mirror of https://github.com/mkerrisk/man-pages
129 lines
3.4 KiB
Groff
129 lines
3.4 KiB
Groff
.\" Copyright (C) 2015 Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(GPLv2+)
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.\"
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.\" This program is free software; you can redistribute it and/or modify
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.\" it under the terms of the GNU General Public License as published by
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.\" the Free Software Foundation; either version 2 of the License, or
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.\" (at your option) any later version.
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.\"
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.\" This program is distributed in the hope that it will be useful,
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.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
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.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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.\" GNU General Public License for more details.
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.\"
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.\" You should have received a copy of the GNU General Public
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.\" License along with this manual; if not, see
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.\" <http://www.gnu.org/licenses/>.
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.\" %%%LICENSE_END
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.\"
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.TH POSIX_MADVISE 3 2017-09-15 Linux "Linux Programmer's Manual"
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.SH NAME
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posix_madvise \- give advice about patterns of memory usage
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.SH SYNOPSIS
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.nf
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.B #include <sys/mman.h>
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.PP
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.BI "int posix_madvise(void *" addr ", size_t " len ", int " advice );
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.fi
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.PP
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.RS -4
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Feature Test Macro Requirements for glibc (see
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.BR feature_test_macros (7)):
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.RE
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.PP
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.BR posix_madvise ():
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.br
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.RS 4
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.ad l
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_POSIX_C_SOURCE >= 200112L
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.RE
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.ad
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.SH DESCRIPTION
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The
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.BR posix_madvise ()
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function allows an application to advise the system about its expected
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patterns of usage of memory in the address range starting at
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.I addr
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and continuing for
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.I len
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bytes.
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The system is free to use this advice in order to improve the performance
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of memory accesses (or to ignore the advice altogether), but calling
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.BR posix_madvise ()
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shall not affect the semantics of access to memory in the specified range.
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.PP
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The
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.I advice
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argument is one of the following:
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.TP
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.B POSIX_MADV_NORMAL
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The application has no special advice regarding its memory usage patterns
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for the specified address range.
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This is the default behavior.
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.TP
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.B POSIX_MADV_SEQUENTIAL
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The application expects to access the specified address range sequentially,
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running from lower addresses to higher addresses.
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Hence, pages in this region can be aggressively read ahead,
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and may be freed soon after they are accessed.
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.TP
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.B POSIX_MADV_RANDOM
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The application expects to access the specified address range randomly.
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Thus, read ahead may be less useful than normally.
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.TP
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.B POSIX_MADV_WILLNEED
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The application expects to access the specified address range
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in the near future.
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Thus, read ahead may be beneficial.
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.TP
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.B POSIX_MADV_DONTNEED
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The application expects that it will not access the specified address range
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in the near future.
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.SH RETURN VALUE
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On success,
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.BR posix_madvise ()
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returns 0.
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On failure, it returns a positive error number.
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.SH ERRORS
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.TP
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.B EINVAL
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.I addr
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is not a multiple of the system page size or
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.I len
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is negative.
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.TP
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.B EINVAL
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.I advice
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is invalid.
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.TP
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.B ENOMEM
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Addresses in the specified range are partially or completely outside
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the caller's address space.
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.SH VERSIONS
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Support for
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.BR posix_madvise ()
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first appeared in glibc version 2.2.
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.SH CONFORMING TO
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POSIX.1-2001.
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.SH NOTES
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POSIX.1 permits an implementation to generate an error if
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.I len
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is 0.
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On Linux, specifying
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.I len
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as 0 is permitted (as a successful no-op).
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.PP
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In glibc, this function is implemented using
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.BR madvise (2).
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However, since glibc 2.6,
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.BR POSIX_MADV_DONTNEED
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is treated as a no-op, because the corresponding
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.BR madvise (2)
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value,
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.BR MADV_DONTNEED ,
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has destructive semantics.
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.SH SEE ALSO
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.BR madvise (2),
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.BR posix_fadvise (2)
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