C/C++ structures and unions are converted to assembly .struct and .union elements. Padding and ending alignments are added as necessary to make the resulting assembly structure have the same size and member offsets as the C/C++ source. The primary purpose is to allow access to members of C/C++ structures, as well as to facilitate debugging of the assembly code. For nested structures, the assembly .tag feature is used to refer to other structures/unions.

The alignment is also passed from the C/C++ source so that the assembly symbol is marked with the same alignment as the C/C++ symbol. (See Section 14.3.3 for information about pragmas, which may attempt to modify structures.) Because the alignment of structures is stored in the assembly symbol, built-in assembly functions like $sizeof( ) and $alignof( ) can be used on the resulting structure name symbol.

When using unnamed structures (or unions) in typedefs, such as:

typedef struct { int a_member; } mystrname;

This is really a shorthand way of writing:

struct temporary_name { int a_member; };
typedef temporary_name mystrname;

The conversion processes the above statements in the same manner: generating a temporary name for the structure and then using .define to output a typedef from the temporary name to the user name. You should use your mystrname in assembly the same as you would in C/C++, but do not be confused by the assembly structure definition in the list, which contains the temporary name. You can avoid the temporary name by specifying a name for the structure, as in:

typedef struct a_st_name { ... } mystrname;

If a shorthand method is used in C to declare a variable with a particular structure, for example:

extern struct a_name { int a_member; } a_variable;

Then after the structure is converted to assembly, a .tag directive is generated to declare the structure of the external variable, such as:

_a_variable .tag a_st_name

This allows you to refer to _a_variable.a_member in your assembly code.