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.\" Copyright (C) 2015 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: GPL-2.0-or-later
.\"
.TH posix_madvise 3 2023-03-30 "Linux man-pages 6.05.01"
.SH NAME
posix_madvise \- give advice about patterns of memory usage
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <sys/mman.h>
.PP
.BI "int posix_madvise(void " addr [. len "], size_t " len ", int " advice );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR posix_madvise ():
.nf
_POSIX_C_SOURCE >= 200112L
.fi
.SH DESCRIPTION
The
.BR posix_madvise ()
function allows an application to advise the system about its expected
patterns of usage of memory in the address range starting at
.I addr
and continuing for
.I 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
.BR posix_madvise ()
shall not affect the semantics of access to memory in the specified range.
.PP
The
.I advice
argument is one of the following:
.TP
.B POSIX_MADV_NORMAL
The application has no special advice regarding its memory usage patterns
for the specified address range.
This is the default behavior.
.TP
.B 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.
.TP
.B POSIX_MADV_RANDOM
The application expects to access the specified address range randomly.
Thus, read ahead may be less useful than normally.
.TP
.B POSIX_MADV_WILLNEED
The application expects to access the specified address range
in the near future.
Thus, read ahead may be beneficial.
.TP
.B POSIX_MADV_DONTNEED
The application expects that it will not access the specified address range
in the near future.
.SH RETURN VALUE
On success,
.BR posix_madvise ()
returns 0.
On failure, it returns a positive error number.
.SH ERRORS
.TP
.B EINVAL
.I addr
is not a multiple of the system page size or
.I len
is negative.
.TP
.B EINVAL
.I advice
is invalid.
.TP
.B ENOMEM
Addresses in the specified range are partially or completely outside
the caller's address space.
.SH VERSIONS
POSIX.1 permits an implementation to generate an error if
.I len
is 0.
On Linux, specifying
.I len
as 0 is permitted (as a successful no-op).
.PP
In glibc, this function is implemented using
.BR madvise (2).
However, since glibc 2.6,
.B POSIX_MADV_DONTNEED
is treated as a no-op, because the corresponding
.BR madvise (2)
value,
.BR MADV_DONTNEED ,
has destructive semantics.
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
glibc 2.2.
POSIX.1-2001.
.SH SEE ALSO
.BR madvise (2),
.BR posix_fadvise (2)
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