about boolean type

[wind0965]

I have one piece of code:

typedef struct{
unsigned int Cb11_defaultstate2 : 1; /* boolean type */
unsigned int Cb10_basicstate2 : 1; /* boolean type */
unsigned int Cb6_basicstate1 : 1; /* boolean type */
unsigned int Cb7_basicstate3 : 1; /* boolean type */
unsigned int Cb8_defaultstate1 : 1; /* boolean type */
unsigned int Cb9_substate2 : 1; /* boolean type */
unsigned int Cb5_substate1 : 1; /* boolean type */
unsigned int Cb3_basicstate4 : 1; /* boolean type */
unsigned int Cb4_substate0 : 1; /* boolean type */
unsigned int Cb2_superstate : 1; /* boolean type */
} BFb7_tp;

... ...
I have not seen something like this, what does the ":1" mean? I guess it implies 1 bit. But will it use just one bit for every element?
As the code shows, the struct BFb7_tp should have the size of 10 bytes. But I use some tool to check the RAM size, and it needs just 2 bytes! That is, 1 byte for 8 elements! How can this happen?

Thank you in advance!

[panayotisk]

It is 10 bits which is 2 bytes (6 bits unused). It will use 1 bit for every element I guess

[wind0965]

Is it legal usage in C like:
unsigned int Cb2_superstate : 1;

[Andreas Masur]

This is a so-called bit field and is a special case of a structure. In those structures the amount of bits for every component of it is separated with the : X.

1. Bit fields may be used inside a structure to pack several members into a single word of memory:

struct Foo
{
  unsigned sevenbits : 7;
  unsigned fourvals  : 2;
} sFoo;

creates a structure with two members: 'sevenbits' which can hold values between 0 (binary: 0000000) and 127 (binary: 1111111), and 'fourvals' which can hold values between 0 (binary: 00) and 3 (binary: 11)

The bit field length may be a constant expression, similar to an array length specifier.

2. Bitfields are addressed just like other structure members:

sFoo.sevenbits = 32;
sFoo.fourvals  /= 2;

3. Bitfields can be either signed or unsigned (though pre-ANSI compilers may only allow unsigned)

4. Bitfields may be unnamed, to act as padding between named fields:

struct Foo
{
  unsigned sea : 3;
  unsigned     : 3;
  unsigned see : 3;
};

5. The & operator cannot be applied to a bitfield, since a bitfield is only a subset of the bits in a word and thus has no address

6. Bitfields may be mixed in with other types in a structure

7. Some machine-level details of bitfields are implementation-defined:

- Bitfields may be stored left to right (big-endian) or right to left (little-endian), depending on the machine architecture
- Some compilers will allow bitfields to overlap a word boundary, others will silently add padding bits
- The maximum size of a bitfield is implementation-defined

8. These implementation-defined aspects make data created with bitfields inherently nonportable between dissimilar machines

[Andreas Masur]

Originally posted by wind0965
As the code shows, the struct BFb7_tp should have the size of 10 bytes. But I use some tool to check the RAM size, and it needs just 2 bytes! That is, 1 byte for 8 elements! How can this happen?

Well...the size is 4 bytes due to the underlying datatype used (-> unsigned int) which is 4 bytes on a windows system...

[wind0965]

Well, you are right. But I am sorry to forget to mention that it is a programm for 16 bits microcontroller.
Anyway, thank you very much.

[Andreas Masur]

Originally posted by wind0965
Well, you are right. But I am sorry to forget to mention that it is a programm for 16 bits microcontroller.
Anyway, thank you very much.

Well...okay...in this case the size of an integer would rather be 16-bit...