posix_madvise(3) Library Functions Manual posix_madvise(3)
NAME
posix_madvise - give advice about patterns of memory usage
LIBRARY
Standard C library (libc, -lc)
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
DESCRIPTION
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 posi-
tive error number.
ERRORS
EINVAL addr is not a multiple of the system page size or len is nega-
tive.
EINVAL advice is invalid.
ENOMEM Addresses in the specified range are partially or completely
outside the caller's address space.
VERSIONS
Support for posix_madvise() was added in glibc 2.2.
STANDARDS
POSIX.1-2001.
NOTES
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 seman-
tics.
SEE ALSO
madvise(2), posix_fadvise(2)
Linux man-pages 6.03 2022-12-04 posix_madvise(3)
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