Atmel CAVR-4 Manual De Usuario

CAVR-4

Descriptions of language extensions

Reference Guide

For example, the structure 

str

 in the following example contains an anonymous union. 

The members of the union are accessed using the names 

b

 and 

c

, for example 

obj.b

.

Without anonymous structure types, the union would have to be named—for example 

u

—and the member elements accessed using the syntax 

obj.u.b

.

struct str 

{

  int a;

  union 

  {

    int b;

    int c;

  };

};

struct str obj;

In ISO/ANSI C, a bitfield must be of the type 

int

 or 

unsigned int

. Using IAR 

language extensions, any integer types and enums may be used. 

For example, in the following structure an 

unsigned

 

char

 is used for holding three 

bits. The advantage is that the struct will be smaller.

struct str 

{

  unsigned

 

char bitOne   : 1;

  unsigned

 

char bitTwo   : 1;

  unsigned

 

char bitThree : 1;

};

This matches G.5.8 in the appendix of the ISO standard, ISO Portability Issues.

The last element of a 

struct

 may be an incomplete array. This is useful because one 

chunk of memory can be allocated for the 

struct

 itself and for the array, regardless of 

the size of the array.

Note: The array may not be the only member of the 

struct

. If that was the case, then 

the size of the 

struct

 would be zero, which is not allowed in ISO/ANSI C.