Maaz Jukaku Bit-Fields : Be โBitโ Careful While Using!
In Embedded systems C programming, bit-fields offer optimization and flexibility benefits but require careful usage to avoid adverse results.
The code snippet below illustrates a structure named “bit_field” with three integer fields: hours (5 bits), mins (6 bits), and secs (6 bits).
Now let us declare a variable alarm of struct time_t and set values as 22, 12 and 20 respectively.
When we print these values using (simple printf statement) what could be the output? At first sight most of us will envision answers will be 22, 12 and 20 for hours, mins and secs field.
Whereas when we actually compile and run the value would be different -10, 12 and 20 (as mentioned in Fig 3 below). Now whatโs wrong with this? Isnโt it obvious to print an integer value?
What and where did we go wrong? Let us give some work to our grey matter!
- We all know that default signed qualifier for the โintโ is โsigned intโ.
- In our case, we reserved โ5โ bits for storing the hours field assuming we are using 24 hrs format. So, among โ5โ bits โ1โ bit is used for storing the sign of the number, which means only 4 bits are now available for storing actual value.
- In this 4 bits we can store the numbers ranging from -16 to +15 according to the formula (-2^k) to (+2^(k-1)) including โ0โ, where โkโ indicating number of bits.
- We will see how 22 is stored in binary form in this โ5โ bits through pictorial representation, shown in
- From the above table, it is very clear that sign bit (b4) is SET which indicates the value is negative. So, when printed using the printf statement we will get -10 (Decimal value of 10110) because of which we are getting unexpected output.
Now that we understand the problem, how do we fix it? It is very simple just qualify โintโ to โunsigned intโ just before the hours in the bit fields in C programming structure as shown below Fig 4 with corrected output in Fig 5.
Bit wise operators definitely provide advantages, which need to be used โbitโ carefully. In Embedded programming environment they might lead to major issues in case of not handling it properly.
| Serial No | Related Blog Posts | Links |
|---|---|---|
| 1. | The Future Scope of Embedded Systems : Trends and Predictions | Click Here |
| 2. | Online Free IoT Internship for Engineering Students | Free Certificates |2023 | Click Here |
| 3. | Emertxe Placements in Embedded Systems and Embedded IoT โ 2022 (Jan-sept) Report | Click Here |
Bit fields in C programming optimize memory usage in Embedded Systems by reducing footprint and improving performance. They provide memory efficiency and compact data structures.
- Plan data structure carefully.
- Use appropriate data types.
- Minimize padding.
- Document bit field layout.
- Consider endianness.
- Thoroughly test and verify.
These practices optimize memory usage and improve efficiency