Subject: Re: [PATCH] bisect: save heap memory. allocate only the required amount

From: Jeff King <[email-masked]>
Date: Tue, 26 Aug 2014 08:14:00 -0400

On Tue, Aug 26, 2014 at 12:57:21PM +0100, Ramsay Jones wrote:

> > +	ret = xcalloc(1, base + extra);
> > +	va_start(ap, fmt);
> > +	vsnprintf(ret + offset, extra, fmt, ap);
> 
> What is the relationship between 'base' and 'offset'?
> 
> Let me assume that base is always, depending on your compiler, either
> equal to offset or offset+1. Yes? (I'm assuming base is always the
> sizeof(struct whatever)). Do you need both base and offset?

It's much more complicated than that. Take "struct name_decoration", for
instance, which looks like this:

  struct name_decoration {
	struct name_decoration *next;
	int type;
	char name[FLEX_ARRAY];
  };

On my 64-bit system using gcc, sizeof() returns 16; it has to pad the
whole thing to 64-bit alignment in case I put two of them in an array.
But offsetof(name) is 12, since the array of char does not need the same
alignment; it can go right after the type and make use of the padding
bits.

As a side note, that means that the original "char name[1]" (before it
became FLEX_ARRAY) was not any less efficient on 64-bit machines (the
1-byte went into the padding, and sizeof() was the same). It did matter
on 32-bit systems, though where it bumped the empty struct size from 12
to 16.

-Peff