Bitfield operation in C.

ongth60 · ‎2005-09-02

Posted on September 02, 2005 at 14:58

ongth60 · ‎2005-08-24

Posted on August 25, 2005 at 01:14

Hi guys,

I would like some advise if its possible to declare a variable of 24 bits and manipulate each single bit like setting bits?

Rgrds,

ongth60

ongth60 · ‎2005-09-01

Posted on September 02, 2005 at 02:30

Hi,

This is using the GPIO as an example, mayb i dun understand how do i use it as a general variable where the size is of the variable is 8 bit and i can manipulate each single bit and when i were to store, i can store the whole byte all together, like creating my own settings:

struct{

u8 Regular_Reporting_Time : 3;

u8 Non_Violation_SMS_Enable : 1;

u8 Vibration_Enable : 1;

u8 Smart_Powersave_Enable : 1;

u8 FLEET_Type_Enable : 1;

u8 Voice_Call_Enable : 1;

}Byte3_Setting;

Regular_Reporting_Time = 0x04;

Vibration_Enable = 0x01;

EEPROM_byte_write(Byte3_Setting);

Rgrds,

ongth60

rsherry · ‎2005-09-02

Posted on September 02, 2005 at 14:58

ongth60,

You first have to allocate a variable for a Byte3_Setting structure. Then you can access the bit fields through normal structure member access (i.e. dot notation)...for example

void SaveSettings(void)

{

struct Byte3_Setting mySettings;

mySettings.Regular_Reporting_Time = 0x04;

mySettings.Vibration_Enable = 0x01;

EEPROM_byte_write(Byte3_Setting);

}

This example isn't particularly practical, but that's how you can use your structure.

In a more practical usage you would probably want to allocate your variable somewhere else and pass a pointer along with settings to fill in but remember then you would access the members using the -> operator like:

void SaveSettings(struct Byte3_Setting *pSettings, int reportingTimeValue, int vibrationEnabled)

{

pSettings->Regular_Reporting_Time = reportingTimeValue;

pSettings->Vibration_Enable = vibrationEnabled;

EEPROM_byte_write(*pSettings);

}

Also check your compiler documentation to make sure you understand how it handles bit-fields and structure packing as this can sometimes be a problematic area.

Hope this helps.

Ryan.