Infinite loop generator in C

Question

I tried this code in C

 unsigned char c= 150;
 for (unsigned char i = 0; i < (2*c); ++i)
 {
    // do something;
 }

The program never halts. Why?

Answer 1

The sizeof(unsigned char) is 1, (i.e. 8 bits), so the maximum value it can hold is (2^8 - 1) which is 255. Hence, unsigned char cannot hold values over 255, after which the count restarts from 0. In your case, the loop condition i < 300 is always true, because c resets to 0 once it reaches 255.

To understand why it resets to zero, see the binary representation of 255. 255 can be written in binary as

11111111

which is 8 bits. So far so good.

If you add 1 to it, then the value becomes

100000000

which is 9 bits. Since unsigned char can store only 1 byte (or 8 bits), the most significant bit 1 is discarded and only the least significant 8 bits get stored in the variable. As a result, 00000000 is what gets stored in c, thus making it 0 again.