What Is Alignment

Posted 2002.12.04 13:14

What Is Alignment?

Simple data types declared as variables are assigned addresses that are multiples of the size (in bytes) of the data type. Thus, variables of type long will be assigned addresses that are multiples of 4 (the bottom 2 bits of the address are 0).

Structures are padded so that each element of the structure will end up at a naturally aligned address. For example:

struct x_
{
   char a;     // 1 byte
   int b;      // 4 bytes
   short c;    // 2 bytes
   char d;     // 1 byte
} foo;

will actually be laid in memory so that it looks like this:

struct x_
{
   char a;            // 1 byte
   char _pad0[3];     // padding to put 'b' on 4-byte boundary
   int b;            // 4 bytes
   short c;          // 2 bytes
   char d;           // 1 byte
   char _pad1[1];    // padding to make sizeof(x_) multiple of 4
}

The result in both cases is that sizeof(struct x_) will return 12. Note that char _pad0[3] is included so that the int b member will be on a 4-byte boundary. The reason char _pad1[1] is included at the end is so that arrays of this structure will all have the same alignment; i.e.:

struct _x bar[3];

so that this array can be laid out as follows:

adr
offset   element
------   -------
0x0000   char a;         // bar[0]
0x0001   char pad0[3];
0x0004   int b;
0x0008   short c;
0x000a   char d;
0x000b   char _pad1[1];

0x000c   char a;         // bar[1]
0x000d   char _pad0[3];
0x0010   int b;
0x0014   short c;
0x0016   char d;
0x0017   char _pad1[1];

0x0018   char a;         // bar[2]
0x0019   char _pad0[3];
0x001c   int b;
0x0020   short c;
0x0022   char d;
0x0023   char _pad1[1];

This is important so that each member of each array element can be accessed naturally.

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