typedef'd enum specifying type

This is a discussion on typedef'd enum specifying type within the C Programming forums, part of the General Programming Boards category; Hi all, I'm having trouble setting the size of an enumerated type when it's set as a typedef:- Code: typedef ...

  1. #1
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    typedef'd enum specifying type

    Hi all,

    I'm having trouble setting the size of an enumerated type when it's set as a typedef:-

    Code:
    typedef enum 
    	{ MT_NULL, MT_ACK, MT_PING, MT_BYE, MT_GETINFO } 
    messageType;
    The above is a condensed version - there are more message types, but way less than 256 of them and I'd like to tell the compiler to treat this as a byte rather than the default int (I'm using GCC).

    Is there a method to do this as inline code? I have other enumerated types which are ints and work fine - I don't want to break those by setting a global compiler value.

    Many thanks.

  2. #2
    Kernel hacker
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    You could try to apply __attribute__ ((packed)) to the enum declaration. I don't guarantee that it works, but it says so in the gcc info pages.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  3. #3
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    That doesn't seem to have the required effect.

    Also on further reading I've found that it is advised not to mess with ENUM sizes unless you're changing them globally (which I don't want to do).

    This is a major drawback with GCC imo and I can't believe I'm the first person to discover incompatibility issues. Almost every other compiler I ever worked with in the past has allowed the specifying of types for ENUM statements.

    Thanks for the help though.

  4. #4
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    I have worked with several compilers that do not allow changing the enum size, but we've obviously used different compilers.

    I think it's quite risky (from a portability aspect) to rely on the size of enums. Instead, I would recommend something like this:
    Code:
    enum BLAH { blah, bleh, bluh ... };
    
    typedef unsigned char u8;
    
    struct somestruct
    {
       ...
       u8 someblah;
       ...
    };
    
    ...
    struct somestruct st;
    ...
    st.someblah = blah;
    This is completely valid code, and it fixes any problem without relying on compiler specific behaviour.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  5. #5
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    we've obviously used different compilers.
    Oh, I'm going back to the 1970's Of course memory usage was far more significant than it is now. If you could do something in a byte, you did it in a byte, not 4.

    Excellent suggestion though Mats - many thanks - I think I'll do something just like that.

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