VxWorks / Tornado II FAQ

There are lots of complex rules on getting structures sized the way one wants on an i960 for consistency with other machines. By default, i960 GCC aligns structures and objects such that they are on the next higher power of 2 boundary (up to a maximum of 16 bytes). We want to change this to pack data tightly. When doing this, we need to avoid one capability of GCC; making enum types only as big as needed to hold the range of values.

GCC allows "short" enum types. The option -fshort-enums turns on this option (it is not on by default unless the compiler is built to emulate the Intel i960 tools which isn't the case with VxWorks). The option -fno-short-enums turns it off. As long as we don't turn on -fshort-enums then enumerated types should be set up as 32 bit words the way we want (regardless of other packing).

There are two ways to affect data packing in GCC; pragmas and attributes. Attributes are GCC specific. The GCC doc says to always use attributes because:

Some people object to the __attribute__ feature, suggesting that ANSI C's #pragma should be used instead. There are two reasons for not doing this.

• It is impossible to generate #pragma commands from a macro.
• There is no telling what the same #pragma might mean in another compiler.

These two reasons apply to almost any application that might be proposed for #pragma. It is basically a mistake to use #pragma for anything.

Attributes are applied after the '}' in typedefs and after the variable name in variable defs.

I did some testing to try to check packing. I took a structure and tried to pack it. I added an extra single byte value to the structure to make it an "odd" size. I found that I got optimal packing by applying "pragma align 1" and "attribute packed" to the structure. Note, I couldn't use the "aligned" attribute to achieve the same effect as the "aligned" pragma because "pragma aligned" is recognized in the i960 specific GCC code as a way to lower the default alignment applied whereas the "aligned" attribute can only be used to increase the minimum alignment. Thus we need to use the alignment pragma and packing attribute.

The packing attribute is "__attribute__((packed))". We can implement this packing attribute string as a macro:

#define PACK_STRUCT __attribute__((packed))
    

This is good since this string screws up the C "indent" program we use if entered directly.

So it seems we should use "#pragma align (1)" and "#pragma align (0)" to bracket portable structure definitions and the macro PACK_STRUCT on the structure definitions. The PACK_STRUCT macro has to be applied to all nested structures too.

To avoid portability problems we should conditionally define PACK_STRUCT for the i960 and bracket all the alignment pragmas as follows:

/* definitions to change alignment to be tightly packed and then restore
 * it to default alignment (16 bytes)
 */
#ifdef I960
#pragma align 1
#endif


#ifdef I960
#pragma align 0
#endif
    

EXAMPLE:

Given this, I would code your example as:

/* set tight packing */
#ifdef I960

#define PACK_STRUCT __attribute__((packed))
#pragma align 1

#else

#define PACK_STRUCT

#endif

struct A {
    short x
    short y
    struct B {
        char a
        char b
        char c
        char d[..]
        } PACK_STRUCT;
} PACK_STRUCT;

/* restore default packing */
#ifdef I960
#pragma align 0
#endif
    

CAVEAT:

So, I hope this explanation helps. I found that I got the results I wanted with it but I always defined the structures with typedefs as in:

  typedef struct {
    ...
  } PACK_STRUCT NAME_T;
    

You didn't include any names in your example but I think if you had, the PACK_STRUCT would go after the object name.

As a last note, there was one little "nasty" thing I came across with this. When I was doing this work (a few years ago so the details are a bit fuzzy), I found that the assembler code generated for accessing packed structures ended up using all byte accesses even where the data was properly aligned for larger access operations. I don't remember if this was affected by optimization or not. I would just suggest not to bother packing things unless you need to.

© J.A. Borkhuis, 2000 - 2005