Bloggy

The “fun” in the title should be read in your most sarcastic tone of voice… Anyways, one of my professors mailed us a PDF of a scanned document to read (and print out) for the next class. Being that is was scanned in (by what appeared to be the professor literally holding it above a scanner) there was a lot of excess black in the picture.

I don’t know about you, but printing 2 large blocks of solid black, for 22 pages, doesn’t sound like a wise investment of toner. But ah! Why don’t I just crop off the excess part of each page so that just the scanned-in text is visible, and print that out? This has to be easy, right?

Unfortunately it wasn’t as easy as I’d hoped (most of the picture editors that can even handle PDFs can’t print out each layer as a separate page, and there’s no way I’m doing the exact same operation 22 times). ImageMagick looked like the thing I needed, even if it would take some trial-and-error to figure out exactly how much to crop off.

Turned out it only took a couple of runs to figure out exactly how much I could get away with cropping. But I had a worse problem than having to do trial runs: The output looked horrible.

I tried reading the man page, going to the website, and the rest, and couldn’t figure out what to do. Using the -density option seemed to be the right idea, but alas I couldn’t get it to work.

I troubleshot further, even getting to the point of running gs manually to see if Ghostview or ImageMagick was the problem (turned out it was myself, I guess). Eventually I realized that Ghostview was rendering the initial image to ImageMagick at a low resolution (72 DPI) but viewing the source in Okular, it was obvious that much better was possible (I’d estimate 200 DPI although I ended up using 300). So if I could figure out how to get ImageMagick to pass the right DPI to Ghostview I should have the problem fixed.

More directed Google searching revealed I’d had the right flag the whole time, -density. I just had it in the wrong spot. Something like this is right: convert -density 300x300 input.pdf -crop ... output.pdf. Instead I’d been using convert input.pdf -density 300x300 -crop ... output.pdf.

I figured I’d put my experience out there in the great Internet Memory Machine in case others have similar troubles.

Posted by mpyne under Computing Troubles & Useful Tricks | 10 Comments »

Knocked out a few minor kdesvn-build bugs in my free time today (even if I am crazy tired now, maybe they won’t be “bugfixes” when I wake up this morning!)

Specifically:

• Fix multiple subdir log filename breakage
• Make global set-envs apply to every module (regression)
• Generate log/latest/build-status again
• Allow source-dir to be changed per module

In other good news, the documentation styling improvements I was pondering a couple weeks ago were added in time to make 4.4. Speaking of documentation, Burkhard Lück noticed that the /trunk documentation for kdesvn-build was significantly out-of-date compared to the kdesvn-build.kde.org ones (since I did not commit during the freeze), and got the updated docs imported. (And he might be able to backport them for 4.4.1 as well)

Finally if you’re as big a fan of the man and info kioslaves as I am you may have noticed that URLs of the form man:foo (or info:foo, etc.) don’t work in KRunner in the release candidate or betas (bug 221371). I think I have a fix for that which I’ll try to get in before the 4.4 tagging, but if that doesn’t happen then you can workaround by using man:/foo or info:/foo URLs until then.

Posted by mpyne under KDE & Programming & Useful Tricks | No Comments »

So I woke up this morning to notice that my kdesvn-build --refresh-build run from last night had a hiccup somewhere:

The problem was kdelibs failed to install, which meant every subsequent module error-ed out during the CMake process. The problem with kdelibs was related to me having an old library installed in my Qt directory since I hadn’t cleared out my Qt/KDE installation directories first (whoops).

It would be really annoying to have to type out each module to build just to skip the first four that had built right. Luckily I don’t have to do that, instead this sufficed:

$ kdesvn-build --no-src --resume-from kdepimlibs
Script started processing at Sat Dec 12 19:04:24 2009
< <<  Build Process  >>>
Building kdepimlibs (1/21)
        Preparing build system for kdepimlibs.
        Removing files in build directory for kdepimlibs
        ...

The magic here is the --resume-from command line option which skips until the given module in the build process. --no-src is only available since kdesvn-build-1.10, use --no-svn if you haven’t upgraded yet.

Posted by mpyne under Computing Troubles & KDE & Useful Tricks & kdesvn-build | 1 Comment »

If you’re a KDE developer using a recent version of glibc (since 2.10), you may have come across strange crashes complaining about memory corruption when running development versions of KDE (especially Okular, KTorrent, KNotify, and other KDE applications using threading).

This had been noticed by quite a few people (and was being tracked by KDE under bug 196207). Apparently OpenSUSE had noticed this as well and had submitted a patch to the glibc maintainers this past June, in sourceware bug 10282. I am happy to be able to say that the patch works for me™. So if you’re been having this issue, see about getting your distribution to update their glibc with this patch (or a better one, I don’t care ;)

Posted by mpyne under Computing Troubles & KDE & Useful Tricks | 1 Comment »

Some minor things:

Temporary confusion: On a mailing list the other day someone was having issues with the following type of code:

const char *name = m_obj.byteArray().constData();
printf("%s", name); // kaBOOM

So what was wrong? In this case, usage of a pointer that was freed. Freed, in this case, by the QByteArray object that was created and destroyed all on the same line. In other words, this part: m_obj.byteArray() returns a new QByteArray (called a temporary object since it is not named by the programmer). The .constData() tacked onto it grabs the address of the data in that QByteArray.

Unfortunately, by the time you’ve used the pointer on the next line, the temporary QByteArray has been destroyed. Now that pointer you have is dangling into unallocated memory and who knows what will happen. Maybe it will work, maybe it won’t, but it’s technically undefined behavior.

The solution is to assign that QByteArray to some local variable first, that way the compiler won’t destroy it on you. Also note that passing this pointer to an enclosing function is also fine, like this:

printf("%s", m_obj.byteArray().constData());

The compiler will make sure that all of the temporaries invoked to handle a function call will be alive until after that function has run (destroying temporaries is done as the last step of that entire expression). This is what makes the qPrintable() function useful.

Also I saw a interesting link about GCC optimization on Hacker News. Basically the -Os (optimize for size) flag ends up being the fastest compilation option in many cases. This is due to the fact that it takes so long to access data that is not in the cache for modern day architectures (in comparison to CPU speed) that it is often better to do a lot of extra work for the CPU if it will mean that the code size is smaller. I’ve used this flag for awhile with no ill effects (on the other hand, no blindingly obvious speedups either ;) It’s something to think about when you’re playing with your compilation flags.

Posted by mpyne under C++ & Tutorial & Useful Tricks | 4 Comments »

From IRC, a quote:

Son, we live in a world that has bugs, and those bugs have to be found by men with debuggers. Who’s gonna do it? You? You, OSS fan boy? I have a greater responsibility than you could possibly fathom…. You have the luxury of not knowing what I know. That the KDE 4.0 release, while tragic, probably saved developer resources. And that my existence, while grotesque and incomprehensible to you, saves developer resources. You don’t want the truth because deep down in places you don’t talk about at parties, you want me on that bug, you need me on that bug.

And in other news, I’m officially on shore duty, at my next duty station. I got to “help” the nice Comcast guy with hooking up cable Internet with linux, but other than that things went smoothly. (Protip:Have a DNS nameserver not part of Comcast memorized before hooking up the Internet. If you’re unlucky you’ll get to a point where your Internet works but DNS does not.)

Posted by mpyne under Computing Troubles & Funny & KDE & Useful Tricks | 2 Comments »

In the spirit of my series of programming tips articles I thought I would go ahead and throw out some more:

• If you are using QAction, QCheckBox, QRadioButton, or similar classes, then you need to be wary of the differences between the toggled(bool) signal and the triggered(bool) (for QAction) or clicked(bool) (for button-like classes) signals.

  These two signals are used in two different situations. toggled(bool) is always for if the value of the action or button being checked changes, for any reason. This means that if an QRadioButton had not been checked, and it is set to being checked, it will then emit a toggled(true). You would use this signal if you needed to update your GUI based on whether an action is checked for instance.

  The other signal (triggered or clicked) are used when you need to perform an action based on a user action. For instance, if you had a "[ ] Show Wikipedia content" on your menu bar, you may want to bring up that window as soon as the user clicks on it. On the other hand, if you were changing the checked status of the action internally to your code but didn’t want the window to pop up, you’d want to use the triggered signal.

• If you’re not continuing to study then you’re not going to get any better at programming. I’ll generally visit programming.reddit.com (and also the C++ and Perl sub-reddits) and sometimes you’ll catch a new technique that is neat. For instance:
• From Mr. Edd’s C++ blog: You can use the not-well-known feature of C++ to give “pointers to member data” to allow for assigning a null pointer to smart pointer classes, while not allowing any other pointers to be assigned. His writeup covers everything so I’ll refer you to that but suffice to say that something like “smart_ptr service = 0 would work, while “smart_ptr service = &newService” would not. This is useful in basically any place where you want to allow a null pointer to be assigned by not any other kind of pointer.
• Another interesting tidbit is the restrict keyword, available in C since the C99 revision. Sadly, it is not available in C++, not even the upcoming C++0x standard, except as a compiler extension. The use of the keyword is fairly low-level, and is explained in this article for Cell processor development.

Hopefully you’ve learned something neat (and useful and not merely contrived!)

Posted by mpyne under C++ & Programming & Useful Tricks | 1 Comment »

I’m sure there’s already a million “Happy New Year” type posts up but I figured I’d throw in my two cents as well (since I’m actually here for this New Year’s Day).

While I’m at it I’ll pass along some things I managed to accomplish over the past week:

• Have some troubleshooting that you need to do that requires you to have a small partition? (In my case, troubleshooting for bug 118594). You don’t need to re-format your hard drive. Instead (at least on Linux) just create a partition on a file:
  1. Create a small-ish file (for instance, using dd if=/dev/zero of=file-name-here bs=1M count=size-in-megabytes).
  2. From there, you need to create a filesystem on it. I used plain ext2. If you’re not running as root you may need to manually run /sbin/mke2fs file-name-here. (since /sbin is normally only in the PATH for root). mke2fs should complain about the file not being a special block device but you can tell it to proceed. Please note that mke2fs is a horribly horribly destructive thing to do on a live filesystem. Make sure to take the extra second to get the filename right, and don’t run mke2fs as root.
  3. From there you can mount the file using the loop device. I had to do the following: mkdir test-dir # Create directory to mount filesystem to
sudo mount -o loop ./file-name-here ./test-dir/.
     On my system root permission is required to run mount even for a single-user mount like this, it may be different on yours.
  4. Now use your new filesystem normally. Any changes that you make to the test-dir/ folder actually affect the file you created for that purpose. This is handy for testing out-of-disk-space problems in your app.
• If you use Gentoo you will want to start shifting over to split ebuilds if you haven’t already. Their KDE-style modules were stopped (for the most part) at KDE 3.5.9, which was fine with me until every emerge world I tried to run started complaining about blocked packages at the various kdelibs requirements started to get inconsistent. I’ve decided to take this opportunity to simply get rid of the majority of the system KDE 3 and upgrade to KDE 4.1.3 instead. So far my wife hasn’t complained about it at all (and she really likes KSudoku as well) so as soon as I’m convinced I haven’t forgotten to install a KDE 4 program I’ll need I’m going to uninstall KDE 3 with the exception of the libraries and a few apps.
• World of Goo is definitely worthy of play. A Linux version is in beta but the Windows demo ran fine for me in WINE. Well, it crashed WINE on shutdown before the video mode had been restored but I suppose that’s good enough nowadays. :)

Posted by mpyne under Computing Troubles & KDE & Personal & Useful Tricks | 1 Comment »

I’m not sure when this changed in KDE 4 but if you move your mouse to the upper-left corner and keep trying to “push” you’ll get the “exposé” effect without having to use the keyboard shortcut, and then click on the window you want. Handy alternative to Alt-Tab.

Posted by mpyne under KDE & Useful Tricks | 12 Comments »

So before I went underway our car had developed a trouble causing the Check Engine light to come in continuously. No abnormal sounds but being nuclear-trained has taught me not to live with “locked-in” alarms or warnings. I wasn’t able to troubleshoot it in the limited time I had before deploying so I asked my wife to make sure it got investigated when she had free time.

She took the car to the dealership. She actually had to make two trips; apparently there were two separate faults that would have caused the light to come in by this time. Nothing mechanical luckily, but apparently at some point while the dealership was troubleshooting the issue they had to unhook the battery. Doing this activated the anti-theft feature of my radio; it would not play music unless I typed in the anti-theft code. This code was included on a card which I was supposed to remove from the car and keep in a safe place. Needless to say, I had removed it from the car, and it is still in some safe place, nowhere to be found.

So my car radio has been silent for about a month and a half now. I finally got the time to really dig into this issue since the dealership apparently is completely unable to figure out what the unlock code should be. A little browsing on the Internet and apparently many people have had the same issue, and the unlock codes are few enough to list in one paragraph.

So I went and wrote them down and typed them in one by one. After about a dozen tries the radio started working again. Yay! I will record the code in my KWallet this time; it is the only thing which I reliably can count on being there. In case anyone else develops this issue with a Chevy Aveo, I found these posts helpful: automotive forums and Yahoo! Answers.

Posted by mpyne under Personal & Useful Tricks | 3 Comments »