speedreader | Fork of anon codepen http : //codepen.io/anon/pen/uyKhm | Animation library

Since stdout is line buffered, putchar doesn't write to the terminal directly; it puts the character into a buffer, which is flushed when a newline is encountered. And the buffer for stdout happens to be located on the heap following your heap_book allocation.

So at some point in your copy, you putchar all the characters of your secretinfo method. They are now in the output buffer. A little later, heap_book[i] is within the stdout buffer itself, so you encounter the copy of secretinfo that is there. When you putchar it, you effectively create another copy a little further along in the buffer, and the process repeats.

You can verify this in your debugger. The address of the stdout buffer, on glibc, can be found with p stdout->_IO_buf_base. In my test it's exactly 160 bytes past heap_book.