c - Find number of bits in a data type

Question

I need to write a macro named CountBitsM. this macro has one parameter and produces a value of type int. The parameter is any expression with an object data type or the literal name of any object data type, so i used int. This macro determines the number of bits of storage used for the data type on any machine in which its run. And i can use a macro from limits.h. Here is what i wrote, does this look right?

#ifndef COUNTBITSM_H
#define COUNTBITSM_H
#include <limits.h>

#define CountBitsM(int) ((int)*(CHAR_BIT))

#endif

Second question was to create a function CountIntBitsF that counts the number of bits used to represent a type int value on any machine. However, i can NOT USE any #define, or #include header files, or any macro. I also can not use any multiplications or divisions. The hint that was given was to start with a value of 1 in a type unsigned int variable and left-shift it one bit at a time, keeping count of number of shifts, until the variables value becomes 0. Here is what i have so far:

int CountIntBitsF(void)
{
    int IntgMax = 8;
    unsigned int count = 1; 
    
    while (IntgMax = IntgMax>>2) count++;

    return count;
}

First off, i am not supposed to use division or multiplication so am i doing the shift properly? And i cant assume a char/byte contains 8 or any other specific number of bits. So how or what should i set my IntgMax to? Thanks for any help. I am new to C.

question from:https://stackoverflow.com/questions/65835593/find-number-of-bits-in-a-data-type

Answer 1

Macro for Bits in a Type

A macro to produce the number of bits used to represent a type in storage is:

#define CountBitsM(x) (sizeof (x) * CHAR_BIT)

However, this produces a result with type size_t (usually). If you really need an int result as stated in the question, convert it (but be aware overflow becomes possible):

#define CountBitsM(x) ((int) (sizeof (x) * CHAR_BIT))

Counting Bits

The second question asks to count the number of bits “to represent a type int value” by shifting bits in an unsigned value. There are two theoretical problems here. One is that the number of bits used to represent a value may including padding bits, and counting the bits by shifting a 1 through them only counts the value bits, not the padding bits. The second is that an int may have more padding bits than an unsigned; it may use fewer bits for the sign and value. Overwhelmingly, modern systems will not have these issues; the number of used bits in an int will be the same as the total number of bits used to store it and the number of bits used in an unsigned.

That said, you can count the number of bits in an unsigned object with:

    int count = 0;
    for (unsigned u = 1; 0 != u; u <<= 1)
        ++count;

This repeatedly shifts the bit in u left until it is shifted out, while counting the number of iterations required to do this. Note that the bits in an int cannot properly be counted this way, because the behavior of left shift is not defined by the C standard when it overflows an int.