BrowserUk has asked for the wisdom of the Perl Monks concerning the following question:

Update:Please ignore (or downvote or consider; whatever's appropriate!). Sorry to have bothered you.

Okay. That's what comes from trying to program when I've just got up!.

Somewhere in my head I had this vague recollection of seeing an 'array of bits' in what I remembered as being C code.

Now I've woken up a bit; what I was thinking of was LLVM's arbitrary bit integer type.

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Re: [Way OT] C: Arrays of bitfields?
by pme (Monsignor) on Jan 18, 2015 at 09:16 UTC
    Hi BrowserUk,

    I think you cannot do that in C. But you can use macro to hide the messy expression: 

    #define getsextet(UV, I) (( (UV) & ( 0x7f << (6*(I)) ) >> 6*(I))

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    and the appropriate 'set' and 'clr': 
    #define clrsextet(UV, I) ( (UV) & ~( 0x7f << (6*(I)) )
#define setsextet(UV, I, S) ( (UV) | ( ((S) & 0x7f) << (6*(I)) )

    [download]
    The sextet have to be cleared before set. I haven't had time to test these macros, please use them carefully.
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      But you can ... hide the messy expression:

      Indeed. Though I'll probably opt for inlined functions.

      With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice. Agile (and TDD) debunked
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Re: [Way OT] C: Arrays of bitfields?
by Anonymous Monk on Jan 18, 2015 at 09:14 UTC

    All addressing in C is by the byte as the smallest unit (sizeof(char) is one by definition). You can't take address of a bitfield, or sizeof a bitfield. So, no array of bitfields. But underneath, bitfield access involves a shift-and-mask in any case, so there's no problem with explicit shifts like n = (uv >> (6*i)) & 0x3f.

    As far as tricks go, well you might use fd_set and related macros for a small array of booleans, but that could be seen as a slipshod misapplication.

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