Question

I have a function that I need to macro’ize. The function contains temp variables and I can’t remember if there are any rules about use of temporary variables in macro substitutions.

long fooAlloc(struct foo *f, long size)
{
   long      i1, i2;
   double   *data[7];

   /* do something */
   return 42;
}

MACRO Form:

#define ALLOC_FOO(f, size) \
{\
   long      i1, i2;\
   double   *data[7];\
\
   /* do something */ \
}

Is this ok? (i.e. no nasty side effect – other than the usual ones : not “type safe” etc). BTW, I know “macros are evil” – I simply have to use it in this case – not much choice.

Answer 1

There are only two conditions under which it works in any “reasonable” way.

1. The macro doesn’t have a return statement. You can use the do while trick.

   #define macro(x) do { int y = x; func(&y); } while (0)
   

2. You only target GCC.

   #define min(x,y) ({ int _x = (x), _y = (y); _x < _y ? _x : _y; })
   

It would help if you explain why you have to use a macro (does your office have “macro mondays” or something?). Otherwise we can’t really help.