Is `kLabelPlaceholder` an absolute or relative address? If it's absolute, you start executing at `kLabelPlaceholder`. If it's relative, you start executing at `buf + kLabelPlaceholder`. You could see the final value if you compile with debug info and run the binary under `gdb` or some other debugger.
It might help to read skeeto's blogs [0][1][2].
[0] https://nullprogram.com/blog/2015/03/19/