align syntax

This is a discussion on align syntax within the C++ Programming forums, part of the General Programming Boards category; Is this code correct? Code: #define GRANULARITY 4 //Bytes #define ALIGN __attribute__ ((aligned (GRANULARITY))) typedef pthread_mutex_t ALIGN MUTEX; typedef struct ...

  1. #1
    Alessio Stella
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    align syntax

    Is this code correct?

    Code:
    	#define GRANULARITY 4 //Bytes
    	#define ALIGN __attribute__ ((aligned (GRANULARITY)))
    
    	typedef pthread_mutex_t ALIGN MUTEX;
    
    typedef struct 
    {
    	MUTEX  Mutex;	
    	unsigned int MeasureActive;
    	unsigned int LastIndex;
    	unsigned int FirstIndex;
    	unsigned int ElementsNr;
    	unsigned long long int Overflow;
    	int64 RecordingInstant[MeasFIFO_SIZE];
    	unsigned int PacketIndex[MeasFIFO_SIZE];	
    	int Value[MeasFIFO_SIZE];
    	
    } ALIGN Type_IntMeasFIFOComm;
    
    typedef class  c_IntWrParCAM_FIFOComm
    {
    public:
    	c_IntWrParCAM_FIFOComm()	{	pthread_mutex_init(&Mutex, NULL);	}
    	~c_IntWrParCAM_FIFOComm()	{	pthread_mutex_destroy(&Mutex);	}
    	
    	MUTEX  Mutex;
    
    	unsigned int LastIndex;
    	unsigned int FirstIndex;
    	unsigned int ElementsNr;	
    	unsigned long long int Overflow;
    	
    	unsigned int TargetIndex[WrFIFO_SIZE];	// DUMMY !!
    	unsigned int ParIndex[WrFIFO_SIZE];
    	int Value[WrFIFO_SIZE];
    	unsigned int ServerIndex[WrFIFO_SIZE];
    	int sock[WrFIFO_SIZE];	
    	
    } ALIGN Type_IntWrParCAM_FIFOComm;
    I ask especially about
    (1) the 4 usages of ALIGN (2 direct usages, 2 indirect via MUTEX)
    (2) the use of the constructor destructor (those classes will have global object instances)
    (3) the use of typedef class ..Type_IntWrParCAM_FIFOComm;

    It is basically a variation of a structure just not to have to remember to call pthread_..init/destroy somewhere in the code (I often forget!!)
    Last edited by mynickmynick; 08-27-2008 at 10:23 AM.

  2. #2
    Kernel hacker
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    I personally wouldn't change the alignment of a pthread_mutex - it is either defined properly elsewhere, or it will work OK without further alignment [or you have a bug to report to the pthread library supplier]. You trying to align it further will never make anything better - but you may break the previous alignment [gcc accepts changes in alignment, as shown in this example]:
    Code:
    #include <stdio.h>
    
    typedef int foo __attribute__ ((aligned (16)));
    typedef foo bar __attribute__ ((aligned (4)));
    
    
    bar y;
    foo x;
    
    void xfunc(foo a, bar b)
    {
      foo x;
      bar y;
    
      printf("Func: x(foo) = &#37;p, y(bar) = %p, a(foo) = %p, b(bar) = %p\n", &x, &y, &a, &b);
    }
    
    
    
    int main()
    {
      bar a = 7;
      foo b = 8;
    
      xfunc(a, b);
      printf("Main: x(foo) = %p, y(bar) = %p, a(bar) = %p, b(foo) = %p\n", &x, &y, &a, &b);
    
      return 0;
    }
    Structures (and thus classes) are ALWAYS aligned to the largest alignment needed by the structure, so aligning the WHOLE class to the same value as you have aligned one of it's members is pointless and reduntant.

    Edit: On closer inspection, you also have int64 and unsigned long long members of your struct, which means that the struct itself SHOULD be aligned to 8 bytes. You are now REDUCING it's alignment (which you are allowed to do, but probably not a good idea unless you are trying to save a few bytes).

    --
    Mats
    Last edited by matsp; 08-27-2008 at 11:50 AM.
    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
    Alessio Stella
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    Quote Originally Posted by matsp View Post

    Structures (and thus classes) are ALWAYS aligned to the largest alignment needed by the structure, so aligning the WHOLE class to the same value as you have aligned one of it's members is pointless and reduntant.

    Edit: On closer inspection, you also have int64 and unsigned long long members of your struct, which means that the struct itself SHOULD be aligned to 8 bytes. You are now REDUCING it's alignment (which you are allowed to do, but probably not a good idea unless you are trying to save a few bytes).

    --
    Mats

    Sorry I don't get this
    Can you give links to read about it? "needed by the struct" : needed for what? why if there is a field of 8 bytes (like int64) the struct should be aligned to 8 bytes? May be the whole size of the struct is not a multiple of 8 bytes (in this special case it is butit is sufficient to add one int dummy; andit is not any more) so I guess it doesn't need it?

  4. #4
    Kernel hacker
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    Consider this:
    Code:
    struct x
    {
       int64 a;
       int b;
    };
    
    struct x arr[10];
    If the entire struct is not aligned to the size of int64, what is the alignment of arr[1].a?

    --
    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
    C++まいる!Cをこわせ! Elysia's Avatar
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    Also note that aligning is compiler dependant. Other compilers require different syntax and some do not even have the ability to align at all.
    Unless you really do need to align, consider not doing so.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  6. #6
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    Quote Originally Posted by Elysia View Post
    Also note that aligning is compiler dependant. Other compilers require different syntax and some do not even have the ability to align at all.
    Unless you really do need to align, consider not doing so.
    I heartily agree with this. I think you are trying to solve something that isn't a problem.

    --
    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.

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