Struct byte alignment

Tip: http://hi.baidu.com/j2ee_cn/blog/item/3819f6bf88582d0e18d81f44.html

1. Alignment of the Data Type itself: Alignment of the basic data type described above.
2. Specify the alignment value: # The alignment value specified when progma pack (value) is used.
3. Alignment value of a struct or class: The value with the largest alignment value among its members.
4. Valid alignment values of data members, structures, and classes: the alignment value itself and the value smaller than the specified alignment value.
with these values, we can easily discuss the data structure members and their alignment. The valid alignment value n is the final value used to determine the data storage address. Valid alignment means "alignment on N", that is, the "Starting address for storing the data % n = 0 ". data variables in the data structure are discharged in the defined order. The starting address of the first data variable is the starting address of the data structure. The member variables of the struct must be aligned and discharged, and the struct itself must be rounded according to its own valid alignment values (that is, the total length occupied by the member variables of the struct must be an integer multiple of the valid alignment values of the struct, ). In this way, you cannot understand the values of the above examples.
Example Analysis:
analysis example B;
struct B
{< br> char B;
int A;
short C;
};
assume that B is discharged from the address space 0x0000. The alignment value is not defined in this example. In the author's environment, this value is 4 by default. The first member variable B's own alignment value is 1, which is smaller than the specified or default alignment value 4. Therefore, the valid alignment value is 1, therefore, the storage address 0x0000 is 0 x 0000% 1 = 0. the alignment value of the second member variable A is 4, so the valid alignment value is 4. Therefore, it can only be stored in the four consecutive bytes from the starting address 0x0004 to 0x0007, review 0 x 0004% 4 = 0, which is close to the first variable. The third variable C has its own alignment value of 2, so the valid alignment value is also 2, which can be stored in the two bytes from 0x0008 to 0x0009, Which is 0 x 0008% 2 = 0. Therefore, B content is stored from 0x0000 to 0x0009. Then, let's look at the alignment value of Data Structure B as the maximum alignment value in its variable (here it is B), so it is 4, so the valid alignment value of the structure is also 4. According to the requirements of the structure, 0x0009 to 0x0000 = 10 bytes, (10 + 2) % 4 = 0. Therefore, 0x0000a to 0x000b is also occupied by struct B. Therefore, B has 12 bytes from 0x0000 to 0x000b, and sizeof (struct B) = 12;