Skip to main content

"Inchworm on a Stick" in PHP

~- is known as the inchworm on a stick operator in Perl obfuscation circles. It isn't really an operator; it's a term coined to describe negating a value and then taking its compliment, the apparent effect of which is to decrement the value. (To understand why, you need to understand how negative numbers are represented in binary.)

In Perl, ~- works for positive values:

for ($i = 3; $i > -3; $i--) {
    print ~-$i . "\n";
}

$ perl test.pl
2
1
0
4294967295
4294967294
4294967293

Interestingly enough, when the same is tried in PHP the results are slightly different...

2
1
0
-1
-2
-3

PHP treats the value as a signed integer where Perl treats it as an unsigned integer. I don't know which way is "better" or which is "more correct," but it is something I found interesting and I thought would be worth sharing.

Labels

Labels:

Comments

  1. AnonymousFebruary 25, 2011 at 9:58 AM

    So why the difference? Clearly it isn't dependent on the processor architecture. Also it makes me wonder if there is a way to modify the default behavior in either case.

    Interesting post!

    ReplyDelete

Post a Comment

Popular posts from this blog

Writing a Minimal PSR-0 Autoloader

An excellent overview of autoloading in PHP and the PSR-0 standard was written by Hari K T over at PHPMaster.com , and it's definitely worth the read. But maybe you don't like some of the bloated, heavier autoloader offerings provided by various PHP frameworks, or maybe you just like to roll your own solutions. Is it possible to roll your own minimal loader and still be compliant? First, let's look at what PSR-0 mandates, taken directly from the standards document on GitHub : A fully-qualified namespace and class must have the following structure \<Vendor Name>\(<Namespace>\)*<Class Name> Each namespace must have a top-level namespace ("Vendor Name"). Each namespace can have as many sub-namespaces as it wishes. Each namespace separator is converted to a DIRECTORY_SEPARATOR when loading from the file system. Each "_" character in the CLASS NAME is converted to a DIRECTORY_SEPARATOR . The "_" character has no special ...
6 comments
Read more

What's Wrong with OOP

Proponents of Object Oriented Programming feel the paradigm yields code that is better organized, easier to understand and maintain, and reusable. They view procedural programming code as unwieldy spaghetti and embrace OO-centric design patterns as the "right way" to do things. They argue objects are easier to grasp because they model how we view the world. If the popularity of languages like Java and C# is any indication, they may be right. But after almost 20 years of OOP in the mainstream, there's still a large portion of programmers who resist it. If objects truly model the way people think of things in the real world, then why do people have a hard time understanding and working in OOP? I suspect the problem might be the focus on objects instead of actions. If I may quote from Steve Yegge's Execution in the Kingdom of Nouns : Verbs in Javaland are responsible for all the work, but as they are held in contempt by all, no Verb is ever permitted to wander about ...
9 comments
Read more

Safely Identify Dependencies for Chrooting

The most difficult part of setting up a chroot environment is identifying dependencies for the programs you want to copy to the jail. For example, to make cp available, not only do you need to copy its binary from /bin and any shared libraries it depends on, but the dependencies can have their own dependencies too that need to be copied. The internet suggests using ldd to list a binary’s dependencies, but that has its own problems. The man page for ldd warns not to use the script for untrusted programs because it works by setting a special environment variable and then executes the program. What’s a security-conscious systems administrator to do? The ldd man page recommends objdump as a safe alternative. objdump outputs information about an object file, including what shared libraries it links against. It doesn’t identify the dependencies’ dependencies, but it’s still a good start because it doesn’t try to execute the target file. We can overcome the dependencies of depende...
Post a Comment
Read more