often easier to express some things in a dynamic language than in Java. If this is the case, you can go the other way; consume Python/Ruby libraries from Java.
There's a performance hit, but many are willing to accept that in exchange for a less verbose, clearer codebase.