🚀 go-pugleaf

lele@itnsg1.cineca.it (Lele Gaifax) writes:

>struct point { int x; int y; };
>
>/* This macro should return a pointer to an initialized
> * struct point. I use it to generate inline points to be passed
> * as argument. If some space gets alloced, I would like it freed
> * automatically... */
>
>#define POINT(_x,_y) ({struct point _p = {_x,_y}; &_p;})
>
>Ok. point.c defines other functions that accept a pointer to a
>struct point as the first argument. The question is about the macro POINT():
>is it correct to use it like
>	...
>	move_to_point (POINT(45,34));
>	...
>??. Please note that *it does work*, but I guess it is not a correct use
>of this GCC feature.

You are right - it is not correct to use it like that.
It might work most of the time, and yet mysteriously fail
at random intervals.

>I tried to use alloca to setup such a macro:
>
>#define POINT(_x,_y) ({struct point * _pp = alloca(sizeof (struct point)); \
>		       *_pp = (struct point){_x,_y}; \
>		       _pp;})
>
>Alloca gets some memory from the stack, and frees it when the function exits...
>or when the block ends?

When the function exits.
Using alloca() should work just fine.

-- 
Fergus Henderson                     fjh@munta.cs.mu.OZ.AU