posix_madvise - Man Page

give advice about patterns of memory usage

Synopsis

#include <sys/mman.h>

int posix_madvise(void addr[.len], size_t len, int advice);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

posix_madvise():

    _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.

The advice argument is one of the following:

POSIX_MADV_NORMAL: The application has no special advice regarding its memory usage patterns for the specified address range. This is the default behavior.
POSIX_MADV_SEQUENTIAL: 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.
POSIX_MADV_RANDOM: The application expects to access the specified address range randomly. Thus, read ahead may be less useful than normally.
POSIX_MADV_WILLNEED: The application expects to access the specified address range in the near future. Thus, read ahead may be beneficial.
POSIX_MADV_DONTNEED: The application expects that it will not access the specified address range in the near future.

Return Value

On success, posix_madvise() returns 0. On failure, it returns a positive error number.

Errors

EINVAL: addr is not a multiple of the system page size or len is negative.
EINVAL: advice is invalid.
ENOMEM: Addresses in the specified range are partially or completely outside the caller's address space.

Versions

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.