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rafi1228 · 5 years ago

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Learn Java In This Course And Become a Computer Programmer. Obtain valuable Core Java Skills And Java Certification

What you’ll learn

Learn the core Java skills needed to apply for Java developer positions in just 14 hours.

Be able to sit for and pass the Oracle Java Certificate exam if you choose.

Be able to demonstrate your understanding of Java to future employers.

Learn industry “best practices” in Java software development from a professional Java developer who has worked in the language for 18 years.

Acquire essential java basics for transitioning to the Spring Framework, Java EE, Android development and more.

Obtain proficiency in Java 8 and Java 11.

Requirements

A computer with either Windows, Mac or Linux to install all the free software and tools needed to build your new apps (I provide specific videos on installations for each platform).

A strong work ethic, willingness to learn, and plenty of excitement about the awesome new programs you’re about to build.

Nothing else! It’s just you, your computer and your hunger to get started today.

Description

You’ve just stumbled upon the most complete, in-depth Java programming course online. With over 260,000 students enrolled and tens of thousands of 5 star reviews to date, these comprehensive java tutorials cover everything you’ll ever need.

Whether you want to:

– build the skills you need to get your first Java programming job

– move to a more senior software developer position

– pass the oracle java certification exam

– or just learn java to be able to create your own java apps quickly.

…this complete Java Masterclass is the course you need to do all of this, and more.

Are you aiming to get your first Java Programming job but struggling to find out what skills employers want and which course will give you those skills?

This course is designed to give you the Java skills you need to get a job as a Java developer. By the end of the course you will understand Java extremely well and be able to build your own Java apps and be productive as a software developer.

Lots of students have been success with getting their first job or a promotion after going through the course.

Here is just one example of a student who lost her job and despite having never coded in her life previously, got a full time software developer position in just a few months after starting this course. She didn’t even complete the course!

“Three months ago I lost my job, came to a turning point in my life, and finally made the drastic decision to completely change course. I decided to switch career path and go into coding. My husband found and gave me your Complete Java Masterclass at Udemy as a gift, and I wholeheartedly dove into it as a life line. Following your course has been absolutely enjoyable (still working on it, not yet finished), and has been a great way of keeping on course, dedicated and motivated. Yesterday, three months after starting the course and honestly to my surprise, I received (and accepted!) a job offer as a full-time developer. I wanted to just drop you a line to say thank you for doing this work, for being such a dedicated teacher, and for putting all this knowledge available out there in such an approachable way. It has, literally, been life changing. With gratitude, Laura”

The course is a whopping 76 hours long. Perhaps you have looked at the size of the course and are feeling a little overwhelmed at the prospect of finding time to complete it. Maybe you are wondering if you need to go through it all?

Firstly, Laura’s story above shows that you do not have to complete the entire course – she was yet to complete the course when she accepted her developer job offer.

Secondly, the course is designed as a one stop shop for Java.

The core java material you need to learn java development is covered in the first seven sections (around 14 hours in total). The Java Basics are covered in those sections. The rest of the course covers intermediate, advanced and optional material you do not technically need to go through.

For example section 13 is a whopping 10 hours just by itself and is aimed at those students who want to build desktop applications with graphical user interfaces. JavaFX (which is the technology used in this section) is something that most java developers will rarely or never need to work on. So you could skip that section entirely. But if you are one of the few that need to build user interfaces, then the content is there and ready for you. And there are other sections you can completely avoid if you wish.

If you want to know absolutely everything about Java, then you can go through the entire course if you wish, but it’s not necessary to do so if you are just looking to learn the essential information to get a java developer position.

Why would you choose to learn Java?

The reality is that there is a lot of computer languages out there. It’s in the hundreds. Why would you choose the Java language?

The number one reason is its popularity. According to many official websites that track popularity of languages, Java is either #1 or in the top 3. Popularity means more companies and their staff are using it, so there are more career opportunities available for you if you are skilled in the language.

The last thing you want to do is pick a language that is not in mainstream use. Java came out in the 1990’s and is still very popular today.

What version of Java should you learn?

Generally speaking you would want to learn the very latest version of a computer programming language, but thats not necessarily the case with Java.

Until recently Java releases were infrequent (one major release in 3 years was common). Companies standardised on specific versions of Java. Right now most companies are still focused on Java 8, which is a relatively old version, dating back to 2015.

Oracle (the owners of Java) are now releasing new versions of Java every six months, and when the new version comes out the old version is no longer supported.

But to cater for most companies who tend to stick to specific versions of Java for a long time, they have marked the current version of Java – Java 11 as LTS – or Long Term support. That means that they guarantee to support this version for the long term – for a number of years at least.

Companies will stick to versions of Java that are supported in the long term. For career purposes you should learn the appropriate versions of Java that your future employer will likely be using. Right now thats Java 8 and Java 11 (Java 9 and Java 10 have been released and already been marked obsolete and are no longer supported).

The good news is that this course is focused on Java 8, and has recently been updated for Java 11.

Will this course give me core java skills?

Yes it will. Core Java is the fundamental parts of the java jdk (the java development kit) that programmers need to learn to move onto other more advanced technologies.

Why should you take this course?

It’s been a best seller since it’s release on Udemy, you would be joining over 260,000 students who are already enrolled in the course.

There are close to 60,000 reviews left by students. It’s rated as the best course to learn Java for beginners.

What makes this course a bestseller?

Like you, thousands of others were frustrated and fed up with fragmented Youtube tutorials or incomplete or outdated courses which assume you already know a bunch of stuff, as well as thick, college-like textbooks able to send even the most caffeine-fuelled coder to sleep.

Like you, they were tired of low-quality lessons, poorly explained topics and all-round confusing info presented in the wrong way. That’s why so many find success in this complete Java developer course. It’s designed with simplicity and seamless progression in mind through its content.

This course assumes no previous coding experience and takes you from absolute beginner core concepts, like showing you the free tools you need to download and install, to writing your very first Java program. You will learn the core java skills you need to become employable in around 14 hours, and if you choose to, can take advantage of all the additional content in the course. It’s a one stop shop to learn java. If you want to go beyond the core content you can do so at any time.

Here’s just some of what you’ll learn

(It’s okay if you don’t understand all this yet, you will in the course)

All the essential Java keywords, operators, statements, and expressions needed to fully understand exactly what you’re coding and why – making programming easy to grasp and less frustrating

You will learn the answers to questions like What is a Java class, What is polymorphism and inheritance and to apply them to your java apps.

How to safely download and install all necessary coding tools with less time and no frustrating installations or setups

Complete chapters on object-oriented programming and many aspects of the Java API (the protocols and tools for building applications) so you can code for all platforms and derestrict your program’s user base (and potential sales)

How to develop powerful Java applications using one of the most powerful Integrated Development Environments on the market, IntelliJ IDEA! – Meaning you can code functional programs easier. IntelliJhas both a FREE and PAID version, and you can use either in this course.

(Don’t worry if you’re used to using Eclipse, NetBeans or some other IDE. You’re free to use any IDE and still get the most out of this course)

Learn Java to a sufficient level to be a be to transition to core Java technologies like Android development, the Spring framework, Java EE (Enterprise edition) in general as well as and other technologies. In order to progress to these technologies you need to first learn core Java – the fundamental building blocks. That’s what this course will help you to achieve.

“AP-what?”

Don’t worry if none of that made sense. I go into great detail explaining each and every core concept, programming term, and buzzwords you need to create your own Java programs.

This truly is Java for complete beginners.

By the end of this comprehensive course, you’ll master Java programming no matter what level of experience you’re at right now. You’ll understand what you are doing, and why you are doing it. This isn’t a recipe book, you’ll use your own creativity to make unique, intuitive programs.

Not only do these HD videos show you how to become a programmer in great detail, but this course includes a unique challenge feature. Each time a core concept is taught, a video presents a challenge for you to help you understand what you have just learned in a real world scenario.

You’ll go and complete the challenge on your own, then come back and see the answers which I then explain in detail in a video, allowing you to check your results and identify any areas you need to go back and work on.

This is a proven way to help you understand Java faster and ensure you reach your goal of becoming a Java Developer in record time. Remember doing those old past exam papers in high school or college? It’s the same concept, and it works.

As your instructor, I have over 35 years experience as a software developer and teacher and have been using Java since the year 2000. Yes, over 18 years (I’ve taught students younger than that). Meaning not only can I teach this content with great simplicity, but I can make it fun too!

It’s no surprise my previous students have amazing results…

See what your fellow students have to say:

“This course was a guiding light in my “Becoming a developer” path from the first step. It helped me become a much more educated developer comparing to my friend who learned to code from trial/error. It’s still a guide for me. every now and then I will come back to this course to learn something new or to improve what I’ve learned somewhere else. A BIG Thanks to “Tim Buchalka” my Master.” – Sina Jz

“I was an absolute beginner when I started this course, and now I can write some good small advanced clean codes. I wrote a code and showed it to a programmer, and he was shocked, he told me that I’m more than ready to start a programming career.” – Amirreza Moeini

“I am taking this class in conjunction with a Java 101 college class. I have learned more in one afternoon of videos from this class than I have in 4 weeks of college class. Tim actually explains what things are and why they do what they do, as opposed to my college instructor that mainly said “go make a program that does *whatever*” and then I had to figure out a program that would meet those requirements but not actually learning why it worked.” – Stacy Harris

It’s safe to say my students are thrilled with this course, and more importantly, their results, and you can be too…

This complete Java course will teach you everything you need to know in order to code awesome, profitable projects,

Is the course updated?

It’s no secret how technology is advancing at a rapid rate. New, more powerful hardware and software are being released every day, meaning it’s crucial to stay on top with the latest knowledge.

A lot of other courses on Udemy get released once, and never get updated. Learning an older version of Java can be counter productive – you could will be learning the “old way” of doing things, rather than using current technology.

Make sure you check the last updated date on the page of any course you plan to buy – you will be shocked to see some have not been updated for years.

That’s why I’m always adding new, up-to-date content to this course at no extra charge. Buy this course once, and you’ll have lifetime access to it and any future updates (which are on the way as we speak).

I’ve continued to do this since the original version of the course came out, and recently have been updating it to Java 11.

With this complete Java Masterclass, you will always have updated, relevant content.

What if I have questions?

As if this course wasn’t complete enough, I offer full support, answering any questions you have 7 days a week (whereas many instructors answer just once per week, or not at all).

This means you’ll never find yourself stuck on one lesson for days on end. With my hand-holding guidance, you’ll progress smoothly through this course without any major roadblocks.

That’s just one reason why I was voted top 10 in the Udemy instructor awards (out of a whopping 18,000 instructors), and quickly became a top-rated, bestselling instructor on the Udemy site.

Student Quote: “This course is a great place to ask questions if you have them or find help if you become stuck in areas.” – Blake S.

There’s no risk either!

This course comes with a full 30 day money-back guarantee. Meaning if you are not completely satisfied with the course or your progress, simply let me know and I’ll refund you 100%, every last penny no questions asked.

You either end up with Java skills, go on to develop great programs and potentially make an awesome career for yourself, or you try the course and simply get all your money back if you don’t like it…

You literally can’t lose.

Ready to get started, developer?

Enrol now using the “Add to Cart” button on the right, and get started on your way to creative, advanced Java brilliance. Or, take this course for a free spin using the preview feature, so you know you’re 100% certain this course is for you.

See you on the inside (hurry, Java class is waiting!)

Who this course is for:

This course is perfect for absolute beginners with no previous coding experience, to intermediates looking to sharpen their skills to the expert level.

Those looking to build creative and advanced Java apps for either personal use or for high-paying clients as a self-employed contractor.

Those who love letting their own creative genius shine, whilst getting paid handsome amounts to do so.

Created by Tim Buchalka, Tim Buchalka’s Learn Programming Academy, Goran Lochert Last updated 7/2019 English English

Size: 37.73 GB

Download Now

https://ift.tt/1lFyAPe.

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robertbryantblog · 6 years ago

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ourdelicatestudentpost-blog · 8 years ago

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ralphmorgan-blog1 · 8 years ago

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Blockchains are the new Linux, not the new Internet

Cryptocurrencies are booming beyond belief. Bitcoin is up sevenfold, to $2,500, in the last year. Three weeks ago the redoubtable Vinay Gupta, who led Ethereums initial release, published an essay entitled What Does Ether At $100 Mean? Since then it has doubled. Too many altcoins to name have skyrocketed in value along with the Big Two. ICOs are raking in money hand over fist over bicep. What the hell is going on?

(eta: in the whopping 48 hours since I first wrote that, those prices have tumbled considerably, but are still way, way up for the year.)

A certain seductive narrative has taken hold, is what is going on. This narrative, in its most extreme version, says that cryptocurrencies today are like the Internet in 1996: not just new technology but a radical new kind of technology, belittled or ignored by by most, which has slowly and subtly grown in power and influence over the last several years, and is about to explode into worldwide relevance and importance with shocking speed and massive repercussions.

(Lest you think Im overstating this, I got a PR pitch the other day which literally began: Blockchains 1996 Internet moment is here, as a preface to touting a $33 million ICO. Hey, whats $33 million between friends? Its now pretty much taken as given that were in a cryptocoin bubble.)

I understand the appeal of this narrative. Im no blockchain skeptic. Ive been writing about cryptocurrencies with fascination for six years now. Ive been touting and lauding the power of blockchains, how they have the potential to make the Internet decentralized and permissionless again, and to give us all power over our own data, for years. Im a true believer in permissionless money like Bitcoin. I called the initial launch of Ethereum a historic day.

But I cant help but look at the state of cryptocurrencies today and wonder where the actual value is. I dont mean financial value to speculators; I mean utility value to users. Because if nobody wants to actually use blockchain protocols and projects, those tokens which are supposed to reflect their value are ultimately well worthless.

Bitcoin, despite its ongoing internal strife, is very useful as permissionless global money, and has a legitimate shot at becoming a global reserve and settlement currency. Its anonymized descendants such as ZCash have added value to the initial Bitcoin proposition. (Similarly, Litecoin is now technically ahead of Bitcoin, thanks to the aforementioned ongoing strife.) Ethereum is very successful as a platform for developers.

But still, eight years after Bitcoin launched, Satoshi Nakamoto remains the only creator to have built a blockchain that an appreciable number of ordinary people actually want to use. (Ethereum is awesome, and Vitalik Buterin, like Gupta, is an honest-to-God visionary, but it remains a tool / solution / platform for developers.) No other blockchain-based software initiative seems to be at any real risk of hockey-sticking into general recognition, much less general usage.

With all due respect to Fred Wilson, another true believer and, to be clear, an enormous amount of respect is due it says a lot that, in the midst of this massive boom, hes citing Rare Pepe Cards, of all things, as a prime example of an interesting modern blockchain app. I mean, if thats the state of the art

Maybe Im wrong; maybe Rare Pepe will be the next Pokmon Go. But on the other hand, maybe the ratio of speculation to actual value in the blockchain space has never been higher, which is saying a lot.

Some people argue that the technology is so amazing, so revolutionary, that if enough money is invested, the killer apps and protocols will come. That could hardly be more backwards. Im not opposed to token sales, but they should follow If you build something good enough, investors will flock to you, not If enough investors flock to us, we will build something good enough.

A solid team working on an interesting project which hasnt hit product-market fit should be able to raise a few million dollars or, if you prefer, a couple of thousand bitcoin and then, once their success is proven, they might sell another tranche of now-more-valuable tokens. But projects with hardly any users, and barely any tech, raising tens of millions? That smacks of a bubble made of snake oil one all too likely to attract the heavy and unforgiving hand of the SEC.

That seductive narrative though! The Internet in 1996! I know. But hear me out. Maybe the belief that blockchains today are like the Internet in 1996 is completely wrong. Of course all analogies are flawed, but theyre useful, theyre how we think and maybe there is another, more accurate, and far more telling, analogy here.

I propose a counter-narrative. I put it to you that blockchains today arent like the Internet in 1996; theyre more like Linux in 1996. That is in no way a dig but, if true, its something of a death knell for those who hope to profit from mainstream usage of blockchain apps and protocols.

Decentralized blockchain solutions are vastly more democratic, and more technically compelling, than the hermetically-sealed, walled-garden, Stack-ruled Internet of today. Similarly, open-source Linux was vastly more democratic, and more technically compelling, than the Microsoft and Apple OSes which ruled computing at the time. But nobody used it except a tiny coterie of hackers. It was too clunky; too complicated; too counterintuitive; required jumping through too many hoops and Linuxs dirty secret was that the mainstream solutions were, in fact, actually fine, for most people.

Sound familiar? Today theres a lot of work going into decentralized distributed storage keyed on blockchain indexes; Storj, Sia, Blockstack, et al. This is amazing, groundbreaking work but why would an ordinary person, one already comfortable with Box or Dropbox, switch over to Storj or Blockstack? The centralized solution works just fine for them, and, because its centralized, they know who to call if something goes wrong. Blockstack in particular is more than just storage but what compelling pain point is it solving for the average user?

The similarities to Linux are striking. Linux was both much cheaper and vastly more powerful than the alternatives available at the time. It seemed incredibly, unbelievably disruptive. Neal Stephenson famously analogized 90s operating systems to cars. Windows was a rattling lemon of a station wagon; MacOS was a hermetically sealed Volkswagen Beetle; and then, weirdly beyond weirdly there was

Linux, which is right next door, and which is not a business at all. Its a bunch of RVs, yurts, tepees, and geodesic domes set up in a field and organized by consensus. The people who live there are making tanks. These are not old-fashioned, cast-iron Soviet tanks; these are more like the M1 tanks of the U.S. Army, made of space-age materials and jammed with sophisticated technology from one end to the other. But they are better than Army tanks. Theyve been modified in such a way that they never, ever break down, are light and maneuverable enough to use on ordinary streets, and use no more fuel than a subcompact car. These tanks are being cranked out, on the spot, at a terrific pace, and a vast number of them are lined up along the edge of the road with keys in the ignition. Anyone who wants can simply climb into one and drive it away for free.

Customers come to this crossroads in throngs, day and night. Ninety percent of them go straight to the biggest dealership and buy station wagons They do not even look at the other dealerships.

I put it to you that just as yesterdays ordinary consumers wouldnt use Linux, todays wont use Bitcoin and other blockchain apps, even if Bitcoin and the the other apps build atop blockchains are technically and politically amazing (which some are.) I put it to you that the year of widespread consumer use of [Bitcoin | Ripple | Stellar | ZCash | decentralized ether apps | etc] is perhaps analogous to the year of [Ubuntu | Debian | Slackware | Red Hat | etc] on the desktop.

Please note: this is not a dismissive analogy, or one which in any way understates the potential eventual importance of the technology! There are two billion active Android devices out there, and every single one runs the Linux kernel. When they communicate with servers, aka the cloud, they communicate with vast, warehouse-sized data centers teeming with innumerable Linux boxes. Linux was immensely important and influential. Most of modern computing is arguably Linux-to-Linux.

Its very easy to imagine a similar future for blockchains and cryptocurrencies. To quote my friend Shannon: It [blockchain tech] definitely seems like it has a Linux-like adoption arc ahead of it: Theres going to be a bunch of doomed attempts to make it a commercially-viable consumer product while it gains dominance in vital behind-the-scenes applications.

But if your 1996 investment thesis had been that ordinary people would adopt Linux en masse over the next decade which would not have seemed at all crazy then you would have been in for a giant world of hurt. Linux did not become important because ordinary people used it. Instead it became commodity infrastructure that powered the next wave of the Internet.

Its easy to envision how and why an interwoven mesh of dozens of decentralized blockchains could slowly, over a period of years and years, become a similar category of crucial infrastructure: as a reserve/settlement currency, as replacements for huge swathes of todays financial industry, as namespaces (such as domain names), as behind-the-scenes implementations of distributed storage systems, etc. while ordinary people remain essentially blissfully unaware of their existence. Its even easy to imagine them being commoditized. Does Ethereum gas cost too much? No problem; just switch your distributed system over to another, cheaper, blockchain.

So dont tell me this is like the Internet in 1996, not without compelling evidence. Instead, wake me up when cryptocurrency prices begin to track the demonstrated underlying value of the apps and protocols built on their blockchains. Because in the interim, in its absence of that value, Im sorry to say that instead we seem to be talking about decentralized digital tulips.

Disclosure, since it seems requisite: I mostly avoid any financial interest, implicit or explicit, long or short, in any cryptocurrency, so that I can write about them sans bias. I do own precisely one bitcoin, though, which I purchased a couple of years ago because I felt silly not owning any while I was advising a (since defunct) Bitcoin-based company.

More From this publisher : HERE

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thothonegan · 8 years ago

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A bug, how processes work, a history of FDs and C, forking gotchas, race conditions that aren't and general Insanity.

So I just had a fun bug to track down. Was going to have an early day and just finish up builds for a release, when I hit a bug doing my OSX build. I've had this bug for probably a month, but it was easy to work around so I've been ignoring it. So I decided to fix it, not realizing it'd take 5 hours and the rest of my sanity. This is the true story. No names were changed.

I have a tool called WolfRez, which is a resource compiler. Its kindof like a specialized make : you hand it a directory, and it uses a bunch of rules in the filesystem to run commands on the resources. A lot are just copying to where they should be, but it will also do things like compress YAML and Wexpr files, crush PNGs, encode WAV files as Opus (including adding song tags), compile lua files, etc.

So for example, this rule tells it to crush all .png files it finds, and the one below is compiling .lua files into .luac files:

- inputGlob: "**/Resources/Endless.tek/**.png" rule: pngCrush # no outputExt: dont change the filename - inputGlob: "**.lua" rule: luaCompile notOSTargets: [ android, ios, emscripten ] outputRegexpCapture: ".*(?'capture'\\.lua)$" outputRegexpReplace: ".luac"

A few months ago, I was tired of its performance not being the best, so I had it multithread its jobs. With the 2500 files currently in Endless's resource directory, and Demon (my computer) having 24 cores, you can easily see it can get a massive speedup. And resources are embarassingly parallel - no real interdependencies at the moment or anything. So I added it, using WolfTask (which is a nice easy task system), and with a few mutexes/semaphores added, it worked perfectly!

Well it did on Linux and Windows at least.

OSX had this odd thing where occassionly it would hang. The log would say it was finished (or close to finished), but it would never finish. In debug mode or manual builds it'd be fine for some reason, but when I did my release builds (which is pretty much automated cause I'm lazy) it almost always would just hang. So of course my first thought is a race condition. Some task isn't reporting its completion properly, or two are stomping on each other somewhere.

And if I ran it again (which 95% of the resources were already compiled from the failed run), it'd be perfectly fine finishing the last few. Weird.

So first, lets pull out our trusty helgrind! Helgrind is a module for valgrind which lets it detect race conditions. Helgrind is really slow, but I have my laptop so lets run it on Linux while we're looking on OSX. Maybe it'll point out the race condition and it'll be easy!

Hint: Nope. Took about an hour, pointed out some races between console statements (that arent a problem in practice since they're in error handling) but no easy race problems under Linux. Damn. Must be platform specific.

So time for debugging! With my trusty console statements, I confirmed that the TaskGroup all the tasks were a part of was getting all of them added, yet it was never being completed. Welp, lets start throwing statements inside TaskGroup. Its old code, it probably fell down right?

An hour later : nope. Its old code, but thats cause its good code.

Instead it seems like the handler isn't ever finishing. Errr. what?

auto task = taskSource->createTask([iter,&outputs,&failedDuringBuilding](WolfTask::Task* /*task*/) { auto output = iter.value; BuildProgress progress; progress.currentOutput = iter.index; progress.numberOfOutputs = outputs.count(); // code made it here... but never left buildIfNeeded() if (!output->buildIfNeeded(progress)) { console.error() << "Error when building" << WolfConsole::EndLine; failedDuringBuilding = true; return; } });

Okkkkk so its a bug inside buildIfNeeded() that only happens on OSX RelWithDebInfo sometimes. And causes hangs. And never was an issue before. Weird, but ok - start tracing through that. At this point I had like 600 console statements outputting task pointers and stuff, and a lot of it interleaving (cause console is not thread safe on purpose). Finally narrowed it down to basically two different tasks that seem to not complete : luac tasks (compiles lua files into binary form), and pngcrush tasks (tries to make png files as small as possible). No pattern at all of which specific job though.

So lets do some tracing within the pngcrush rule!

WolfType::Bool PNGCrushRule::runWithAction (Action* action, const WolfYAML::Node& customProperties) { // ... elided : set up params, etc ... // code makes it fine here auto res = RuleHelpers::runProgramWithArguments( pngCrushLocation.ref().filePath().zref(), args ); // and this never returns. return res; }

Ok, so its blaming runProgramWithArguments. Thats just a wrapper around WolfSystem::Process. And WolfSystem::Process is code thats used practically everywhere for a long time, so its gotta be good, right? Well thats where the bug says it is, so down another rabbit hole! Can't make any assumptions about whats good code or not.

So Process is a wrapper around running an external program. 99% of programs (including WolfRez) just uses the nice wrapper function around it which looks like this:

WolfType::Vector<WolfType::String> stdoutVec; WolfType::Vector<WolfType::String> stderrVec; WolfSystem::ExitStatusCode returnValue = WolfSystem::ExitStatusCodeSuccess; // Run the given program, with the given arguments, and store its output // in our vectors. auto res = WolfSystem::Process::runTillFinished( programPath, args, &stdoutVec, &stderrVec, &returnValue );

Pretty simple right? Even get the output back easily. Theirs a lot more control if you need it to run async or whatever, but this will just run till the program finishes, then hand back what it got.

Since I think its a multithreaded issue, lets add a mutex right here! Make it so only one thread at a time can run a process. Slow as hell, but it'd show if its a race issue right? Welp, added it, slowed everything by 20x, and while it didnt happen as often, the hang still happened. Sigh, time to go deeper.

Lets talk about how Process works! Unfortunately, most OS level things for running processes dont work the nice simple way the above API does. Most programmers will know about system() which let you run a program, but you can't get the output from it. And that's about it for C standards of running programs! Amazing isn't it. C++ has a technical group I think working on a better interface, but for now it's OS specific. So you need to go even lower to POSIX/WinAPI. So here's how processes work on UNIX (Windows is similar, but has some core differences : if you want me explain it, send me a msg).

Lets say you want to start a new process with the file descriptors (FDs) (such as stdout/stdin) redirected so your program can read them. Here is the list of things you need to do:

First, you need to create the new process. In POSIX, you use fork() to do this. Fork clones the current process, and tells you if you're the parent or the child. Congratulations on your new process!

Second, you need to redirect your FDs. You use something like dup2() to make copies. In Wolf's Process, we use pipes to talk between the parent[application] and child[what will become the program]. Importantly, dup2 needs file numbers, so you can use something like fileno() to get them.

Lastly, you need to run one of the exec functions (such as execve()). This replaces the current process with the new process image at the given location. If you know of Julie-Su (my IRC bot) that could restart herself, this is how she did it.

So in other terms: its like making a clone of yourself, making sure it knows how to talk, then wiping its memory with whatever you want it to do. Programming is weird.

So now you have a child process running the program you want, along with a pipe to your main process to communicate. In the case of runTillFinished, you then just sit on that pipe select()ing, recording any data from stdout/stderr, and waiting for the pipes to close. Once they're closed, you know the program finished and can return all that juicy information.

Back to the bug, I started adding more console statements within WolfSystem::Process. You can't put much debug things within the child process after fork, since you get rid of your FDs, but the parent process basically ends up just waiting on select() for the child process to call. Cue spending 30 minutes playing with arguments to select's timeout which seem to affect it hanging or not, but ultimately was a red herring. The problem is the child never sends messages to its parent, but it also never closed the pipes even though it finished. Err.. it did finish, right?

While I was in the middle of recompiling for the 30 billionth time, I decided to setup a small script running pstree on the process. pstree is just a simple version of ps which shows a process and its children as a nice tree. On Linux, theirs no distinction between processes and threads (somewhat known as light weight processes), so you can easily watch your program to see how its threading or spawning processes. So open another terminal, and lets do:

while true; do clear; pstree PID; sleep 1; done

Now on OSX, threads aren't processes so they dont show up. But as soon as you fork() (which creates a new process), it shows up in pstree. And even nicer, when you exec it shows the new commandline in pstree! Neat! In retrospect this makes sense, since fork()/exec() is how shells work too.

Within pstree though there were two interesting things that showed up which made me curious.

First, zombie processes show up like (pngcrush). A zombie process is a process which has exited, but hasn't been cleaned up by its parent yet. You'd mostly see them for a split second before they get reaped (yes, those are technical terms). However, 1 or 2 of the stuck tasks were showing up as zombies.

Second, I had quite a few child processes which were still named 'WolfRez'. This only should happen for the few microseconds between the fork() (creating the process) and execve() calls (replacing it with the new process). It definitely should not be hanging in that state, theirs literally 5 lines of code there and no loops or anything for it to get stuck in.

What the heck is going on? So lets pull out our debugger (and not in the normal mode). See, if you're a normal process debugging is easy. You start up your program, and can access all the threads and so on. But a debugger generally doesn't let you get to child processes (or more likely, I don't know how to do it in LLDB). Luckily though, like all good debuggers, we can attach to arbitrary processes! So lets try attaching to things.

Attaching to pngcrush doesn't work at all. Mostly cause its a zombie [yes that took me a while to figure out]. Attaching to the WolfRez child processes work fine, and you can see the entire call stack, even pre-fork! (since the fork() copies everything, it even has its old callstack). However the top is a bit odd - its waiting inside a mutex called from the dup2() lines.

if (dup2(m_stdoutPipes[1], fileno(stdout)) == InvalidPipe) std::cout << "[Process::execAndPipe] Unable to dup stdout" << std::endl;

Now the dup2() is used to duplicate file descriptors. If you want to get the output from the process (say STDOUT, also known as FD1), you need some way to feed it back to your parent program. So the easiest way is to create a pipe. The parent program has one end (call it FD4) and the child has the other (call it FD5). Output from one end moves to the other end, and bam: easy communication! But how do we trick the program to write its output on FD5 instead of FD1? Thats where dup2() comes in. dup2() copies a file descriptor into another FD, making both of them act the same. So you can duplicate FD5 into FD1 and now anything written to FD1 is the same as writing to FD5! Program writes to FD1, ends up in FD5, goes to FD4, and we're good. Goes over our pipe to the parent, allowing storing in the vector, yadda, yadda.

So lets talk a bit more about forking [I'm sure you're excited about Yet Another Tangent]. See, forking and multithreading dont get along. At all. Forking makes a copy of the process's data space, but ONLY the current thread's callstack. All other threads just dissapear in the new space. Its kindof like a camera that takes a normal picture, except it will only show one person (the active one). Everyone else was never there : if they dug a hole, the hole is still there but thats it. So whats bad about this? Consider this: you're a good programmer (cause of course you are!), and you make things thread safe. Therefore, you have mutexes, critical sections, semaphores, and other constructs to make sure multiple threads dont mess with stuff at the same time. So consider this: a thread enters a mutex, locks it, then BAM: another thread forks. Data is copied, but only the thread that forks is in the new process. What happens to that mutex? Well, its still locked! And the thread that locked it can't unlock it, cause it never existed [in the new process's view at least]. So if your new thread now tries to use anything that needs that mutex, welp, you're screwed : the lock is frozen shut. Meet Mr. Deadlock.

Sounds like fun, eh? So theirs some pretty big warnings about not calling things between fork() and execve() if you're multithreaded. You can't guarantee the state of anything at all, because so many things are trying to be thread-safe. Even simple things like malloc() or printf() might use a global lock somewhere, and could get locked. So maybe it's dup2()s fault! After all it doesn't say its async-signal safe (which is what the man pages says is the only guaranteed working functions). But the dup2() thing is used by a lot of better programmers than I, so either:

they have the same issue and never ran into it

its something else.

So after another half hour of messing with things, I finally looked at the LLDB trace a bit closer for the child process.

The mutex being locked is within fileno(). .....What?

fileno() should be a pretty brain dead function. It gets the FD number for a FILE* pointer. See, UNIX always has used integers for FDs, such as FD1 being stdout. However, when it came time to make C portable (especially to Windows), it needs to hide details like that. So C uses FILE* (the same thing functions like fopen() do) as its main file handle. So stdout for example, is a FILE* to remain portable. Luckily on POSIX, they have fileno() to convert from the C form to the actual int code, which other POSIX functions like dup2() want. So fileno(stdout) is an easy way to get the number 1 (since its always file descriptor 1).

Here's glibc's version of the function for an example:

// this code block is GPL from glibc-2.25. // not that theirs any reason to copy it. #define _IO_fileno(FP) ((FP)->_fileno) int __fileno (_IO_FILE *fp) { CHECK_FILE (fp, EOF); if (!(fp->_flags & _IO_IS_FILEBUF) || _IO_fileno (fp) < 0) { __set_errno (EBADF); return -1; } return _IO_fileno (fp); }

Simple! It just accesses the struct backing 'fp', and grabs the fileno from it (with error handling and safety things around it).

Well, until you find out that on OSX, fileno() appears to take a mutex. I'm not sure if its a global mutex or one tied to the FD's backing data, but either way it locks a mutex. The same mutex that output uses. A mutex which quite possibly is locked because another thread before forking was outputting progress information or rules to the screen.

So the events to cause this was basically:

WolfRez runs, starts up multiple tasks [24 on this computer].

One thread outputs to the screen its normal messages. This takes an internal mutex by the OS/standard library.

Another thread starts running a task requiring it run an external program (say luac or pngcrush).

That causes WolfSystem::Process to run, causing a fork(), freezing the mutex closed.

Post fork, the process tries to duplicate stdout. To do that, it needs the number so it calls fileno().

fileno() tries to take the lock that was taken prior to the fork.

And since the fork happened, the lock is frozen and will never be unlocked. Deadlock.

Sigh...

So the fix? POSIX also defines the simple defines STDOUT_FILENO STDIN_FILENO STDERR_FILENO which has the explicit numbers (1 2 3) for the standard streams. Which is all that was needed in the first place. So its an easy change. Rerun everything, and its all perfectly happy now.

And that my friends is why you're not supposed to do anything complex at all between a fork() and its execve(). You get to spend 5 hours with Xcode, lldb, pstree, and man pages. You get to question your abilities, what the hell is happening, and try to remind yourself that computers are supposidly logical. And get to listen to all of She's albums over and over and over. On the plus side though, learned a TON about how lldb works, and way too much about how OSX works. Now to forget it all, because after all : programming is about solving problems, then having them nicely wrapped so you can forget it all.

Programming in a nutshell : 5 hours of work for 5 seconds of typing to fix it. And the fix was something you should have done in the first place.

Now for some 酒 and some massively needed sleep.

#debugging #development #wolfrez #wolf #osx

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rafi1228 · 6 years ago

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