Are Bit Fields Portable

Danielle Fletcher
• Monday, 07 December, 2020
• 8 min read

TeaLeef teaLeef1,67922 gold badges1111 silver badges2424 bronze badges Bit fields are portable, in the sense that they are a part of the C language as specified in the standard (C11 section

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Any compiler that fails to recognize code that uses bit fields is not standard-compliant. What they probably mean is that the fields themselves may be packed unpredictably in location and order (allowed by the standard, previous ref.

This means that a struct with e.g. four bit fields of size 4, 12, 13 and 3 does not necessarily take up 32 bits, and they won't necessarily be placed within the struct in that order; the compiler can place them where it likes. This means that the struct cannot be treated as an actual component-wise representation of an underlying binary object.

In contrast, bit masks applied manually to integers exist exactly where you put them. Of an unsigned integer, the fields will actually apply to the bits, in order and in position (assuming you know the endangers, anyway).

You can use integers to write a portable program, but you cannot expect to send a binary representation of int as is to a remote machine and expect it to interpret the data correctly. Therefore, integers can be used to communicate across network, as long as these details are taken care of.

Everything about the actual allocation details of bit fields within the class object Unlike endangers, it is not trivial to convert “everything about the actual allocation details” to a canonical form.

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TL;DR bit fields are not a portable way to communicate between computers. Bit fields are non- portable in the sense that the ordering of the bit is unspecified.

This prevents the use of bit fields in applications like toggling bits in memory-mapped hardware registers. But, you will see hardware vendor use bit fields in the code they release (like microchip for instance).

In microchip case for instance, the license for their source code mandates you to use their own compiler (for 8 bits low-end devices) After I had proven to myself that serializing data with a template meta-program was feasible, I started to research what features would be required to make this a useful library.

I have also worked in my share of code that implemented these packet formats using the bit -field feature in C/C++. I decided support for accessing values at the bit -level needed to be a feature of Alchemy.

My plan was to attempt use the same approach with the Hg::Airfield, to allow the user to access each field with a specific variable name. If that did not work, my fallback plan was to simply convert the variable-like syntax to use a function instead.

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Convert to compile-time The only part of the expression above that currently requires run-time processing is the std::pow() function. Two values will need to be passed into the definition of the Hg::Airfield template declaration, 1) the size of the field, 2) the offset.

Templatestruct Airfield{stations T k_offset = Offset;stations T k_size = County;// The Airfield Mask can be described with this formula://// Mask = (2 ^ (number of bits))-1 << shifted to the left // by offset bits.//stations T k_mask = T(((Hg::pow::value)-1)<< k_offset);/// ... This is the point where I need to front-load my explanation with an excuse because it appears so obvious to me now.

However, in the experimentation phase, it is most important to even find a working solution. Spoiler alert, I reworked this class when I reached the benchmark phase and witnessed its abysmal performance.

I am trying to create slightly shorter entries, so they can be digested more quickly, and I will be able to update more frequently. This entry described the intended purpose and design goals of the Hg::Airfield class.

In my next Alchemy entry I will show how I integrated the bit -field instance into its container. Then I will describe the hurdles that I started to encounter as my data types became more complex.

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A struct reserves enough memory to contain all the fields at the same time (Figure 1) but a union only reserves enough memory to contain the largest field (Figure 2). A struct allocates enough memory to contain all the fields at the same time.

Smaller items share this memory, which implies that a union can hold many data types but only one at a time. Like structs, unions form a new type specifier.

In this article, the author tells embedded C programmers why writing code involving bit fields needs to be done carefully, failing which the results may not be as expected. However, bit fields are handy as they involve only low level programming and result in efficient data storage.

Of course, the usage of bit fields is easy’ and comes handy, especially during low level programming. Though considered as one of the unique features of C programming, bit fields do have some limitations.

We reserved 5 bits for storing the hours field assuming we were using the 24-hour format. We will see how 22 is stored in binary form in 5 bits through pictorial representation (Figure 2).

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From the table(as shown in Figure 2), it is very clear that sign bit (b4) is SET, which indicates the value is negative. So, when printed using the print statement, we will get -10 (the decimal value of 10110), because of which we got an unexpected output.

Figure 2 : Pictorial representation of binary value of 24 in ‘5’ snow that we have understood the problem, how do we fix it? It is very simple; just qualify into to unsigned into just before the hours in the bit field structure, as shown below.

In the embedded programming environment, they might lead to major issues in case they are not handled properly. Little Indian machines pack bit fields from the least significant byte to the most.

Figure 8 helps us understand how the bits ordering takes place. From the last two examples, it is very clear that bit fields pose serious portability issues.

Low level programming The biggest advantage of bit fields is that one does not have to keep track of how flags and masks actually map to the memory. Once the structure is defined, one is completely abstracted from the memory representation as in the case of bit -wise operations, during which one has to keep track of all the shifts and masks.

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Also, the bit field length has a high dependency on word size. The author is a member of the embedded software team at Emerge Information Technologies.

His area of interest lies in embedded C programming combined with data structures and microcontrollers. He likes to experiment with C programming in his spare time to explore new horizons.

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