Re: data structure question

[Glynn Clements <glynn@gclements.plus.com>]

gumbold wrote:

> > > why somebody wants to write such thing
> > > struct abc {
> > >     __u16 a;
> > >     __u16 b;
> > >     char abc[0];
> > > } __attribute__((packed));
> > > Specialy for zerod array of chars.
> > 
> > A structure which ends with a zero-length array is intended for use as
> > the "header" for a variable-sized block of data, e.g.:
> > 
> > 	struct abc *new_abc(__u16 a, __u16 b, const char *string)
> > 	{
> > 		int len = strlen(string);
> > 		struct abc *p = malloc(sizeof(struct abc) + len + 1);
> > 	
> > 		p->a = a;
> > 		p->b = b;
> > 		memcpy(p->abc, string, len + 1);
> > 	
> > 		return p;
> > 	}
> > 
> >   
> 
> So if my compiler can't handle such code, can i change it to
> struct abc {
>     __u16 a;
>     __u16 b;
>     char abc[1];
> } __attribute__((packed));

Yes, although you would need to adjust the size calculation if you
need to calculate the exact size of the overall block.

Zero-length arrays aren't valid in C89, but are in C99. Also, gcc has
supported them as an extension since long before it started supporting
C99.

-- 
Glynn Clements <glynn@gclements.plus.com>