Karl Coder

Karl is an aspiring C programmer, and is excited by the risks and rewards of low-level manual memory management. In the program he currently develops, he stores a string containing $N$ non-zero bytes into a buffer named "buf". By mistake he accidentally made the buffer $2 N$ bytes in size. The last $N$ bytes of the buffer consists of only zero-bytes.

Now Karl needs to know the value $N$, the size of the string, in a separate part of the program. Traditionally you would recover the length of a string using the strlen-function, which reports the position of the first zero-byte in the provided buffer using a linear scan. However, Karl finds that this is much too slow, and that it defeats the advantage of using C in the first place. Can you help Karl efficiently recover $N$ without crashing his program?

The contents of the buffer in sample interaction 3 are shown here.