posix_madvise - give advice about patterns of memory usage
#include <sys/mman.h> int posix_madvise(void *addr, size_t len, int advice);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
_POSIX_C_SOURCE >= 200112L
The posix_madvise() function allows an application
to advise the system about its expected patterns of usage of memory in
the address range starting at
addr 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.
advice 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 error number.
addr is not a multiple of the system page size or
len is negative.
advice is invalid.
Addresses in the specified range are partially or completely outside the caller's address space.
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 corresponding madvise(2) value, MADV_DONTNEED, has destructive semantics.
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