Maps file system object into virtual memory. This function is
    reentrant.

    Format

      #include  <types.h>

      #include  <mman.h>

      void mmap  (void *addr, size_t len, int prot, int flags, int
                 filedes, off_t off); (X/Open, POSIX-1)

      void mmap  (void *addr, size_t len, int prot, int flags, int
                 filedes, off_t off ...); (DEC C Extension)