__unaligned before and after the * (64bit)

Discussion in 'C Programming' started by Udi, Mar 19, 2008.

  1. Udi

    Udi Guest

    Hi All,
    I'm not sure I understand the difference between placing the
    __unaligned before or after the *:
    I was trying to handle the C4366 warning - "The result of the unary
    '&' operator may be unaligned") and used the '__unaligned' modifier as
    suggested,
    but ended up with C4090 - "different '__unaligned' qualifiers". (See
    below)
    However, moving the __unaligned keyword after the ' * '
    solved the warning but I'm not sure I solved the probelm.


    with no __unaligned keyword --> Warning C4366
    ```````````````````````````````````````````````````````````````````````
    void List_Clear(List *pList) ;
    :
    List * pSubscribersList = NULL;

    pSubscribersList = (List *)&(p->subscribersList); //C4366: The result
    of the unary '&' operator may be unaligned
    List_Clear(pSubscribersList);




    __unaligned before the * --> warning C4090
    ```````````````````````````````````````````````````````````````
    void List_Clear(List *pList) ;
    :
    List __unaligned * pSubscribersList = NULL;

    pSubscribersList = (List __unaligned *)&(p->subscribersList);
    List_Clear(pSubscribersList); //warning C4090: 'function' : different
    '__unaligned' qualifiers


    __unaligned after the * --> no warnings
    ````````````````````````````````````````````````````````
    void List_Clear(List *pList) ;
    :
    List * __unaligned pSubscribersList = NULL;

    pSubscribersList = (List * __unaligned)&(p->subscribersList);
    List_Clear(pSubscribersList); // OK - no warning



    Can anyone explain what's the difference between the last two
    examples?
    I'm using VS2005 compiling to 64 bit.
    Thanks!
     
    Udi, Mar 19, 2008
    #1
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  2. __unaligned isn't part of standard C. You needd to ask ona compler
    specific news group. Try I Microsoft related ng.
     
    Nick Keighley, Mar 19, 2008
    #2
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  3. Better yet, re-write the code to avoid the need to use __unaligned.
    The original poster appears to be writing code for a linked list.
    There's no need to use any implementation-specific extensions for such
    a relatively straightforward task.

    Apparently the compiler's warning "The result of the unary '&'
    operator may be unaligned" included a suggestion to use __unaligned.
    I strongly suspect that suggestion was a poor one.
     
    Keith Thompson, Mar 19, 2008
    #3
  4. I guess, it works like 'const', which applies to the left unless it is at
    the leftmost side, then it applies to the right. However, it is, as others
    pointed out, non-standard, so you have to consult the compiler docs.
    I don't think so...
    Well, the first problem here is that you are using casts, which is typically
    a sign that something's wrong. Remove those, and you won't need any
    unaligned attributes. If it doesn't compile then, your types simply don't
    match, but adding them doesn't change that. If you can't do it yourself or
    want to verify the solution is correct, please boil your problem down to a
    minimal but complete example, in particular guessing
    what 'p->subscribersList' could be is pretty hart.

    Uli
     
    Ulrich Eckhardt, Mar 19, 2008
    #4
  5. Udi

    Udi Guest

    Thanks All,
    I agree I shouldn't use the __unaligned, that's why I'm writing to
    figure out how.

    Here are the full types and example:

    typedef struct
    {
    HANDLE hAccess; // Mutex
    HANDLE hCanRead; // Event
    HANDLE hCanWrite; // Event
    HANDLE hWriterMutex; // Mutex
    DWORD dwTlsSlot;

    DWORD dwTotalReaderThreads;
    DWORD dwTotalWriterThreads;
    } tReadWriteLock;

    typedef struct _ListItem
    {
    void *pData;
    struct _ListItem *pNextItem;
    } ListItem;

    typedef struct
    {
    ListItem *pHead;
    DWORD dwLength;
    tReadWriteLock *pLock;
    } List;

    typedef struct
    {
    DWORD id;
    List subscribersList;
    } IndicatorSubscribers;


    with no __unaligned keyword --> Warning C4366
    ```````````````````````````````````````````````````````````````````````
    void List_Clear(List *pList) ;
    :
    List * pSubscribersList = NULL;


    pSubscribersList = (List *)&(p->subscribersList); //C4366: The result
    of the unary '&' operator may be unaligned
    List_Clear(pSubscribersList);


    __unaligned before the * --> warning C4090
    ```````````````````````````````````````````````````````````````
    void List_Clear(List *pList) ;
    :
    List __unaligned * pSubscribersList = NULL;


    pSubscribersList = (List __unaligned *)&(p->subscribersList);
    List_Clear(pSubscribersList); //warning C4090: 'function' : different
    '__unaligned' qualifiers


    __unaligned after the * --> no warnings
    ````````````````````````````````````````````````````````
    void List_Clear(List *pList) ;
    :
    List * __unaligned pSubscribersList = NULL;


    pSubscribersList = (List * __unaligned)&(p->subscribersList);
    List_Clear(pSubscribersList); // OK - no warning




    As you can see these are all pretty simple typedefs.
    So the 'p' in 'p->subscribersList' is of type 'IndicatorSubscribers'.

    Any ideas now?
    (Still I didn't get the difference between placing the __unaligned
    before and after the *.)

    Thanks,
    Udi
     
    Udi, Mar 23, 2008
    #5
  6. Udi

    Flash Gordon Guest

    Udi wrote, On 23/03/08 07:11:

    Start off by getting rid of ALL of the casts. Doing the sort of thing it
    looks like you are trying to do does not require casts if you do it
    correctly. Then produce a small *complete* compilable example that gives
    the warning, preferably without using "//" style comments as they do not
    survive line wrapping. Your example obviously is not complete as you
    have code that is not inside a function.
     
    Flash Gordon, Mar 23, 2008
    #6
  7. But you left the cast.
    You did not show the definition of p. If it is of type pointer to
    IndicatorSubscribers, and the line in question gives you an error when
    you remove the cast (it is not needed -- trust me!) then the problem
    is elsewhere.

    Most likely, something like a #pragma elsewhere or a compiler option
    is causing the compiler to pack the structure in a way that violates
    the assurances given by the language standard. If that is the case
    (and you can't change it) you will have to use something non-standard
    to solve the problem.
     
    Ben Bacarisse, Mar 23, 2008
    #7
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