[Developers] macro as macro argument

[David Rideout]

I have a C macro which employs internal variables, e.g.

#define foo(arg1) { \
  int internal_var; \
  ... code(arg1) ... }

Often the macro is invoked multiple times in a single source file, which leads 
to a compiler warning about identical variables in scope or something like 
this.  I tried a hack:

#define foo(arg1, XX) { \
  int internal_var##XX; \
  ... code(arg1) ... }

which I wanted to call with

foo(bar, __LINE__)

to give each internal variable a unique name, but this instead makes each 
internal variable into 'internal_var__LINE__'.

Can someone more familiar with the workings of the C preprocessor suggest a 
way to avoid this compiler warning?

(I am trying to build an API in Cactus which uses macros, so in this sense the 
question is on-topic...)

Many thanks,
David