🚀 go-pugleaf

mjs@hubcap.clemson.edu (M. J. Saltzman) writes:

>In a previous article (now expired at my site), I asked if the
>notorious "struct hack" was still conformant if the tail array in the
>struct was of type other than char.  Since no-one ever replied, I'll
>ask one more time.

>My reading of the earlier thread on this topic was that the hack was
>indeed legal, but all the previous examples used an array of char in
>their examples.  Thus, is there any reason it would not be legal to 
>declare:

>	struct vector {
>		int  size;
>		double v[1];
>	} *vec;

>and then code (for example):

>	vec = malloc(sizeof(struct vector) + (n - 1) * sizeof(double));
>	vec->size = n;

>	for ( i = 0;  i < vec->size;  i++ )  vec->v[i] = 0.0;

I don't see that the type of the array makes any difference.

I do believe that the hack is not strictly conforming, however, since
you would be using array indices outside the declared array bounds
(char or double makes no difference).  The results of doing so are
undefined.  Nonetheless, I don't know of any implementations where this
would not work.

I have thought of a plausible implementation where the struct hack
would fail, however.

An implementation is allowed to add padding to the end of a struct.
Suppose that the implementation adds space to the end of each struct
type where it encodes information used for run-time error checking.
The struct hack would fail in this case, since it would overwrite data
used by the run-time system.  I believe the implementation would still
be conforming, since we are in the arena of undefined behavior.
-- 

Steve Clamage, TauMetric Corp, steve@taumet.com