What is the meaning of attribute packed aligned 4

Question

It is C language It is written that   typedef struct   attribute    packed  aligned 4    Ball       float2 delta      float2 position        float3 color      float size        int arcID        float arcStr    Ball t  Ball t  balls    Please tell me what is the meaning of it and how to use this keyword

User · Answer

The attribute packed means that the compiler will not add padding between fields of the struct  Padding is usually used to make fields aligned to their natural size  because some architectures impose penalties for unaligned access or don t allow it at all   aligned 4  means that the struct should be aligned to an address that is divisible by 4

User · Answer

Before answering  I would like to give you some data from Wiki    Data structure alignment is the way data is arranged and accessed in computer memory  It consists of two separate but related issues  data alignment and data structure padding   When a modern computer reads from or writes to a memory address  it will do this in word sized chunks  e g  4 byte chunks on a 32-bit system   Data alignment means putting the data at a memory offset equal to some multiple of the word size  which increases the system s performance due to the way the CPU handles memory   To align the data  it may be necessary to insert some meaningless bytes between the end of the last data structure and the start of the next  which is data structure padding     gcc provides functionality to disable structure padding  i e to avoid these meaningless bytes in some cases  Consider the following structure   typedef struct        char Data1       int Data2       unsigned short Data3       char Data4    sSampleStruct    sizeof sSampleStruct  will be 12 rather than 8  Because of structure padding  By default  In X86  structures will be padded to 4-byte alignment    typedef struct        char Data1         3-Bytes Added here       int Data2       unsigned short Data3       char Data4         1-byte Added here    sSampleStruct    We can use   attribute    packed  aligned X    to insist particular X  sized padding  X should be powers of two  Refer here  typedef struct        char Data1       int Data2       unsigned short Data3       char Data4      attribute    packed  aligned 1    sSampleStruct      so the above specified gcc attribute does not allow the structure padding  so the size will be 8 bytes   If you wish to do the same for all the structures  simply we can push the alignment value to stack using  pragma    pragma pack push  1     Structure 1           Structure 2          pragma pack pop

User · Answer

packed means it will use the smallest possible space for struct Ball - i e  it will cram fields together without padding aligned means each struct Ball will begin on a 4 byte boundary - i e  for any struct Ball  its address can be divided by 4   These are GCC extensions  not part of any C standard

[c] What is the meaning of "attribute((packed, aligned(4))) "

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[c] What is the meaning of "attribute((packed, aligned(4))) "