A Weird Imagination

Rarely drawing screen in SDL

The problem

Working on the same screensaver as yesterday, we want to minimize CPU usage. Since the screensaver is a clock showing hours and minutes, there's no need to do anything except once a minute to change the time display. Optimally, the process would only be scheduled once a minute, exactly when the minute changed, to draw the screen.

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Rarely drawing screen in SDL

The problem

Working on the same screensaver as yesterday, we want to minimize CPU usage. Since the screensaver is a clock showing hours and minutes, there's no need to do anything except once a minute to change the time display. Optimally, the process would only be scheduled once a minute, exactly when the minute changed, to draw the screen.

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SDL screensaver hangs on exit

The problem

I was modifying a screensaver written using SDL and noticed that sometimes there were many instances of it left running, even after unlocking the screen. Another bug was causing the screensaver to use 100% CPU, resulting in it using up all of my processing power just for a simple screensaver.

The solution

Make the program exit immediately when it receives a SIGTERM signal by including the function

void exitImmediately(int sig) {
    abort();
}

and making the SIGTERM signal handler call it:

signal(SIGTERM, exitImmediately);

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SDL screensaver hangs on exit

The problem

I was modifying a screensaver written using SDL and noticed that sometimes there were many instances of it left running, even after unlocking the screen. Another bug was causing the screensaver to use 100% CPU, resulting in it using up all of my processing power just for a simple screensaver.

The solution

Make the program exit immediately when it receives a SIGTERM signal by including the function

void exitImmediately(int sig) {
    abort();
}

and making the SIGTERM signal handler call it:

signal(SIGTERM, exitImmediately);

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Pi in shell

Posted in

Calculating π the hard way

In honor of Pi Day, I was going to try to write a script that computed π in shell, but given the lack of floating point support, I decided it would be too messy. If you want to see hard to follow code to generate π, I highly recommend the IOCCC entry westley.c from 1998, the majority of which is an ASCII art circle which calculates its own area and radius in order to estimate π. The hint file suggests looking at the output of

$ cc -E westley.c

The 2012 entry, endoh2 is also a pretty amazing π calculator.

Getting π

Instead, I will just generate π the shell way: using another program.

$ python -c 'import math; print(math.pi)'
3.14159265359

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Some things not allowed are compulsory

Physics

The totalitarian principle is a concept in physics that states

Everything not forbidden is compulsory.

In order words, any observable outcome not forbidden by a physical law will occur. The many-worlds interpretation of quantum mechanics suggests an even stronger statement that for every such outcome, there is some alternate world in which it does occur.

Programming language design

While

Everything not forbidden is compulsory.

may sound absurd, its contrapositive

Anything not mandatory is forbidden.

sounds more like a rule you would expect in a computer system, although it may not always be desirable. While it does not fully apply, programming languages and command lines are infamously picky about their inputs:

If you forget a comma in your English essay, your teacher doesn’t hand it back and say, I can’t understand any of this.

In contrast, HTML parsers have historically been quite flexible in what they accept. There's reason to believe this lack of strictness was a strength: less technical users could create their web pages using incorrect HTML that would still work. Somewhat related, browser vendors were also able to add their own extensions and explore what could be added to HTML. On the other hand, the result was large amounts of invalid HTML to the point that HTML5 had to add explicit rules for parsing invalid HTML.

Law and security

Alternatively, the weaker statement,

Everything which is not forbidden is allowed.

enshrines the principle of law that citizens are free to do whatever they will except when explicitly forbidden by a law.

On the other hand, the same principle applied to computer security is called enumerating badness and is widely held to be a bad idea: put simply, while you as the user of your computer want the freedom to do whatever you want on it, you probably don't want arbitrary code which may have been written by malicious actors to have those same freedoms and it's unfeasible to list all the bad programs as much as you may try.

Instead, many modern systems support the reverse,

Everything which is not allowed is forbidden.

or enumerating goodness, in the form of software repositories and app stores. Although some of these implementations unfortunately go against user freedom.

.NET

While it seems like we have exhausted the variants of this phrasing, as the title of this post suggests, some software systems follow yet another one:

Some things not allowed are compulsory.

Yes, you read that right.

Using the Windows Azure .NET SDK v2.3, in a web role, the web.config file contained the following automatically generated XML:

  <system.diagnostics>
    <trace>
      <listeners>
        <add type="Microsoft.WindowsAzure.Diagnostics.DiagnosticMonitorTraceListener, Microsoft.WindowsAzure.Diagnostics, Version=2.3.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" name="AzureDiagnostics">
          <filter type="" />
        </add>
      </listeners>
    </trace>
  </system.diagnostics>

I was cleaning up warnings in this project hoping narrow down an unrelated issue and saw this warning on the <filter type="" /> tag:

The 'type' attribute is not allowed.

Naturally, I removed the type="" attribute which didn't seem to be doing anything since the warning said it wasn't even allowed. To my surprise, when I ran the code, it failed to run due to the initialization code throwing an exception with the following message:

Required attribute 'type' not found.

Hence, the type attribute is apparently both not allowed and compulsory.

As the message telling me it is not allowed was merely a warning and not an exception, I put it back and decided not to worry too much about it. Removing the <filter /> tag entirely also seemed to work and eliminate the warning as well.