C Programming - Structures, Unions, Enums - Discussion

@Prema.

The main purpose of reserving bits in declaring them is the memory allocation for the variables can be used for what we want.

So by allocating required number of bits, we can save the memory.

What is the purpose of reserving the bits for a variable?

As the size of integer is 2 bytes or 4 bytes depending on the compiler why we should reserve 5 bits in this code?

What is bit field? explain it in detail.

You can test this.

The minimum size of a structure variable will be the size of the biggest data type in the structure.

i.e.,

In this example (*replace float with int).

sizeof (struct a) will be a minimum of 4 in 32-bit turboc.

Please correct me if I'm wrong.

@Prashant O_O: Friend I ran this code on gcc compiler after making few changes, here is my code:

#include<stdio.h>

int main()
{
struct a
{
int category:5;
char scheme:4;
};

printf("size=%d", sizeof(struct a));
return 0;
}

And according to your suggestion output must be 9 but it is 8 even take any values in the bit field still the output will be 8. Can you please explain me this.

When we run this program it will show an error that "Bit Field must be an Integral Type". This means that bit fields must be of type signed int or unsigned int and char. We cannot define float data type. Therefore,

int a:3; /* valid */

float b:3; /* Invalid */

One more thing, If we do not mention the type then also it will show an error.

struct a
{ /*
category: 5; Invalid code
scheme: 4; */
}

How float category: 5 is wrong? can anyone explain this?

prashant

very nice explaination...!!

What do you mean by un/reserved bits?

Bit field can not be used for floating values
it can be used with => signed int ,unsigned int and char
the meaning of "char scheme:4;"
is only 4-bit are reserved for "scheme" not 4-byte(people always make that confusion) here is one example
[1][1][1][1][1][1][1][1] -->8-bit field for char
but for char scheme:4;
the first 4 are not reserved and the last 4 will be reserved for "scheme".
like this (assume 0 for unreserved and 1 for reserved)
[0][0][0][0][1][1][1][1]