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*(char*)0 = 0; – What does the C++ programmer intend with this code? [video]

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Re: *(char*)0 = 0; – What does the C++ programmer intend with this code? [video]

#61

Post by ecshafer »

zh3 wrote: post #55
vore wrote: post #42

Earlier quoted context omitted.

You say that as if that's a bad thing – I absolutely need to kept safe from myself, because I need all the help I need to make sure I'm doing things right!

I'd agree with that, to the extent that understanding how badly your tools can hurt you is an important thing to learn. Consider how a surgeon would respond if told not to use a scalpel because of the risk of accidental injury when using a sharp tool. We learn from our mistakes - to which I'd add, sometimes we can afford to make mistakes (home programs) and other times we can't (safety-critical code).

I think a more accurate analogy would be some kind of scalpel that would not let the surgeon cut deeper than they needed. So if they needed a 1 cm incision, it would somehow stop the blade form going in 1.1 cm. In that case, some experienced surgeon may say "But I know how to make a 1cm incision!", but I think that the reality is that preventing a mistake from happening is valuable.

You see the same arguments against static analysis, unit tests, strong type systems, etc. The evidence seems to favor systems which prevent errors over artisinal expertise.

Re: *(char*)0 = 0; – What does the C++ programmer intend with this code? [video]

#63

Post by Kamq »

ecshafer wrote: post #61
zh3 wrote: post #55

Earlier quoted context omitted.

I'd agree with that, to the extent that understanding how badly your tools can hurt you is an important thing to learn. Consider how a surgeon would respond if told not to use a scalpel because of the risk of accidental injury when using a sharp tool. We learn from our mistakes - to which I'd add, sometimes we can afford to make mistakes (home programs) and other times we can't (safety-critical code).

I think a more accurate analogy would be some kind of scalpel that would not let the surgeon cut deeper than they needed. So if they needed a 1 cm incision, it would somehow stop the blade form going in 1.1 cm. In that case, some experienced surgeon may say "But I know how to make a 1cm incision!", but I think that the reality is that preventing a mistake from happening is valuable. You see the same arguments against…

I think an even more accurate analogy would be if a surgeon had some kind of scalpel with a 1 cm limit, but if a surgery suddenly changes in the middle, and suddenly requires a deeper incision, the surgeon has to spend a non-zero amount of time re-configuring the scalpel.

This casts flexibility vs safety as a tradeoff, which it is.

Re: *(char*)0 = 0; – What does the C++ programmer intend with this code? [video]

#64

Post by tzs »

stefan_ wrote: post #28
dekhn wrote: post #25

When I see that code my head immediately translates to "store a byte with value 0 at memory address zero". 0 the integer needs to be cast to a char * to treat it as a byte address (rather than a word) and the * means "assign to the memory location given by the pointer. I would probably use something like this when hacking on an Apple II with a zero page. I have also written programs that ran on VMS which dumped memor…

On ATmega processors, the register file is mapped to 0, so this would be writing 0 to r0.. which GCC reserves as a dedicated 0 register anyway.

The PDP-10 did that. There was actually core memory at addresses 0-15 but if you had registers they overlaid that.

Yes, I said if you had registers--if you were cheap you could buy your PDP-10 without registers and all the instructions that referenced registers would use core instead.

Since the registers were in effect just 16 words of fast semiconductor memory overlaying the first 16 words of slow core memory you could do anything in them that you could do in regular memory, including running code out of them.

If you had a small loop that did a lot of iterations you could sometimes get a significant speed boost by copying the loop into the registers and running it there.

Re: *(char*)0 = 0; – What does the C++ programmer intend with this code? [video]

#65

Post by jinushaun »

yodsanklai wrote: post #62

Not a big fan of the format of the talk. The guy is a good orator, but it takes ages to go to the point. A bit annoying if you're actually interested in the technical part rather than the jokes.

Same here. This talk reminds me why I stopped going to tech meetups with tech talks.

Re: *(char*)0 = 0; – What does the C++ programmer intend with this code? [video]

#66

Post by inkyoto »

sargstuff wrote: post #52
wvenable wrote: post #26

Earlier quoted context omitted.

Back in the day there were no protections. You could write to any address whether it was used by the CPU for interrupt vectors, part of the OS, hardware addresses, anything.

pre-MMU, unless using DEC box.

It is still true even today for microcontrollers – many of them come with a miniscule amount of RAM, no MMU and generally unpredictable memory maps.

Re: *(char*)0 = 0; – What does the C++ programmer intend with this code? [video]

#67

Post by kevin_thibedeau »

layer8 wrote: post #29
sacnoradhq wrote: post #12

This statement would be technically legal on its own in x86 real mode if the compiler didn't do null pointer checks. However it would set the divide-by-zero IRQ handler to itself 0000:0000, and when the next division by zero happened, the machine run into UB (likely a reset or halt) because it would jump there, do 4x ADD byte ptr [BX + SI], AL (or ADD byte ptr [EAX], AL) followed by running the remaining interrupt ve…

Not quite, I think. Since this is a char pointer being used, only the first byte of the interrupt address would be zeroed. Since in real mode those are far pointers, the lower byte of the segment would be zeroed. So xx00:xxxx. But yes, the interrupt table was my first thought when reading the headline.

Char can be the same size as short or int. You can't assume it is one byte.

Re: *(char*)0 = 0; – What does the C++ programmer intend with this code? [video]

#70

Post by interroboink »

Some comments about this being valid on old-school computers, but even in more modern contexts, there are such cases. When programming SPUs on the Cell processor (okay, still a bit old), address 0 was valid memory you could use, in the 256K per-SPU local store.
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