Re: [Paparazzi-devel] I need an expert in C

[Famille Chateauvieux-Hellwig]

Hi Cédric,

What compiler are you using? With GCC you can say:
struct test_t {
  int  a;
  char b;
  int  c;
} __attribute__((__packed__));

If you can choose the order in which the items are placed in the structure, 
put the biggest at the front, and the shortest at the back.

typedef struct {
  long l; 
  long l2;
  short s1; 
  char c1;
  char c2;
} naturally_packed_t

Regards
Martial

Le vendredi 4 septembre 2009 08:47:41 Cédric Marzer, vous avez écrit :
> Hello,
>
>
>
> My OSD makes good progress but I am stuck with some C trick.
>
> The messages are stored in an array each char, short or long following each
> other.
>
> I want to map this message into different structures (for the gps message,
> the batterie message, etc.).
>
>
>
> The crude way to do that (it works), is the following :
>
> gps.utm_north=MAKE32(array[12],array[11],array[10],array[9];
>
> etc.
>
> But that means that I will write a dozen of this for each message…
>
>
>
> The (not so smart) way is to do that :
>
>
>
> gpspointer=&array+4
>
>
>
> but there is a trap…
>
> When you make a structure, the compiler makes sure that the short and long
> are aligned in the memory
>
> and add gaps after let's say a char to make sure that the next long is well
> aligned in the memory.
>
> That means that the data in my array and in the structure are not aligned.
> The process to put the gaps is called "padding".
>
>
>
> Some compiler and processor allow one to disable "padding". I doubt that
> this is the case with arm.
>
>
>
> So does anyone of you have a smart idea for a clean work around ? Otherwise
> I will just go the crude way…
>
>
>
> Cheers
>
>
>
> Cédric