Category Archives: Programming

Material related to computer programming.

Target then source

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I’m surprised it took me this long to figure this out. But there’s a very big problem with a command like this:

rsync var/data/ target:/var/data/

The problem is that in order to type that command, you first have to type the first part of the command, which includes this command:

rsync var/data/ target:/

That first part of the command is in fact a valid command, and if you pressed ENTER accidentally before you had finished typing the whole command, then rsync would begin to replace your entire root file system with the contents of var. That’s the sort of thing that will ruin your day.

So from now on, when I write command-line tools, I will nominate the target *before* the source, and if both the target and source are not specified then I will return an error.

p.s. Yes, the reverse problem exists, that when you nominate the target first you can still fuck up with the source, but given that you are operating on the target from the source, if you get the source wrong, you can just fix your command and run it again and it will be fixed up.

Blowfish

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I found myself reminiscing about my implementation of Blowfish today. It earned me the great honour of being listed on Bruce Schneier’s web page. My implementation made Blowfish compatible across 32-bit signed ints, 64-bit signed ints, and 64-bit floats. I wrote this code coming up on 12 years ago… how time flies.

RAM

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I learned a few things about RAM today. Things began with the realisation that a CPU can read 32GB of RAM (e.g. all of it) in about 10 seconds. That surprised me at first, and then I thought about it (for the first time ever, not usually a big fan of thinking, can lead to discomfort).

Then I learned that DDR4 has a 64-bit data bus whereas DDR5 has a 128-bit data bus.

And then I learned that today’s SDRAM used to be asynchronous DRAM.

Intuitionism

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Recently I read about Intuitionism and I can tell you unreservedly that I do not believe in it. I believe that objective things can exist independent of human minds and the most salient example I could give is in your computers. In your computer you can calculate 1 + 1 = 2 and you can do so quite independently of a human mind. 1 + 1 = 2 is an objective truth which can easily be modelled in a machine. Certainly mathematics can *also* be the result of constructive mental activity in human minds, but it is not limited to that. Also it might be interesting to note that machines will be able to find objective mathematical truths which are beyond the capacity of a human mind alone, in fact I’m quite sure that will have been done already (if you can think of an example feel free to let me know!).