user3116473 user3116473 - 1 month ago 6
C Question

Confusion in the Bitwise operation

I just started to learn c language. I have a question about the result of bitwise operations. In the following code, why we get a different result from c and d? How do we have 72 as the value of d?

char a = 41;//101001
char b = (a<<5);//32 or 100000
char c = (b>>2);//8 or 1000
char d = (a<<5)>>2;//72 or 1001000
printf("a= %d , b=%d, b=%d , d=%d\n", a, b,c,d);

Answer

When any type or arithmetic operation is performed on a type smaller than int, the value is promoted to int within the expression. Then when the result is saved to a char, all but the lowest order byte are truncated.

This covered in section 6.3.1.1 of the C standard:

2 The following may be used in an expression wherever an int or unsigned int may be used:

— An object or expression with an integer type (other than int or unsigned int) whose integer conversion rank is less than or equal to the rank of int and unsigned int.

— A bit-field of type _Bool, int, signed int, or unsigned int.

If an int can represent all values of the original type (as restricted by the width, for a bit-field), the value is converted to an int; otherwise, it is converted to an unsigned int. These are called the integer promotions. All other types are unchanged by the integer promotions.

In the case of c, you first do a<<5. Before this happens, the value of a in this expression is promoted in int, so the result can be larger than a char.

This results in the binary value 10100100000 (decimal 1312). This is then saved to b which has type char, so only the lower 8 bits (00100000) are retained.

Then we perform b>>2 giving us binary 00001000 (decimal 8) which is saved in c.

In the case of d, we perform a<<5 as before (with the value of a being promoted to int) getting binary 10100100000. Now this value is shifted right by 2 resulting in 00101001000 (decimal 328). This is then saved to d which is of type char, so only the lower 8 bits are saved (01001000).

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