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madvise() - Unix, Linux System Call
madvise - give advice about use of memory
int madvise(void *start, size_t length, int advice);
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
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.
madvise() returns zero. On error, it returns -1 and
errno is set appropriately.
A kernel resource was temporarily unavailable.
The map exists, but the area maps something that isnt a file.
len is negative,
start is not page-aligned,
advice is not a valid value, or the application is attempting
to release locked or shared pages (with MADV_DONTNEED).
(for MADV_WILLNEED) Paging in this area would exceed the processs
maximum resident set size.
(for MADV_WILLNEED) Not enough memory: paging in failed.
Addresses in the specified range are not currently
mapped, or are outside the address space of the process.
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).
madvise() function first appeared in 4.4BSD.
posix_madvise() with constants POSIX_MADV_NORMAL, etc.,
with a behaviour close to that described here. There is a similar
posix_fadvise() for file access.