How to find the most significant bit of a signed integer in C

Question

I need to find the most significant bit of signed int N and save it in signBitN. I want to do this using bitwise only operations. Also, how would I make signBitN extend so that all its bits are equal to its significant bit. i.e. if it's significant bit was zero, how would I extend that to be 00000...00? The closest I've gotten is signBitN=1&(N>>(sizeof(int)-1));

Answer 1

Portable expression:

1 & (x >> (CHAR_BIT * sizeof(int) - 1))

Latest C standards put 3 standards on representation of ints.

• sign and magnitude
• one complement
• two complement See section 6.2.6.2 Integer types of C11 standard.

Only the third option is relevant in practice for modern machines. As specified in 6.2.6.1:

Values stored in non-bit-field objects of any other object type consist of n x CHAR_BIT bits, where n is the size of an object of that type, in bytes.

Therefore int will consist of sizeof(int) * CHAR_BIT bits, likely 32. Thus the highest bit of int can be read by shifting right by sizeof(int) * CHAR_BIT - 1 bits and reading the last bit with bitwise & operator.

Note that the exact value of the int after the shift is implementation defined as stated in 6.5.7.5.

On sane machines it would be:

int y = x < 0 ? -1 : 0;

The portable way would be casting between int and an array of unsigned char and setting all bytes to -1. See 6.3.1.3.2:

if the new type is unsigned, the value is converted by repeatedly adding or subtracting one more than the maximum value that can be represented in the new type until the value is in the range of the new type.

And 6.2.6.1.2

Values stored in unsigned bit-fields and objects of type unsigned char shall be represented using a pure binary notation.

You can use memset() for that.

int x;
memset(&x, (x < 0 ? -1 : 0), sizeof x);