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Handling of structures with elements of different size

Sivan Ramachandran
Prodigy 10 points

What are the guidelines for defining structures with elements of variable element sizes ( 32 bit, 16 bit 8 bit and boolean)?

Should any specific alignment needs to be taken care? There is a #pragma directive called STRUCT_ALIGN available in the help, but Iam not sure what purpose does it serve.

  • 0
    Guru 17650 points

    Hi Sivan,

    if you're using CCE or CCS have a look a the C compiler Users Guide (http://focus.ti.com/lit/ug/slau132c/slau132c.pdf) - par. 6.2.1.2 and ongoing. There is an excelent explaination.

    Rgds
    aBUGSworstnightmare

  • 0
    TI__Intellectual 2765 points

    It depends upon the device family you are using. Another answer pointed to a discussion of how structures are handled by the MSP430. If, on the other hand, you were using a device which uses the C6000 CPU, then I would recommend looking at two sections from the C Compiler Users Guide (spru187o.pdf); specifically:

    • 7.2.1.8 Structures and Arrays
    • 6.8.19 The STRUCT_ALIGN pragma

    As with the other response, the Users Guide provides a pretty good discussion of how structures are handled by the compiler. Here are a few notes regarding structure alignment:

    • If you create a C structure using the compiler, it should have no problem reading that structure. (You have to be much more careful when creating data structures in assembly.)
    • Structures are automatically aligned on the largest datatype they contain.
    • The start of Arrays of chars and shorts (outside of structures) are aligned on 8-byte boundaries on most C6000-based devices, which helps with optimization. This allows the processor hardware to better utilize the 64-bit data buses.
    • Arrays (of chars and shorts) contained within structures are not automatically aligned to 8-byte boundaries. If this is desirable (for optimization purposes), then you would need to force the alignment of the array by preceding the array element with a 'double' or 'long long'; or, alternatively you could force alignment using a union. 

    Hope this helps!