Thread: #pragma pack(1) workaround?

  1. 05-31-2008 #1
    l3iggs
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    #pragma pack(1) workaround?

    Hey everyone, I'm new here.

    I'm writing C code for an embedded platform where I have to stream specific messages (that I do not define) in and out an interface to another device.

    The compiler I must use does not support the #pragma pack directive (or any other directive to tweak structure member alignment).

    I have (about 50) structures set up that define the messages that will be passed in and out of the interface. But when I try to lay my structures onto the byte streams, things don't work out correctly because of the padding that the compiler is inserting between structure members.

    Does anyone have any ideas about how I can convert my data to and from padded structures to un-padded byte arrays without having to write separate stream in/out functions for each structure?

    By the way, my code has to be very portable (read 100% ANSI C).

    I'd really appreciate any insight anyone has here. Thanks!
  2. 06-01-2008 #2
    Salem
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    > By the way, my code has to be very portable (read 100% ANSI C).
    You've answered your own question - you have to serialise it yourself.

    Even if your current compiler had a pragma, that wouldn't help you since (as you already know), there are other compilers which don't support it.

    It's a little strange though, most compilers for embedded targets usually have a few toys to play with to make the realities of physical interfaces a little easier to deal with.

    Besides, as well as padding and alignment, there is endian as well, and there are no directives on any compiler which can fix that problem.

    > I have (about 50) structures set up that define the messages that will be passed in and out of the interface
    Perhaps http://en.wikipedia.org/wiki/Externa...Representation
    There are some tools (or perhaps even create your own) which given a struct can auto-generate the encode/decode functions.
    Certainly with that many, it would be worth looking into IMO.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
  3. 06-01-2008 #3
    robwhit
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    You could use offsetof to make an array of the layout of the struct. But if you have bitfields then that wouldn't work.
  4. 06-01-2008 #4
    l3iggs
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    Thanks for the reply Salem.

    I looked into the XDR (external data representation) standard. After reading RFC 4506 it looks like for XDR encoding all members of structures land on four byte boundaries.

    Since I can't force the device I am communicating with to use XDR decoding, XDR is not an option here.
  5. 06-01-2008 #5
    l3iggs
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    My problem has been solved by a suggestion by a user on another forum. If you're interested check out the solution here: http://www.codeguru.com/forum/showth...errerid=294270

    Thanks for your suggestions!
  6. 06-02-2008 #6
    Salem
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    Seems like a lot of editing of structures with macros to me.

    Personally, I would have written a perl script to parse a struct and emit all the code necessary to do the work without having to maul the source code with macro hell.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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