BYTE-aligned Baidu Encyclopedia

The memory space in modern computers is divided by Byte, in theory it seems that access to any type of variable can start from any address, but the reality is that when accessing a particular type of variable, it is often accessed at a specific memory address, which requires all types of data to be spatially arranged according to certain rules, Instead of sequentially one by one emissions, that's the alignment.

The effect and reason of alignment: the processing of storage space varies greatly with each hardware platform. Some platforms can only access certain types of data from certain specific addresses. For example, some architectures have an error when accessing a variable that is not aligned, so programming must ensure byte alignment in this architecture. Other platforms may not have this, but the most common is the loss of access efficiency if the data storage is not aligned according to their platform requirements. For example, some platforms read each time from the even address, if an int (assuming 32-bit system) if the location of the place where the even address begins, then a read cycle can be read out of the 32bit, and if it is stored at the beginning of the odd address, it takes 2 read cycles, The 32bit data can be obtained by piecing together the high and low bytes of the two read-out results.

In fact, byte alignment details are related to a specific compiler implementation, but in general, three criteria are met:

1) The first address of a struct variable can be divisible by the size of its widest base type member;

2) The offset of each member of the struct relative to the first address of the struct is an integer multiple of the member size, and if necessary the compiler adds padding bytes between the members, such as the address space of the second struct variable above.

3) The total size of the struct is an integer multiple of the size of the structure's widest base type member, and if necessary the compiler will add padding bytes after the last member. For example, the first struct-body variable above.