Re: [PATCH 0/5] obstacks again

[Paul Eggert <eggert at cs dot ucla dot edu>]

On 11/02/2014 11:07 PM, Alan Modra wrote:
> Really, it's a fiction that the size argument of obstack_blank_fast has a type.

Yes and no.  It's a fiction in the sense that obstack_blank_fast is a macro, so 
its argument doesn't actually have a C type.  But it's not a fiction in the 
sense that obstack_blank_fast can be documented as if it were a function, and if 
a program calls it with valid arguments that a function would have, it behaves 
like that function.

In this sense, obstack_blank_fast is like putchar.  Although putchar is a macro 
and strictly speaking its argument doesn't have a C type, it is documented to 
take a value of type int, and if you call it according to its specification it 
behaves properly.

> I've added: "Earlier versions of obstacks allowed you to use 
> @code{obstack_blank} to shrink objects. This will no longer work."

But this doesn't suffice for obstack_blank_stack, which *can* shrink objects and 
where user code relies on this feature.  E.g., the following code (taken from 
GCC's finish_regexp_representation) should work if the current object size fits 
into 'int', and should continue to work if we change its 'int' to 'size_t' and 
allocate objects larger than what 'int' can represent:

   int length = obstack_object_size (&irp);
   obstack_blank_fast (&irp, -length);


> There another gnulib issue too. AC_FUNC_OBSTACK is no longer correct. The 
> system libc may support obstacks, but not the current version. It seems to me 
> the easiest solution is to always include obstack.o in libgnu.a.

Thanks, I wrote a "harder" solution, which builds obstack.o only if the system 
version doesn't work with arbitrary size_t values.  This is always true now but 
won't be once these fixes make their way into glibc.

> Ianother small tweak: Don't use alignof.h if __IBM__ALIGNOF__ is defined 
> (alignof.h just uses __alignof__ in that case), or when __alignof__ is defined 
> (for oddball systems, or testing). 

I'd rather insulate obstack from that internal detail of alignof.  I gave a shot 
at doing this in a different way, which preserves the insulation better.

A couple more things.  obstack_chunkfun and obstack_freefun should also return 
void.  And the code should avoid casting function values from one calling 
convention to another: this sort-of-works with the old API on glibc's current 
platforms but is likely to cause problems with the transition to the new one.

I installed the attached patches to gnulib to try to implement the above; please 
let me know of any issues.

Attachment: 0001-obstack-port-to-platforms-that-define-__alignof__.patch
Description: Text document

Attachment: 0002-obstack-do-not-assume-system-supplied-obstack-is-siz.patch
Description: Text document

Attachment: 0003-obstack-fix-macro-return-values.patch
Description: Text document

Attachment: 0004-obstack-avoid-potentially-nonportable-function-casts.patch
Description: Text document