Tips on writing C macros

http://www.commonsense4commonpeople.net/2008/11/tips-on-writing-c-macros.html

by adamo • 17 years ago

17 18 17 years ago

What's that nonsense with the do-while loop? Why not just enclose the macro body in both curly braces and then in parentheses?

Also, more useful (GCC only, though) than just enclosing parameters in parentheses, because it makes macros side-effect safe: http://en.wikipedia.org/wiki/C_preprocessor#Multiple_evaluat... Of course, in the macro in the article, the first 2 parameters need to be modifiable lvals anyway, so for those the whole deal with parentheses and the typeof construct is a moot point.

I'd write the proposed macro as:

  #define MYMACRO(a,b,c) \
    ({ \
      typeof (c) _c = (c); \
      a = b + _c; \
      b = _c * 2; \
    })

greyboy• 17 years ago

Excuse my lack of C knowledge, but using your proposed macro, what would be the result of calling:

  MYMACRO(x,y,x+y)

with your inclusion of typeof()?

paulbaumgart5• 17 years ago

typeof is pretty smart. Since in C all types are known at compile-time, the compiler can put in there in whatever type typeof's parameter resolves to- even if it's an expression.

As an example:

  int main () {
    typeof (1.0f + 10) a;
    int * b = &a;
  }

-----

  $ g++ test.c
  test.c: In function ‘int main()’:
  test.c:3: error: cannot convert ‘float*’ to ‘int*’ in initialization

I'm using g++ because it actually spits out type information for incompatible pointers- probably something having to do with it being an error in C++ but valid in C (with a warning, at least in GCC).

michaelfairley• 17 years ago

With your code, placing a semicolon after the macro leads to a compile error. Semicolons are not valid after arbitrary blocks, but it is acceptable to place them after do-while loops.

It's much nicer to be able to call "MYMACRO(1, 2, 3);" than it is to have to remember to leave the semi-colon off.

paulbaumgart5• 17 years ago

It compiles fine on GCC. For other compilers, the Wikipedia page I linked offers the caveat:

"This construct is not legal ANSI C; both the typeof keyword, and the construct of placing a compound statement within parentheses, are non-standard extensions implemented in the popular GNU C compiler (GCC)."

tb• 17 years ago

The do-while "nonsense" is a pretty standard construct, so much so that I'd never seen your approach - which I admit looks nicer. How well is that supported across different compilers? Is it part of C99, for example?

paulbaumgart5• 17 years ago

No, it's not in C99: http://www.redhatrenewals.com/docs/manuals/enterprise/RHEL-4...

Admittedly I have little experience with compilers other than GCC, but all things considered, I'd shy away from using anything but really basic macros without the compound-statement-in-parens and typeof() extensions that GCC provides.

Perhaps I spoke too harshly, calling it "nonsense". :-)

tedunangst• 17 years ago

It's only supported by gcc and some edg based compilers. edg has trouble with it in some c++ code. Just think of how adding this complicates your grammar.

gchpaco• 17 years ago

Curly braces and then parens is a GCC extension. If you're going to do complicated macrology, GCC is the way to go because of typeof and a bunch of related extensions designed to make macro use work. But it's still a staggering amount of effort and not worth it in the long run.

abecedarius• 17 years ago

This ({ ... }) syntax was not in the original ANSI C or K&R. Is it in C99 or in GNU C only? I've only ever seen it in GNU C.

sketerpot• 17 years ago

What if you already have a variable called _c and you pass it to MYMACRO? Is there any way of generating a guaranteed-unique symbol, similar to GENSYM in lisp?

yvueywa• 17 years ago

Tip 1, don't

Tip 2, see tip 1

neilc• 17 years ago

That is silly; there are plenty of situations in which macros are useful in C. I'd be inclined to avoid complex multi-line macros that multiply-evaluate their arguments unless there's a compelling need for them, however.

_sh• 17 years ago

How do you step through multi-line macros in a debugger?

shadytrees• 17 years ago

GDB has some bare-essentials macro support for the preprocessor macros, and I imagine people can get fancier with their own GDB scripts and functions.

http://sourceware.org/gdb/current/onlinedocs/gdb_11.html

ori_b• 17 years ago

Use inline functions instead of macros whenever possible. They don't have all the tricky gotchas, and they generally have no performance impact over macros.

dspeyer• 17 years ago

> Passing MyArray[x+3] as a macro argument would lexically copy this expression in multiple locations in the macro expansion, causing the generated code to needlessly evaluate *(Myarray+x+3) again and again and again.

Wouldn't any decent optimizing compiler eliminate those common subexpressions? That's one of the basics, isn't it?

Of course, if you're in a tight enough loop to worry about that, check the assembly by hand.

Tips on writing C macros

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