3.9.2 Unions

• The syntax of a union declaration is identical to that for structures, but its semantics are very different

 

• A common method is to introduce an enumer- ated type defining the different possible choices for the union, and then create a structure containing a tag field and the union:
• Unions can also be used to access the bit patterns of different data types

 

3.9.3 Data Alignment

 

• The x86-64 hardware will work correctly regardless of the alignment of data. However, Intel recommends that data be aligned to improve memory system performance. 

• As a result, j has offset 8, and the overall structure size is 12 bytes. Furthermore, the compiler must ensure that any pointer p of type struct S1* satisfies a 4-byte alignment.

 

3.10 Combining Control and Data in Machine-Level Programs

 

3.10.1 Understanding Pointers

• Every pointer has an associated type
• Every pointer has a value.
• Pointers are created with the ‘&’ operator.
• Pointers are dereferenced with the ‘*’ operator.
• Arrays and pointers are closely related.
• Casting from one type of pointer to another changes its type but not its value.
• Pointers can also point to functions.