The madvise system call advises the kernel about how to handle paging
input/output in the address range beginning at address start and with
size length bytes. It allows an application to tell the kernel how it
expects to use some mapped or shared memory areas, so that the kernel can
choose appropriate read-ahead and caching techniques. This call does not
influence the semantics of the application (except in the case of
MADV_DONTNEED), but may influence its performance. The kernel is free
to ignore the advice.
The advice is indicated in the advice parameter which can be
No special treatment. This is the default.
Expect page references in random order. (Hence, read ahead
may be less useful than normally.)
Expect page references in sequential order. (Hence, pages
in the given range can be aggressively read ahead, and may be freed soon
after they are accessed.)
Expect access in the near future. (Hence, it might be a
good idea to read some pages ahead.)
Do not expect access in the near future. (For the time
being, the application is finished with the given range, so the kernel can
free resources associated with it.) Subsequent accesses of pages in this
range will succeed, but will result either in re-loading of the memory
contents from the underlying mapped file (see mmap) or
zero-fill-on-demand pages for mappings without an underlying file.
The current Linux implementation (2.4.0) views this system call more as a
command than as advice and hence may return an error when it cannot do what it
usually would do in response to this advice. (See the ERRORS description
above.) This is nonstandard behaviour.
The Linux implementation requires that the address start be page-aligned,
and allows length to be zero. If there are some parts of the specified
address range that are not mapped, the Linux version of madvise ignores
them and applies the call to the rest (but returns ENOMEM from the
system call, as it should).