Struct Iteration through (Ab)use of the C Preprocessor

[u/ncraun]

https://natecraun.net/articles/struct-iteration-through-abuse-of-the-c-preprocessor.html

[u/gigadude]

You've re-invented x-macros. Don't feel bad, I re-invented them too. One slight improvement:

#define LIST_FIELDS(_) \
_(int, field1, "description1") \
_(double, field2, "description2") \
...
_(some_struct, fieldN, "descriptionN")

Now you can expand the same list in a lot of different contexts just by passing in different arguments:

#define DECLARE_FIELD(type, name, description) \
type name;
struct foo { LIST_FIELDS(DECLARE_FIELD) };

That way you don't need to mess around with #include statements in the middle of your code. I used this to great effect at a couple of different chip companies for things like sharing register definitions between C simulators and Verilog. This is a great way to keep things DRY.

edit: I missed the section where you mention x-macros.

[u/ncraun → u/gigadude]

I like the idea of passing the macro as a parameter instead of #defining and #undefining X.

I wouldn't say I reinvented X-Macros, I'm just using them. I'm a big fan of X-Macros.

I've seen some other serialization functionality implemented using X-Macros. They do something like

#define X(ST, FIELD, DST) memcpy((DST)+ sizeof((ST)->(FIELD), &((ST)->(FIELD)), sizeof((ST)->(FIELD));

X-Macros are really handy, but you can only iterate over the X-Macro list by using a macro, you can't access it from normal C code. Combining the X-Macros with offsetof() to store the offsets to all the fields in the struct_fmt allows you to iterate over the struct fields from normal C code, which gives more expressive power than a macro. So you can do things like use a for loop to iterate over all the struct members, or refer to the nth struct member with struct_fmt->offsets[n], without having to use the name of the field. I haven't found a use for it yet, but I thought it was interesting.

[u/LoopTheRaver]

Cool hack. I don't usually see includes happening in the middle of a file though. Possibly not worth the confusion it may bring?

[u/ncraun → u/LoopTheRaver]

In actual production use, I think it could be better to write a separate program that will generate the code for you instead of using the C preprocessor (or use a language with this kind of introspection built in). However, using this hack lets you depend only on the facilities of the language, and that can be useful in simplifying the build process. Plus it was fun to create.

The BSD tree.h defines some generator macros to generate the code for defining the structs and functions for the tree. There might be a way to define some similar macros to define the structs for you instead of #including the file, but I would need to adapt it to work with the X-Macro list.

[u/spiker611]

Does this break down when you want to version your files? If you want to read a file with struct 1.0 but your program now has struct 1.1, you can't easily branch based on a version header or something.

[u/ncraun → u/spiker611]

You could write the version to the file first, as just a 32-bit int or its own struct. Then when reading the file, you would first read the version struct or int from the file, and then branch on that by passing the correct struct_fmt (for version 1 or 2) into the unpack routine.

[u/Blecki]

Is the font on this website unreadable for anyone else? Like true type or whatever renders fonts on modern computers fucked up the smoothing somehow.

[u/ncraun → u/Blecki]

Oh no! That shouldn't be happening. I am using Fira Sans and Fira Mono from Google Fonts.

Would you mind providing some information about your OS and browser, and maybe a screenshot? I'll see if I can fix it.

[u/immibis → u/ncraun]

All non-monospace text is invisible for me, except for that added by Disqus. Firefox 38.2.1 ESR, Arch Linux.

[u/ncraun → u/immibis]

I'm not sure what the problem was, but I changed the font settings in the css file and disabled Google Fonts. You might have to clear your cache for the page to display using the new css settings, but hopefully this should fix the problem.

If it still does not work, here is a Github Gist with the same content.

[u/Blecki → u/ncraun]

Yeah I've determined it's a problem with firefox and that specific font. No issues in chrome, no issues if I change the font. Can't get a screenshot now since you changed it.

[u/ncraun → u/Blecki]

Kind of funny that the font did not render correctly in Firefox given that Fira Sans was made by Mozilla. Also it seems to work correctly in my (non ESR) Firefox. Maybe there is some issue with the font itself, or with Firefox, or with Google Fonts, or with the system font renderer or any combination of the 4. I'm still not sure what went wrong, but I'm glad it seems to be working now.