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SQLite with two python processes accessing it: one reading, one writing

- by BBnyc

I'm developing a small system with two components: one polls data from an internet resource and translates it into sql data to persist it locally; the second one reads that sql data from the local instance and serves it via json and a restful api. I was originally planning to persist the data with postgresql, but because the application will have a very low-volume of data to store and traffic to serve, I thought that was overkill. Is SQLite up to the job? I love the idea of the small footprint and no need to maintain yet another sql server for this one task, but am concerned about concurrency. It seems that with write ahead logging enabled, concurrently reading and writing a SQLite database can happen without locking either process out of the database. Can a single SQLite instance sustain two concurrent processes accessing it, if only one reads and the other writes? I started writing the code but was wondering if this is a misapplication of SQLite.

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Do we set the bar too high by requiring that code tests not suffer from buffer overflow?

- by brice

We are currently recruiting for a Junior Developer position working mainly in C on Linux. As part of the process, we require candidates to complete a code test at their leisure in C. So far we have rejected two candidates on the basis that their code, although readable and in one case rather idiomatic, suffered from buffer overflow errors due to unbounded buffer writes. Are buffer overflows acceptable from a graduate developer? Are we setting the bar too high? What is the expected capability of graduate/Junior engineers? [Edit]: We explicitly ask for error-checked, production quality code. We provide a test & build framework for the candidates

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Windows Azure XDrive

- by kaleidoscope

This allows your Windows Azure compute applications running in our cloud to use the existing NTFS APIs to store their data in a durable drive. The drive is backed by a Windows Azure Page Blob formatted as a single NTFS volume VHD. The Page Blob can be mounted as a drive within the Windows Azure cloud, where all non-buffered/flushed NTFS writes are made durable to the drive (Page Blob). If the application using the drive crashes, the data is kept persistent via the Page Blob, and can be remounted when the application instance is restarted or remounted elsewhere for a different application instance to use. Since the drive is an NTFS formatted Page Blob, you can also use the standard blob interfaces to uploaded and download your NTFS VHDs to the cloud. More details can be found at: http://microsoftpdc.com/Sessions/SVC14 Anish, S

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"Reverse" encryption? - EncFS the other way around?

- by stwissel

Currently I'm using EncFS to encrypt my directory "confidential" to ".encconfidential" and sync that encrypted directory using an online service (e.g. Dropbox, UbuntuOne etc). However my entire disk is already LUKS encrypted, so the double encryption takes a toll on performance. I wonder is there an "inverted" EncFS option? An unencrypted directory gets mounted and in the mounted directory you only see encrypted files. So I could work with the unencrypted documents while the sync tool sees and read/writes the encrypted files only.

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What You Said: Giving an Old Laptop a New Life

- by Jason Fitzpatrick

Earlier this week we asked you to share your tips and tricks for breathing life into an old laptop, now we’re back to share your junk-bin sparing methods. Many of you worked to keep old laptops from getting scrapped by dusting them off and donating them. Mark writes: My acquaintances & friends give me their old computers when they buy a new one. So I disassemble, clean, install an opsys,and get internet working. I also upgrade memory, wireless, etc. from my parts bin. Then I give it to a poor person who needs a computer. Usually a single working mom with kids. I also do the same with old desktops as well. They really appreciate them and It gives me the satisfaction of resurrecting an old computer. Wbrown does the same: How To Delete, Move, or Rename Locked Files in Windows HTG Explains: Why Screen Savers Are No Longer Necessary 6 Ways Windows 8 Is More Secure Than Windows 7

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ZFS for Database Log Files

- by user12620111

I've been troubled by drop outs in CPU usage in my application server, characterized by the CPUs suddenly going from close to 90% CPU busy to almost completely CPU idle for a few seconds. Here is an example of a drop out as shown by a snippet of vmstat data taken while the application server is under a heavy workload. # vmstat 1 kthr memory page disk faults cpu r b w swap free re mf pi po fr de sr s3 s4 s5 s6 in sy cs us sy id 1 0 0 130160176 116381952 0 16 0 0 0 0 0 0 0 0 0 207377 117715 203884 70 21 9 12 0 0 130160160 116381936 0 25 0 0 0 0 0 0 0 0 0 200413 117162 197250 70 20 9 11 0 0 130160176 116381920 0 16 0 0 0 0 0 0 1 0 0 203150 119365 200249 72 21 7 8 0 0 130160176 116377808 0 19 0 0 0 0 0 0 0 0 0 169826 96144 165194 56 17 27 0 0 0 130160176 116377800 0 16 0 0 0 0 0 0 0 0 1 10245 9376 9164 2 1 97 0 0 0 130160176 116377792 0 16 0 0 0 0 0 0 0 0 2 15742 12401 14784 4 1 95 0 0 0 130160176 116377776 2 16 0 0 0 0 0 0 1 0 0 19972 17703 19612 6 2 92 14 0 0 130160176 116377696 0 16 0 0 0 0 0 0 0 0 0 202794 116793 199807 71 21 8 9 0 0 130160160 116373584 0 30 0 0 0 0 0 0 18 0 0 203123 117857 198825 69 20 11 This behavior occurred consistently while the application server was processing synthetic transactions: HTTP requests from JMeter running on an external machine. I explored many theories trying to explain the drop outs, including: Unexpected JMeter behavior Network contention Java Garbage Collection Application Server thread pool problems Connection pool problems Database transaction processing Database I/O contention Graphing the CPU %idle led to a breakthrough: Several of the drop outs were 30 seconds apart. With that insight, I went digging through the data again and looking for other outliers that were 30 seconds apart. In the database server statistics, I found spikes in the iostat "asvc_t" (average response time of disk transactions, in milliseconds) for the disk drive that was being used for the database log files. Here is an example: extended device statistics r/s w/s kr/s kw/s wait actv wsvc_t asvc_t %w %b device 0.0 2053.6 0.0 8234.3 0.0 0.2 0.0 0.1 0 24 c3t60080E5...F4F6d0s0 0.0 2162.2 0.0 8652.8 0.0 0.3 0.0 0.1 0 28 c3t60080E5...F4F6d0s0 0.0 1102.5 0.0 10012.8 0.0 4.5 0.0 4.1 0 69 c3t60080E5...F4F6d0s0 0.0 74.0 0.0 7920.6 0.0 10.0 0.0 135.1 0 100 c3t60080E5...F4F6d0s0 0.0 568.7 0.0 6674.0 0.0 6.4 0.0 11.2 0 90 c3t60080E5...F4F6d0s0 0.0 1358.0 0.0 5456.0 0.0 0.6 0.0 0.4 0 55 c3t60080E5...F4F6d0s0 0.0 1314.3 0.0 5285.2 0.0 0.7 0.0 0.5 0 70 c3t60080E5...F4F6d0s0 Here is a little more information about my database configuration: The database and application server were running on two different SPARC servers. Storage for the database was on a storage array connected via 8 gigabit Fibre Channel Data storage and log file were on different physical disk drives Reliable low latency I/O is provided by battery backed NVRAM Highly available: Two Fibre Channel links accessed via MPxIO Two Mirrored cache controllers The log file physical disks were mirrored in the storage device Database log files on a ZFS Filesystem with cutting-edge technologies, such as copy-on-write and end-to-end checksumming Why would I be getting service time spikes in my high-end storage? First, I wanted to verify that the database log disk service time spikes aligned with the application server CPU drop outs, and they did: At first, I guessed that the disk service time spikes might be related to flushing the write through cache on the storage device, but I was unable to validate that theory. After searching the WWW for a while, I decided to try using a separate log device: # zpool add ZFS-db-41 log c3t60080E500017D55C000015C150A9F8A7d0 The ZFS log device is configured in a similar manner as described above: two physical disks mirrored in the storage array. This change to the database storage configuration eliminated the application server CPU drop outs: Here is the zpool configuration: # zpool status ZFS-db-41 pool: ZFS-db-41 state: ONLINE scan: none requested config: NAME STATE ZFS-db-41 ONLINE c3t60080E5...F4F6d0 ONLINE logs c3t60080E5...F8A7d0 ONLINE Now, the I/O spikes look like this: extended device statistics r/s w/s kr/s kw/s wait actv wsvc_t asvc_t %w %b device 0.0 1053.5 0.0 4234.1 0.0 0.8 0.0 0.7 0 75 c3t60080E5...F8A7d0s0 extended device statistics r/s w/s kr/s kw/s wait actv wsvc_t asvc_t %w %b device 0.0 1131.8 0.0 4555.3 0.0 0.8 0.0 0.7 0 76 c3t60080E5...F8A7d0s0 extended device statistics r/s w/s kr/s kw/s wait actv wsvc_t asvc_t %w %b device 0.0 1167.6 0.0 4682.2 0.0 0.7 0.0 0.6 0 74 c3t60080E5...F8A7d0s0 0.0 162.2 0.0 19153.9 0.0 0.7 0.0 4.2 0 12 c3t60080E5...F4F6d0s0 extended device statistics r/s w/s kr/s kw/s wait actv wsvc_t asvc_t %w %b device 0.0 1247.2 0.0 4992.6 0.0 0.7 0.0 0.6 0 71 c3t60080E5...F8A7d0s0 0.0 41.0 0.0 70.0 0.0 0.1 0.0 1.6 0 2 c3t60080E5...F4F6d0s0 extended device statistics r/s w/s kr/s kw/s wait actv wsvc_t asvc_t %w %b device 0.0 1241.3 0.0 4989.3 0.0 0.8 0.0 0.6 0 75 c3t60080E5...F8A7d0s0 extended device statistics r/s w/s kr/s kw/s wait actv wsvc_t asvc_t %w %b device 0.0 1193.2 0.0 4772.9 0.0 0.7 0.0 0.6 0 71 c3t60080E5...F8A7d0s0 We can see the steady flow of 4k writes to the ZIL device from O_SYNC database log file writes. The spikes are from flushing the transaction group. Like almost all problems that I run into, once I thoroughly understand the problem, I find that other people have documented similar experiences. Thanks to all of you who have documented alternative approaches. Saved for another day: now that the problem is obvious, I should try "zfs:zfs_immediate_write_sz" as recommended in the ZFS Evil Tuning Guide. References: The ZFS Intent Log Solaris ZFS, Synchronous Writes and the ZIL Explained ZFS Evil Tuning Guide: Cache Flushes ZFS Evil Tuning Guide: Tuning ZFS for Database Performance

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Almost Realtime Data and Web application

- by Chris G.

I have a computer that is recording 100 different data points into an OPC server. I've written a simple OPC client that can read all of this data. I have a front-end website on a different network that I would like to consume this data. I could easily set the OPC client to send the data to a SQL server and the website could read from it, but that would be a lot of writes. If I wanted the data to be updated every 10 seconds I'd be writing to the database every 10 seconds. (I could probably just serialize the 100 points to get 1 write / 10 seconds but that would also limit my ability to search the data later). This solution wouldn't scale very well. If I had 100 of these computers the situation would quickly grow out of hand. Obviously I am well out of my league here and I have no experience with working with a large amount of data like this. What are my options and what should I research?

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What You Said: Your Favorite Remote Desktop Access Tools and Tips

- by Jason Fitzpatrick

Earlier this week we asked you to share your favorite remote desktop access tools and tips; now we’re back to highlight your favorite tools and how you use them. The two prevailing themes among all the tools suggested were pricing and ease of deployment. On that front, LogMeIn had a strong following. Mtech writes: I use Logmein and am amazed the free version can be used even for business purposes. I also felt so bad and wanted to pay for the Pro version just out of gratitude but they called me personally from the USA and said why pay when the free version does all you need! What a company. HTG Explains: Why Do Hard Drives Show the Wrong Capacity in Windows? Java is Insecure and Awful, It’s Time to Disable It, and Here’s How What Are the Windows A: and B: Drives Used For?

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AngularJS: structuring a web application with multiple ng-apps

- by mg1075

The blogosphere has a number of articles on the topic of AngularJS app structuring guidelines such as these (and others): http://www.johnpapa.net/angular-app-structuring-guidelines/ http://codingsmackdown.tv/blog/2013/04/19/angularjs-modules-for-great-justice/ http://danorlando.com/angularjs-architecture-understanding-modules/ http://henriquat.re/modularizing-angularjs/modularizing-angular-applications/modularizing-angular-applications.html However, one scenario I have yet to come across for guidelines and best practices is the case where you have a large web application containing multiple "mini-spa" apps, and the mini-spa apps all share a certain amount of code. I am not referring to the case of trying to have multiple ng-app declarations on the same page; rather, I mean different sections of a large site that have their own, unique ng-app declaration. As Scott Allen writes in his OdeToCode blog: One scenario I haven't found addressed very well is the scenario where multiple apps exist in the same greater web application and require some shared code on the client. Are there any recommended approaches to take, pitfalls to avoid, or good sample structures of this scenario that you can point to?

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Programmaticaly change lightdm autologin session type

- by marcinpz

I have problem with lightdm setting autologin session type (using my config tool). Lightdm logs into previously selected session. I see that autologin-session is UNIMPLEMENTED in version 1.2.1. Maybe I can somehow cheat lightdm? but it seems program writes actual state when stopping and overwrites my changes to /home/user/.dmrc and /var/cache/lightdm/user.dmrc. At the current I have two sessions: default and kiosk. in /etc/lightdm/lightdm.conf I have line: user-session=desktop but after I logged into kiosk session it starts all the time. Changing .dmrc and /var/cache/lightdm/dmrc/user.dmrc doesn't work.

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Would you hire a computer scientist which refuses to use computers? [closed]

- by blueberryfields

Imagine that you're interviewing a brilliant CS grad. He's just finished school, has very high grades, and has been performing very well on the interview so far. You reach a point near the end, where you're starting to speak about terms of employment, salary, etc.., and you're trying to show off the environment he'll be working in. When you mention that programmers at your company have systems with two monitors, the interviewee stops you and informs you that he won't need a computer. He only ever writes code by hand, in a note-book, and relies on his phone for sending/receiving email. This is not something he's willing to budge on. Would you still hire him? How good would he have to be for you to hire him? What would you hire him to do, if you do hire him? (the student is modelling himself on E.W. Djikstra)

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Is it really a security problem to have non secure assets on an ssl page?

- by blockhead

My understanding is that this is just an example of being overly cautious, but if my checkout form contains an unsecure asset on it, that doesn't endanger anybody's credit card numbers from being caught by a man-in-the-middle. I'm asking this because every once in while, maybe because of cached content or whatnot, somebody writes in saying that they are seeing this "error" (even though there are no unsecure assets on my page), but they want an explanation. So yes, I can tell all about encryption and certificates and trust and men-in-the-middle. But what do I tell them about this. How do I convince them that the site is 100% safe (and if it isn't let me know that I'm mistaken!)

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How often should saving to disk occur in an automatically saving text editor?

- by lelandmiller

I am developing a simple text editor and would like the application to save the text automatically. In other words, the user would never have to press a save button. I have seen other applications that do this, and was wondering how often is it safe to write files to disk? From a user experience standpoint, it seems that the more frequently this happens the better, but I am worried about performance and possible disk wear (especially on writes to SSDs). It seems like the operating system disk caching might help avoid these problems, but I also don't know if its safe to rely on that for an application like this. I was planning on writing the whole document to disk at each save, but this just seems terribly inefficient if the OS ends up writing it to disk to frequently, but relying on program unload may lose data in the case of a crash. Does anyone have any experience dealing with this that might be able to help?

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What is the most reliable session storage in PHP: Memcache, database or files?

- by user1179459

What is the best and most safest way to handle PHP sessions. Is the best way to store sessions in: Database (more reliable, but high bottleneck, slow speed, not good for high database usage websites)? Memcache (super fast, but distributed more security problems, chances of loosing data when the server restarted and chances of loosing data when the cache is full)? Files (default option, I guess slow since it reads and writes from file I/O, less security, etc). Which method is the best? What are the problems and good things of each of those approaches?

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Permissions depending on parameters passed to application

- by Engineiro

I have a java app that writes to a very important table I have. This is sudo protected, but this app takes a parameters for where the output should go. The default is the REAL DEAL but the "output" parameter takes priority over the default, so if a user provides a path that they have write permission for, say: /home/username/mystuff then I would like for users to be allowed to write to these directories. The app is launched in a shell script, so I am wondering if I can check that the string does not equal /path/to/real/deal and not empty (since /path/to/real/deal is the default) and allow users to write to places they have permissions for. My impression of linux permissions is that it is tied to the application itself and cannot be conditionally approved, but I wonder if there is some bash fu or alternative method I can perform to get me my desired result. Using Ubuntu 10.04.4 LTS

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What's the difference or purpose of a file format like ELF when flat binaries take up less space and can do the same thing?

- by Sinister Clock

I will give a better description now. In Linux driver development you need to follow a specification using an ELF file format as a finalized executable, i.e., that right there is not flat, it has headers, entry fields, and is basically carrying more weight than just a flat binary with opcodes. What is the purpose or in-depth difference of a Linux ELF file for a driver to interact with the video hardware, and, say, a bare, flat x86 16-bit binary I write that makes use of emulated graphics mode on a graphics card and writes to memory(besides the fact that the Linux driver probably is specific to making full use of the hardware and not just the emulated, backwards compatible memory accessing scheme). To sum it up, what is a difference or purpose of a binary like ELF with different headers and settings and just a flat binary with the necessary opcodes/instructions/data to do the same thing, just without any specific format? Example: Windows uses PE, Mac uses Mach-O/PEF, Linux uses ELF/FATELF, Unix uses COFF. What do any of them really mean or designate if you can just go flat, especially with a device driver which is system software.

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How to use music in a simple game?

- by Aerovistae

It's like this: I've got this very simple game in mind, and I happen to be lucky enough to know this guy at my college who is the best musician I've ever met in person who wasn't already on a stage. He writes these beautiful songs on piano, just meandering and mysterious. They'd add so much as background music. But here's my dilemma: say I record a 5 minute long song from him. How do I use it? Do I set it playing, and then make it start over as soon as it ends? Do I leave a 5 minute period of silence and then start it over again? Or do I find other music and just have continuous music playing? What do other people usually do for this sort of thing?

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Display Select in an ASP.NET DropDownList

A purchaser of my book writes: I would like a drop down list to display the text: "-Select-" initially instead of the first value of the data it is bound to. Here you go <%@ Page Language="VB" %> <script runat="server">...(read more)...Did you know that DotNetSlackers also publishes .net articles written by top known .net Authors? We already have over 80 articles in several categories including Silverlight. Take a look: here.

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VMWare Player pauses often

- by pascal

I'm using a 64bit Windows 8 inside vmplayer, with 2 virtual processor cores, virtual hard disk resides on a fast local disc and is not preallocated; host CPU is Intel i7 3770, should be capable of hardware virtualisation but I don't know if VMWare uses it; NAT networking; Sound card connected, USB connected, accelerated 3D graphics (NVidia 313.30 on host) My problem is, that the VM often pauses for a few seconds, and then speeds up for a few seconds to reach real time again. Time in the VM actually moves faster after the pause, for example all animations using timers speed up. When running, the vmware-vmx process shows ~150% CPU usage in top, but 0% when pausing (and D state i.e. waiting for IO). iotop shows normal disk writes from vmware-vmx threads, but during pauses, the flush kernel thread uses 99%. Are there some options to try so that VMWare doesn't wait for IO? I've tried a few things available from the GUI but the issue never went away…

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Setting up lvm with HDD and SSD

- by stonegrizzly

My current hard drive is just about full and rather than just toss it and get a new one (since it works fine), I want to get a new drive and set them both up using lvm. While I'm at it, I also want to get an SSD to install the OS and applications on. This is my plan: Put / on the SSD (one partition) Put /tmp on a ram disk Put /var on a partition on my new drive Put /home on the rest of the new drive and my current drive using lvm. My goals are: Speed up boot time and application launch Minimize unnecessary writes to the SSD Never have to worry about which disk/partition to store my files on. I want the OS & lvm to take care of that Does this make sense? I'm fairly experienced with Ubuntu but I've never dealt with lvm before.

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Can't install ubuntu along windows 7

- by Mark

I boot from a DVD the last version of ubuntu 12.04 x64(because i have 8g. of ram) ,hit install Ubuntu ,check Install Along windwos 7(or something like this ,anyway the first option) and the button in the bottom right corner says "Restart to install" or just restart...i cant remember. So ,i hit the button than ,the screen goes black ,some proceses are stopped and on the middle of the screen it writes like this " Please remove installation media and close the tray (if any) then press ENTER: " The DVD rom opens and i remove the dvd ,close the tray ,pushing enter and the windows is starting ,no instalation ,no file copied to my PC ,nothing.

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Is sudo dd taking too long to wipe hard drive?

- by Adam133718

I have a 200gb HDD which I removed from a macbook due to several corrupt files in startup. One thing led to another and I decided that I needed to format the drive. I used the command sudo dd if=/dev/zero of=/dev/sdb which is supposed to wipe everything off of the hard drive. It is my understanding that the command writes 0's over every bit on the drive, which I would imagine must take a while. The process has been going for about 18 hours now. I can use other functions of operating system like the web browser and I can even use another terminal window, so I know the system is not frozen. Should I restart the process or let it continue on? Any advice will help. Thanks. By the way, I already noticed a post similar to this that was previously answered though the user was not using the same command as I was.

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Do you charge a client for email and chat communication as a freelancer? [closed]

- by skyork

For a project that is billed by hours, should a freelancer charge the client for the amount of time he/she spends on email/chat correspondence? For example, the client sends an email to the the freelancer, outlining the requirements. Should the freelancer charge the client for the time during which he/she reads the email and writes a reply. The same goes for chat conversations for clarifying the requirements. In particular, if the freelancer's English is not very good, so that he/she spends extra time on understanding what the client wants and explaining him/herself (e.g. copying and pasting into Google Translate), should such time be charged to the client too?

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O'Reilly Deal of the day - 5/JSep/2012 - Programming Windows 8 Applications with C#

- by TATWORTH

Today's deal of the day from O'Reilly at http://shop.oreilly.com/product/0636920024200.do?code=DEAL is Programming Windows 8 Applications with C# ."With Early Release e-books, you get books in their earliest form — the author's raw and unedited content as he or she writes — so you can take advantage of these technologies long before the official release of these titles. You'll also receive updates when significant changes are made, new chapters as they're written, and the final e-book bundle. If you want to build Windows 8 applications for desktops and the forthcoming Microsoft Surface tablet PC, this book will show you how to work with the Metro design language and the Windows RT operating system. You’ll learn this new landscape step-by-step, including the minute system details and design specifications necessary to innovate and build a variety of Windows 8 apps. It’s ideal for .NET developers who use C#."

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Difference between '\n' and '\r\n'

- by sukhbir

Yes yes, I am aware that '\n' writes a newline in UNIX while for Windows there is the two character sequence: '\r\n'. All this is very nice in theory, but my question is why? Why the carriage return character is extra in Windows? If UNIX can do it in \n why does it take Windows two characters to do this? I am reading David Beazley's Python book and he says: For example, on Windows, writing the character '\n' actually outputs the two- character sequence '\r\n' (and when reading the file back, '\r\n' is translated back into a single '\n' character). Why the extra effort? I will be honest. I have known the difference for a long time but have never bothered to ask WHY. I hope that is answered today. Thanks for your time.

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