What's the meaning of "Algn" in "objdump -h"?

[Nick Clifton]

Hi Zuxy,

>> Does it mean that uninitialized vars in such objects may get
>> misaligned when linked together?

Yes.

>> Or the "aligned" attribute takes
>> effect only with -fno-common under mingw32?

Also yes.

> But for a static var
> 
> C:\MSYS\home\jimeng>type x.c
> static short b __attribute__((aligned(16)));

> C:\MSYS\home\jimeng>objdump -h x.o

> Sections:
> Idx Name          Size      VMA       LMA       File off  Algn
>  0 .text         00000020  00000000  00000000  0000008c  2**4
>                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
>  1 .data         00000000  00000000  00000000  00000000  2**2
>                  ALLOC, LOAD, DATA
>  2 .bss          00000010  00000000  00000000  00000000  2**3
>                  ALLOC
> 
> Is this by design or a bug?

A bug.  A gcc bug though, not a binutils one.  Have a look at the 
assembler output gcc generates for the definition of "b":

   .lcomm _b,16

Which assigns 16 bytes of space, unaligned, in the .bss section.  16 
bytes are assigned, rather than 2 because that is the largest alignment 
supported by the pe-i386 port of gcc.  (See assemble_static_space() in 
gcc/varasm.c for this).

It looks like the definition of ASM_OUTPUT_LOCAL in 
gcc/config/i386/bsd.h needs to be fixed, or maybe the file should be 
extended to include a definition of ASM_OUTPUT_ALIGNED_LOCAL.

Cheers
   Nick