Understanding What Memory Alignment Means

Understanding What Memory Alignment Means

Most CPUs require that objects and variables reside at particular offsets in the system’s memory. For example, 32-bit processors require a 4-byte integer to reside at a memory address that is evenly divisible by 4. This requirement is called “memory alignment”. Thus, a 4-byte int can be located at memory address 0x2000 or 0x2004, but not at 0x2001. On most Unix systems, an attempt to use misaligned data results in a bus error, which terminates the program altogether. On Intel processors, the use of misaligned data is supported but at a substantial performance penalty. Therefore, most compilers automatically align data variables according to their type and the particular processor being used. This is why the size that structs and classes occupy is often larger than the sum of their members’ size:

 struct Employee{  int ID;  char state[3]; //CA, NY etc. + terminating null  int salary;};

Apparently, Employee should occupy 11 bytes (4+3+4). However, most compilers add an unused padding byte after the field ‘state’ so that it aligns on a 4 byte boundary. Consequently, Employee occupies 12 bytes rather than 11. You can examine the actual size of an aggregate by using the expression sizeof(Employee).


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