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Pit-falls of Bit-fields


In the past few posts we have been checking various aspects related to bit fields. In this blog, let us focus on the  disadvantages of using bitfields in memory

Portability is basically about having the same program or application running across various processor architecture. When it comes to embedded systems portability plays a very important role as they have diversified set of hardware. 

To start withlet us take a simple program as given below in Fig 1:

disadvantages of using bitfields in memory


Fig 1: Bit-length pose and portability issue. 

After running the above code, output is shown in the given below Fig 2:


Note: source code is tested under gcc (Ubuntu/Linaro 4.7.2-2ubuntu1) 4.7.2 version.

 Fig 2: Output of the source code given in Fig 1.

From the above figure 2, it is clear that when we declare the bit-length more than the sizeof(int)[ In our case , it is 4 bytes] , which is compiler dependent we are getting the error at the compile time. The above code perfectly works fine , if the machine WORD size is 64 bits, but fails to work with lower WORD size, which results in portability issue.


Pointers & Bit-fields

I want to focus on one more problem, why pointers cannot be applied for the bit-fields. let me practically explain this problem by taking an example shown in the fig 3 below,

Fig 3: Pointer operation and bit fields

The output of the above source code is given below,

 Fig 4: Output of the source code shown in fig 3.

One can observe from the above fig 4, when tried to reference the bit-field structure variable, compiler is generating an error, this is because direct addressing of the bit-fields structure variables is not possible in C, since the smallest unit of addressable memory in C is a sizeof(char)i.e byte addressable not an bit addressable. This is one of the disadvantages of using bitfields in memory in C programming.


From the above two examples, we can conclude that how portability issue is major concern with the bit-fields, and also we saw direct addressing of the bit-fields is not possible in C programming. Apart from this two issues, even sizeof() operator cannot also be applied for the bit-fields since sizeof alwaysreturns size in bytes not in terms of bits. Use bit-wise operators instead, they are 100% safe and portable.


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