posix_madvise - give advice about patterns of memory usage
int posix_madvise(void *addr, size_t len, int advice);
Feature Test Macro Requirements for glibc (see feature_test_macros
_POSIX_C_SOURCE >= 200112L
() function allows an application to advise the system
about its expected patterns of usage of memory in the address range starting
and continuing for len
bytes. The system is free to use
this advice in order to improve the performance of memory accesses (or to
ignore the advice altogether), but calling posix_madvise
() shall not
affect the semantics of access to memory in the specified range.
argument is one of the following:
- The application has no special advice regarding its memory usage patterns
for the specified address range. This is the default behavior.
- The application expects to access the specified address range
sequentially, running from lower addresses to higher addresses. Hence,
pages in this region can be aggressively read ahead, and may be freed soon
after they are accessed.
- The application expects to access the specified address range randomly.
Thus, read ahead may be less useful than normally.
- The application expects to access the specified address range in the near
future. Thus, read ahead may be beneficial.
- The application expects that it will not access the specified address
range in the near future.
On success, posix_madvise
() returns 0. On failure, it returns a positive
- addr is not a multiple of the system page size or len is
- advice is invalid.
- Addresses in the specified range are partially or completely outside the
caller's address space.
Support for posix_madvise
() first appeared in glibc version 2.2.
POSIX.1 permits an implementation to generate an error if len
is 0. On
Linux, specifying len
as 0 is permitted (as a successful no-op).
In glibc, this function is implemented using madvise
(2). However, since
glibc 2.6, POSIX_MADV_DONTNEED
is treated as a no-op, because the
(2) value, MADV_DONTNEED
, has destructive