madvise, posix_madvise —
give advice about use of memory
Standard C Library (libc, -lc)
#include <sys/mman.h>
int
  
  madvise(void
    *addr, size_t len,
    int behav);
int
  
  posix_madvise(void
    *addr, size_t len,
    int advice);
The madvise() system call allows a process that has
  knowledge of its memory behavior to describe it to the system. The
  posix_madvise() interface is identical and is provided
  for standards conformance.
The known behaviors are:
  - MADV_NORMAL
- Tells the system to revert to the default paging behavior.
- MADV_RANDOM
- Is a hint that pages will be accessed randomly, and prefetching is likely
      not advantageous.
- MADV_SEQUENTIAL
- Is a hint that pages will be accessed sequentially, from the lower address
      to higher address. It might cause the VM system to depress the priority of
      pages immediately preceding a given page when it is faulted in.
- MADV_WILLNEED
- Is a hint that pages will be accessed in the near future. It might cause
      the VM system to make pages that are in a given virtual address range to
      temporarily have higher priority, and if they are in memory, decrease the
      likelihood of them being freed. It might immediately map the pages that
      are already in memory into the process, thereby eliminating unnecessary
      overhead of going through the entire process of faulting the pages in. It
      might or might not fault pages in from backing store.
- MADV_DONTNEED
- Is a hint that pages will not be accessed in the near future. It might
      allow the VM system to decrease the in-memory priority of pages in the
      specified range.
- MADV_FREE
- Gives the VM system the freedom to free pages, and tells the system that
      information in the specified page range is no longer important.
Portable programs that call the
    posix_madvise() interface should use the aliases
    POSIX_MADV_NORMAL,
    POSIX_MADV_SEQUENTIAL,
    POSIX_MADV_RANDOM,
    POSIX_MADV_WILLNEED, and
    POSIX_MADV_DONTNEED rather than the flags described
    above.
Upon successful completion, a value of 0 is returned. Otherwise, a value of -1
  is returned and errno is set to indicate the error.
madvise() will fail if:
  - [EINVAL]
- Invalid parameters were provided.
Theposix_madvise() system call is expected to conform
  to the IEEE Std 1003.1-2001 (“POSIX.1”)
  standard.
The madvise system call first appeared in
  4.4BSD, but until NetBSD 1.5
  it did not perform any of the requests on, or change any behavior of the
  address range given. The posix_madvise() call was
  added in NetBSD 5.0.