Structure member Alignment & Padding

Structure and union variable declarations


A structure or union declaration has the same form as a definition except the declaration does not have a brace-enclosed list of members. You must declare the structure or union data type before you can define a variable having that type.


Read syntax diagramSkip visual syntax diagramStructure or union variable declaration syntax


.-----------------------------.
V |
>>---+-------------------------+-+--+-struct-+------------------>
+-storage_class_specifier-+ '-union--'
'-type_qualifier----------'


>--tag_identifier--declarator--;-------------------------------><

Bit Fields Alignment

If a series of bit fields does not add up to the size of an int, padding can take place. The amount of padding is determined by the alignment characteristics of the members of the structure.


The following example demonstrates padding, and is valid for all implementations. Suppose that an int occupies 4 bytes. The example declares the identifier kitchen to be of type struct on_off:


struct on_off {
unsigned light : 1;
unsigned toaster : 1;
int count; /* 4 bytes */
unsigned ac : 4;
unsigned : 4;
unsigned clock : 1;
unsigned : 0;
unsigned flag : 1;
} kitchen ;


The structure kitchen contains eight members totalling 16 bytes. The following table describes the storage that each member occupies:
Member Name Storage Occupied
light 1 bit
toaster 1 bit
(padding -- 30 bits) To the next int boundary
count The size of an int (4 bytes)
ac 4 bits
(unnamed field) 4 bits
clock 1 bit
(padding -- 23 bits) To the next int boundary (unnamed field)
flag 1 bit
(padding -- 31 bits) To the next int boundary


All references to structure fields must be fully qualified. For instance, you cannot reference the second field by toaster. You must reference this field by kitchen.toaster.


The following expression sets the light field to 1:


kitchen.light = 1;


When you assign to a bit field a value that is out of its range, the bit pattern is preserved and the appropriate bits are assigned. The following expression sets the toaster field of the kitchen structure to 0 because only the least significant bit is assigned to the toaster field:


kitchen.toaster = 2;

Structure padding

Main article: data structure alignment


To calculate the sizes of user-defined types, the compiler takes into account any alignment space needed for complex user-defined data structures. This is why the size of a structure in C can be greater than the sum of the sizes of its members. For example, on many systems, the following code will print 8:


struct student{
char grade; /* char is 1 byte long */
int age; /* int is 4 bytes long */
};


printf("%zu", sizeof (struct student));


The reason for this is that most compilers, by default, align complex data-structures to a word alignment boundary. In addition, the individual members are also aligned to their respective alignment boundaries. By this logic, the structure student gets aligned on a word boundary and the variable age within the structure is aligned with the next word address. This is accomplished by way of the compiler inserting "padding" space between two members or to the end of the structure to satisfy alignment requirements. This padding is inserted to align age with a word boundary. (Most processors can fetch an aligned word faster than they can fetch a word value that straddles multiple words in memory, and some don't support the operation at all[3]).




Referenes:
http://www.ibm.com/developerworks/library/pa-dalign/#N100FE — best explanation
http://en.wikipedia.org/wiki/Sizeof#Structure_padding
http://publib.boulder.ibm.com/infocenter/macxhelp/v6v81/index.jsp?topic=%2Fcom.ibm.vacpp6m.doc%2Flanguage%2Fref%2Fclrc03defbitf.htm