Re: getdirentries_args and other kernel syscall structures

From: rookie (asmrookie_at_gmail.com)
Date: 11/23/05

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    Date: Wed, 23 Nov 2005 12:23:07 +0100
To: Daniel Rudy <dr2867@pacbell.net>

    2005/11/23, Daniel Rudy <dr2867@pacbell.net>:
    >
    > Ok, I'va got a little question here. In the structure
    > getdirentries_args, there seems to be duplicated fields that I'm not
    > entirely sure what they do. Here's the definition of a structure
    > verbatim from sys/sysproto.h:
    >
    > struct getdirentries_args {
    > char fd_l_[PADL_(int)]; int fd; char fd_r_[PADR_(int)];
    > char buf_l_[PADL_(char *)]; char * buf; char buf_r_[PADR_(char *)];
    > char count_l_[PADL_(u_int)]; u_int count; char
    > count_r_[PADR_(u_int)];
    > char basep_l_[PADL_(long *)]; long * basep; char
    > basep_r_[PADR_(long *)];
    > };
    >
    > Now my question is what does the l and r variables do? It seems that
    > they do something with padding the data based on the endian of the
    > machine? I look through this header file, and I see all the structures
    > have similar constructs. Is it something that can be safely ignored

    It just pads in the right way (according with endianism) the structure
    to the right word. For example, x86 gots sizeof(long *) == 4. If you
    want to have a syscall structure like that:

    struct example_sys
    {
       char f;
       short p;
       int g;
    };

    it is misaligned. In order to get a proper padded structure (all 32
    bits entries) to speed-up accesses to the members, this little trick
    is used.

    Attilio 

    --
Peace can only be achieved by understanding - A. Einstein
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