How does posix_memalign with alignment size of 4096 bytes differ from mmap? Does it internally use mmap or some other mechanism?
posix_memalign is a higher-level API than mmap, designed to interoperate with malloc, free and realloc. mmap usage is more complicated because it offers more functionality than posix_memalign (mapping files into a process's address space). How it is implemented (in terms of mmap or otherwise) is left unspecified by the POSIX standard.
Use posix_memalign where you'd use malloc if you didn't have alignment restrictions.