GENERAL: UNION INVOLVING A BITFIELD IS THE WRONG SIZE

Information in this article applies to:

• C166 Version 3.12
• C166 Version 4.02
• C251 Version 2.14
• C51 Version 5.50

QUESTION

I have declared the following union:

union
{
  struct
  {
    unsigned x : 4;
    unsigned y : 4;
  } nibble;
  unsigned char abyte;
} value;

When I build my project and examine the map file, I see that the union occupies two bytes, but I want it to only occupy one. Why is the compiler wasting a precious byte of data memory?

ANSWER

The reason is that you declared bitfields as 'unsigned'. This defaults to 'unsigned int'.

A union's size accommodates the largest object. In this case, the structure contains an unsigned int (only one unsigned int because there are less than 16 bits specified in total). The size of an unsigned int is two bytes.

To use memory more efficiently, declare your union specifying unsigned chars for the bitfields:

union
{
  struct
  {
    unsigned char x:4;
    unsigned char y:4;
  } nibble;
  unsigned char abyte;
} value;

It then occupies a single byte.

MORE INFORMATION

Refer to a good C book, such as "The C Programming Language" by Kernighan & Ritchie, for more information on using bitfields, structures and unions.

FORUM THREADS

The following Discussion Forum threads may provide information related to this topic.

• The length of struct
• assigning struct to variable
• constant bit-fields in struct
• Address bits in XDATA
• union, bit field and bdata
• how many mem does union cost?
• Union

Last Reviewed: Tuesday, July 19, 2005