C Preprocessor Hell

http://jacquesmattheij.com/C+Preprocessor+Hell

by emwa • 15 years ago

46 29 15 years ago

jacquesmattheij.com

kzrdude• 15 years ago

The ## in front of __VA_ARGS__ is a gcc extension.

Jabbles• 15 years ago

It's in 6.10.3.3 of the C standard.

ge0rg2• 15 years ago

The code in the article is actually less horrible than I expected from the title. However, it seems like a situation which could be more elegantly solved with X-Macros (http://drdobbs.com/cpp/184401387), provided that you consider X-Macros elegant.

X-Macros: you have a header file consisting of lines in the form of "FOO(name, type, defvalue);" and include it several times with different definitions for FOO.

jacquesm2• 15 years ago

I don't particularly like include files in the middle of structs, but it looks like X macros might work.

Thanks for the tip, I'll have a look at them to see if there is a way around that so that it would work from a 'top' included file as well.

The project I'm working on has a single 'master include' file and I'd hate to break that convention.

drblast• 15 years ago

I don't know why people don't seem to think of this when confronted with the limitations of the standard C preprocessor, but Perl or Lisp both make excellent C preprocessors.

You aren't required to only use the C source code transformation tools that a default install of GCC provides.

jerf• 15 years ago

It introduces a new dependency on a new language that has no relationship to C. For local projects, sure, for projects you want to distribute it's risky.

If you're about to reply it's a risk you'd take... me too, in general. CPP sucks. But not wanting to introduce that risk is a valid choice in many cases.

anon_d4• 15 years ago

Yep. Why try to force a tool into a role it was never designed for? You use m4 or just write a preprocessor in any language. If you really want to do complicated lisp-style meta-programming stuff with C, just write another C program to pre-process your C program. It's more work, but it gives you complete control over the syntax.

angersock• 15 years ago

...or just use Python, Ruby, or Lisp directly?

Doing weird things to a C file (while occasionally useful) is just increasing the technical debt of your teammates later.

(though, we do have a closure-ish macro in our codebase that's pretty nifty, so do as I say not as I do etc.)

plorkyeran• 15 years ago

Using external programs to generate code nontrivially increases the required complexity of your build system, and unless you're targeting Linux only, it isn't really given that users will already have perl installed, much less a lisp interpreter.

anon_d4• 15 years ago

nontrivially increases the required complexity of your build system.

No it doesn't. It only requires a single rule in your Makefile. Something like:

    %.c: %.cpre preprocess.pl; ./preprocess.pl <$^ >$@

epenn• 15 years ago

True but this also creates extra dependencies for any project doing this. If you need to be able to compile on various platforms and/or machines doing so can introduce unnecessary complications as a result.

jacquesm2• 15 years ago

For valid reasons the build system is very minimal and the number of external dependencies is limited to 'absolutely necessary', adding another interpreter to the list of dependencies is simply not an option.

But sure, if that's not one of the limitations you're working under then code generation through another language is definitely a possibility.

cube132• 15 years ago

Because it moves you to a whole new circle of hell from the very common C preprocessor one that every C developer on the planet has been in at one point or another.

If you're using VS, you're going to want to use the included pre-build steps in the vcproj to generate the source. Unfortunately, you can't just edit the source in place, unless your team is absolutely in love with your SCS' rollback capability. So that means that in order to edit the code, you need to open it up in either another project, or open it outside the IDE's solution environment.

It also removes the ability to do partial rebuilds, since every file will be necessarily touched by the preprocessing script before compiling, so it's a "new" file as far as the IDE is concerned.

Debugging becomes a chore, because the code in source control is not the code that was compiled, breaking line counts and breakpoints.

Finally, it's a deployment nightmare for your build staff, since they need to make sure that everyone is standardized on the same perl version, maintain the scripts in addition to the makefiles, and make sure that everyone's dev environment works with it.

ctdonath• 15 years ago

"Lisp programmers should stop reading right now because they'll likely suffer severe injury of the jaw muscles as they laugh themselves silly at how hard it is to do some things in C."

Naive question: in Lisp, how would you set the byte at address 0xDEADBEEF to 0x42?

bunderbunder3• 15 years ago

There are some who would suggest that allowing you to commit segment violations is not generally a desirable language feature.

BudVVeezer2• 15 years ago

Except when 0xDEADBEEF happens to be the memory address overlaying the custom hardware registers, and setting it to 0x42 turns the blinky light on. There are some cases where manipulating static memory locations is not only a good thing, but the only way to do something (at least for embedded programming). Not saying that your point is invalid, btw.

groovy2shoes• 15 years ago

The ability to commit segment violations is a consequence of the broader abilities you get with pointers as a language feature. In many cases, neither of those things are desirable, but there are cases (e.g., systems programming) where they are very desirable.

wglb• 15 years ago

Define a C type and use FFI to set the value in memory.

Quite easy.

Loke• 15 years ago

It's very simple:

(setf (cffi:mem-ref (cffi:make-pointer #xDEADBEEF) :int) #x42)

cube132• 15 years ago

Why not just make an internal API function? In my experience with high performance, multiplatform C, macro usage is usually the last thing we do. Macros are absolutely horrendous to debug, and tends to lead to less readable code. They're useful to alias specific platform implementations to a standard interface, or for tiny functions that absolutely need to be inlined for performance.

In this specific case, it might make more sense to have the programmer tell you how many arguments to expect and work with it that way, rather than going through this chain of macros. C doesn't allow function arguments to change dynamically, so that might be a slightly better approach. It would be easier to understand, but a bit harder to maintain code that uses it.

C Preprocessor Hell

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