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`three = 1` in the Linux sourcecode (2014)

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Re: `three = 1` in the Linux sourcecode (2014)

#91

Post by [deleted] »

inetknght wrote: post #48
hannasanarion wrote: post #39

Earlier quoted context omitted.

Why is the reason for doing this obvious? I can't see any benefit. I get why you would prefer to name constants for their use, like `numIterations=3` or whatever, but renaming every integer seems senseless.

> but renaming every integer seems senseless. When the integer itself is self-explainatory but you can't just assign the integer as a name. For example: constexpr int 42 = 42; // compiler error So instead: constexpr int fourtytwo = 42; // yay! However the obviousness of what 42 means is debatable. int universe = fourtytwo; // why? int sum = fourtytwo; // sum... of what? int magic = fourtytwo / 7;

[deleted]

Re: `three = 1` in the Linux sourcecode (2014)

#92

Post by akimball »

hinoki wrote: post #3
post-factum wrote: post #2

705 /* 706 * Iterate through the groups which hold BACKUP superblock/GDT copies in an 707 * ext4 filesystem. The counters should be initialized to 1, 5, and 7 before 708 * calling this for the first time. In a sparse filesystem it will be the 709 * sequence of powers of 3, 5, and 7: 1, 3, 5, 7, 9, 25, 27, 49, 81, ... 710 * For a non-sparse filesystem it will be every group: 1, 2, 3, 4, ... 711 */ 712 static unsigned…

Yeah, maybe calling them pow3, pow5, pow7 would be a bit clearer? But a 10s search for a comment cleared it up, so I don’t really see the problem...

threes_cursor fives_cursor sevens_cursor

Re: `three = 1` in the Linux sourcecode (2014)

#93

Post by dyingkneepad »

I don't know about the ext4 code specifically, but in hardware programming it is common to have stuff like "3 means 5". If for whatever reason a field of a register may only accept values that actually mean 1, 5 and 7 (to program whatever bits of the hardware, like clock divisors or whatnot), the hw register will probably only contain 2 bits, and be encoded in such a way where if you write 00b you mean 1d, if you write 01b you mean 5d and if you write 10d you mean 7. And 11b is reserved. This is a common thing, it's encoding to pack a sparse set of possible values into a few bits as you can.

But then, you should probably do something such as "#define CLOCK_DIVISOR_3 0x1" instead of simply "int three = 1".

Re: `three = 1` in the Linux sourcecode (2014)

#94

Post by l0b0 »

CWuestefeld wrote: post #36

Many years ago in an age of klocs and flowcharts, at a large three-letter computer company, there were iron-clad coding rules that must be obeyed, no questions allowed. In general you could see their reasoning, but such bureaucratic reasoning doesn't pay off. One example of this was that all numeric values used in a program must be factored out as symbolic constants. The reason for doing this is obvious, but it faile…

  #define zero 0
is just bad programming, barring some philosophical code which cares about "zero-ness". Names need to reflect what they represent, not their precise values. Are we looking up the first index? Then how about

  #define first_index 0
? Are we summing up values which are coerced to zero when empty? Then how about

  #define empty_value_integer 0
or

  #define additive_identity 0
?

Yes, that means there might be more than one variable with a value of zero, and that's completely fine since we now have several megabytes available for both source code and for compilation.

To be clear, I'm not endorsing the rule, just saying that there are more productive ways of dealing with it.

Re: `three = 1` in the Linux sourcecode (2014)

#95

Post by a1369209993 »

jerf wrote: post #76
waynesonfire wrote: post #66

Earlier quoted context omitted.

'zero' and '0' are the same label, for the digit 0. There is no advantage to re-defining it, in fact, it does the opposite. It's a good practice to define labels for magic numbers to promote more maintainable code through the DRY / single source of truth principle, document the magic number through the label name, and improve code archeology by enabling searching by label. These principles hold when the label given a…

Heh, try for #define 0 0

  > #define 0 0
That actually works on some implementations (the preprocessor grabs a token without checking that it's a identifier). I've seen:

  #define $ SOME_LINE_NOISE
  #define && HACKY_GARBAGE
  x = &&(y $ z);
as well.

Re: `three = 1` in the Linux sourcecode (2014)

#96

Post by a1369209993 »

kortex wrote: post #26
ronyeh wrote: post #11

Earlier quoted context omitted.

I see a problem: https://github.com/torvalds/linux/blob/master/fs/ext4/resize... Upon a quick code review, these lines look buggy: unsigned three = 1; unsigned five = 5; unsigned seven = 7; Without digging deeper, the reader of this code thinks "The first line surely must be a bug, right???"

unsigned three = 1; To me, that is a 20 foot tall neon orange flashing Chesterton's fence. I think, "surely there is a very strong reason for this." Maybe not a good reason, but definitely a compelling one.

> To me, that is a 20 foot tall neon orange flashing Chesterton's fence.

It's not a Chesterton's fence, though. The whole point of Chesterton's fence is that whoever put up the fence that you want to take down couldn't be bothered to add signage explaining why the fence was present, thus either it isn't important to keep the fence up long term, or they're a idiot, in which case there's no reason to belive they were correct in deciding that having a fence there was useful in the first place.

With this fence, if you walk a few meters to the side (read, "select 'ext4_list_backups' and hit ^F and ^G a few times); et viola, there's your signs.

Re: `three = 1` in the Linux sourcecode (2014)

#97

Post by 8note »

hyperman1 wrote: post #47
CWuestefeld wrote: post #36

Many years ago in an age of klocs and flowcharts, at a large three-letter computer company, there were iron-clad coding rules that must be obeyed, no questions allowed. In general you could see their reasoning, but such bureaucratic reasoning doesn't pay off. One example of this was that all numeric values used in a program must be factored out as symbolic constants. The reason for doing this is obvious, but it faile…

An application I touched a few weeks ago has a DB column named 'type' with values 1,2,3. So on to the source code we go: enum RecordType {TypeOne(1),TypeTwo(2),TypeThree(3); RecordType(int dbValue) ...} As it happens, I know an end user of this particular beast, so I show her some record IDs of each type and ask in what way they differ. She looks a few seconds, then says: 'This is clearly a type one record, the next…

One benefit there is that you don't get left with only context for what type one records used to be like before some evolving. The only baggage kept is by people aliasing that name in their own memory.

You don't get screwed because you called something apple to start with, but it now represents a race car

Re: `three = 1` in the Linux sourcecode (2014)

#98

Post by inetknght »

bluesmoon wrote: post #72
inetknght wrote: post #48

Earlier quoted context omitted.

> but renaming every integer seems senseless. When the integer itself is self-explainatory but you can't just assign the integer as a name. For example: constexpr int 42 = 42; // compiler error So instead: constexpr int fourtytwo = 42; // yay! However the obviousness of what 42 means is debatable. int universe = fourtytwo; // why? int sum = fourtytwo; // sum... of what? int magic = fourtytwo / 7;

until someone runs a spellcheck on this code and changes all locations to fortytwo

Fourtytwo: a number equaling four amounts of ten plus two singular amounts.

Fortytwo: a defensive fortification with a nickname. See also Boaty McBoatface.

Re: `three = 1` in the Linux sourcecode (2014)

#99

Post by trhway »

to me it is a great message that if you have attitude that your predecessors were stupid morons and you don't understand what happens here nor willing to bother to dig enough deep to understand it, you shouldn't be doing changes here. Attempt to correct it is a kind of warning signal for a PR containing the correction.

Re: `three = 1` in the Linux sourcecode (2014)

#100

Post by thrwyoilarticle »

l0b0 wrote: post #94
CWuestefeld wrote: post #36

Many years ago in an age of klocs and flowcharts, at a large three-letter computer company, there were iron-clad coding rules that must be obeyed, no questions allowed. In general you could see their reasoning, but such bureaucratic reasoning doesn't pay off. One example of this was that all numeric values used in a program must be factored out as symbolic constants. The reason for doing this is obvious, but it faile…

#define zero 0 is just bad programming, barring some philosophical code which cares about "zero-ness". Names need to reflect what they represent , not their precise values. Are we looking up the first index? Then how about #define first_index 0 ? Are we summing up values which are coerced to zero when empty? Then how about #define empty_value_integer 0 or #define additive_identity 0 ? Yes, that means there might be m…

>barring some philosophical code which cares about "zero-ness"

But that is the intrinsic meaning OP was talking about. Plenty of mathmatics deals with 0 or 1 as important constants. If you have a function that mods by a parameter, you need error handling for zero. Calling it value_that_causes_undefined_behaviour isn't going to improve code clarity. Beyond that;

for (i = 0; i , or some variation thereof, is very idiomatic C.

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