-mstructure-size-boundary problem

-mstructure-size-boundary problem

[Shaun Jackman]

I compiled the following snippet with
-mstructure-size-boundary=8
Unfortunately, sizeof( s) and sizeof( s0) both report 16 (should be 14).
Through inspection with GCC, there's no padding between the members, so I
assume the padding is being added to the end.
I thought this is what -ms.s.b. was supposed to prevent.
Help?

typedef struct {
    int src;
    int dst;
    char mbz;
    char protocol;
    short length;
    short checksum;
} s;
 
int
main()
{
  s s0;
  int i;
  i = sizeof( s);
  i = sizeof( s0);
  return 0;
}

--
host=i686-pc-linux-gnu
target=arm-elf
--

Thanks,
Shaun



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-mstructure-size-boundary problem

[Doug Evans]

Shaun Jackman writes:
 > I compiled the following snippet with
 > -mstructure-size-boundary=8
 > Unfortunately, sizeof( s) and sizeof( s0) both report 16 (should be 14).
 > Through inspection with GCC, there's no padding between the members, so I
 > assume the padding is being added to the end.
 > I thought this is what -ms.s.b. was supposed to prevent.

Not quite.

 > Help?
 > 
 > typedef struct {
 >     int src;
 >     int dst;
 >     char mbz;
 >     char protocol;
 >     short length;
 >     short checksum;
 > } s;

If you want to make this 14 bytes, you also have to mark it as packed.

 typedef struct {
     int src;
     int dst;
     char mbz;
     char protocol;
     short length;
     short checksum;
 } s __attribute__ ((packed)); // IIRC

-ms.s.b. applies after packing concerns.  Note that on other
targets where structure-size-boundary is 8 (e.g. sparc)
sizeof (s0) == 16 which is what it should be.  The padding
is for alignment requirements: if there's an array of them,
you want each array element properly aligned.

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