Understandably confusing for a non-C-programmer, but it is idiomatic C. It's not code golf; it's just the way one does machine independent bit twiddling.
A mask is used to test each bit position in turn. The tests look like this if written using binary literals:
0b1000 & x
0b0100 & x
0b0010 & x
0b0001 & x
The '&' is the bitwise AND opperator in C. Of course, we'd have to do as many of these as the word size so 1010111 uses a for loop that starts with the first mask, a 1 in the leftmost position, and shifts it right one position every time through the loop (using the C right shift operator >> on an unsigned mask value). When the one bit is eventually shifted out the right side of the mask, the mask is all zeros so the loop terminates because zero acts like false in the for loop test.
The only other tricky thing is initializing the mask. To set only the leftmost bit in a word the code uses the bit complement operator ~ of C. Breaking it down for a four bit example looks like:
This is the expression that appears in the for loop initializing the mask value, and it works for any word size.
The original article's code was definitely not idiomatic, efficient, or safe (the memory allocation for an array of characters could fail and segfault). The book Hacker's Delight is a great reference for those wanting to understand how to do low level coding, a requirement for close to the hardware work like writing device drivers.
It uses a bit mask from the leftmost bit to the rightmost and prints the respective bits in x. Is that more complicated than allocating memory on the heap, storing the bits, and then printing them in reverse?
Also the code above does not contain anything C specific, with the exception of 'putchar' let's say.
I think it's =much= easier to read compared to the one in the article.
Although the usual approach is bit shifting the input and checking only the lowest/highest bit.
Here is an example in Java:
A mask is used to test each bit position in turn. The tests look like this if written using binary literals:
The '&' is the bitwise AND opperator in C. Of course, we'd have to do as many of these as the word size so 1010111 uses a for loop that starts with the first mask, a 1 in the leftmost position, and shifts it right one position every time through the loop (using the C right shift operator >> on an unsigned mask value). When the one bit is eventually shifted out the right side of the mask, the mask is all zeros so the loop terminates because zero acts like false in the for loop test.The only other tricky thing is initializing the mask. To set only the leftmost bit in a word the code uses the bit complement operator ~ of C. Breaking it down for a four bit example looks like:
This is the expression that appears in the for loop initializing the mask value, and it works for any word size.The original article's code was definitely not idiomatic, efficient, or safe (the memory allocation for an array of characters could fail and segfault). The book Hacker's Delight is a great reference for those wanting to understand how to do low level coding, a requirement for close to the hardware work like writing device drivers.