1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
|
.\" Copyright 2003 Abhijit Menon-Sen <ams@wiw.org>
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date. The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein. The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\" %%%LICENSE_END
.\"
.\" 2005-04-08 mtk, noted kernel version and added BUGS
.\" 2010-10-09, mtk, document arm_fadvise64_64()
.\"
.TH POSIX_FADVISE 2 2014-05-03 "Linux" "Linux Programmer's Manual"
.SH NAME
posix_fadvise \- predeclare an access pattern for file data
.SH SYNOPSIS
.nf
.B #include <fcntl.h>
.sp
.BI "int posix_fadvise(int " fd ", off_t " offset ", off_t " len \
", int " advice ");"
.fi
.sp
.ad l
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.BR posix_fadvise ():
.RS 4
_XOPEN_SOURCE\ >=\ 600 || _POSIX_C_SOURCE\ >=\ 200112L
.RE
.ad
.SH DESCRIPTION
Programs can use
.BR posix_fadvise ()
to announce an intention to access
file data in a specific pattern in the future, thus allowing the kernel
to perform appropriate optimizations.
The \fIadvice\fP applies to a (not necessarily existent) region starting
at \fIoffset\fP and extending for \fIlen\fP bytes (or until the end of
the file if \fIlen\fP is 0) within the file referred to by \fIfd\fP.
The \fIadvice\fP is not binding;
it merely constitutes an expectation on behalf of
the application.
Permissible values for \fIadvice\fP include:
.TP
.B POSIX_FADV_NORMAL
Indicates that the application has no advice to give about its access
pattern for the specified data.
If no advice is given for an open file,
this is the default assumption.
.TP
.B POSIX_FADV_SEQUENTIAL
The application expects to access the specified data sequentially (with
lower offsets read before higher ones).
.TP
.B POSIX_FADV_RANDOM
The specified data will be accessed in random order.
.TP
.B POSIX_FADV_NOREUSE
The specified data will be accessed only once.
.TP
.B POSIX_FADV_WILLNEED
The specified data will be accessed in the near future.
.TP
.B POSIX_FADV_DONTNEED
The specified data will not be accessed in the near future.
.SH RETURN VALUE
On success, zero is returned.
On error, an error number is returned.
.SH ERRORS
.TP
.B EBADF
The \fIfd\fP argument was not a valid file descriptor.
.TP
.B EINVAL
An invalid value was specified for \fIadvice\fP.
.TP
.B ESPIPE
The specified file descriptor refers to a pipe or FIFO.
(Linux actually
returns
.B EINVAL
in this case.)
.SH VERSIONS
Kernel support first appeared in Linux 2.5.60;
the underlying system call is called
.BR fadvise64 ().
.\" of fadvise64_64()
Library support has been provided since glibc version 2.2,
via the wrapper function
.BR posix_fadvise ().
.SH CONFORMING TO
POSIX.1-2001.
Note that the type of the
.I len
argument was changed from
.I size_t
to
.I off_t
in POSIX.1-2003 TC1.
.SH NOTES
Under Linux, \fBPOSIX_FADV_NORMAL\fP sets the readahead window to the
default size for the backing device; \fBPOSIX_FADV_SEQUENTIAL\fP doubles
this size, and \fBPOSIX_FADV_RANDOM\fP disables file readahead entirely.
These changes affect the entire file, not just the specified region
(but other open file handles to the same file are unaffected).
\fBPOSIX_FADV_WILLNEED\fP initiates a
nonblocking read of the specified region into the page cache.
The amount of data read may be decreased by the kernel depending
on virtual memory load.
(A few megabytes will usually be fully satisfied,
and more is rarely useful.)
In kernels before 2.6.18, \fBPOSIX_FADV_NOREUSE\fP had the
same semantics as \fBPOSIX_FADV_WILLNEED\fP.
This was probably a bug; since kernel 2.6.18, this flag is a no-op.
\fBPOSIX_FADV_DONTNEED\fP attempts to free cached pages associated with
the specified region.
This is useful, for example, while streaming large
files.
A program may periodically request the kernel to free cached data
that has already been used, so that more useful cached pages are not
discarded instead.
Pages that have not yet been written out will be unaffected, so if the
application wishes to guarantee that pages will be released, it should
call
.BR fsync (2)
or
.BR fdatasync (2)
first.
.SS Architecture-specific variants
Some architectures require
64-bit arguments to be aligned in a suitable pair of registers (see
.BR syscall (2)
for further detail).
On such architectures, the call signature of
.BR posix_fadvise ()
shown in the SYNOPSIS would force
a register to be wasted as padding between the
.I fd
and
.I offset
arguments.
Therefore, these architectures define a version of the
system call that orders the arguments suitably,
but otherwise is otherwise exactly the same as
.BR posix_fadvise ().
For example, since Linux 2.6.14, ARM has the following system call:
.PP
.in +4n
.nf
.BI "long arm_fadvise64_64(int " fd ", int " advice ,
.BI " loff_t " offset ", loff_t " len );
.fi
.in
.PP
These architecture-specific details are generally
hidden from applications by the glibc
.BR posix_fadvise ()
wrapper function,
which invokes the appropriate architecture-specific system call.
.SH BUGS
In kernels before 2.6.6, if
.I len
was specified as 0, then this was interpreted literally as "zero bytes",
rather than as meaning "all bytes through to the end of the file".
.SH SEE ALSO
.BR readahead (2),
.BR sync_file_range (2),
.BR posix_fallocate (3),
.BR posix_madvise (3)
.\" FIXME . Write a posix_fadvise(3) page.
.SH COLOPHON
This page is part of release 3.74 of the Linux
.I man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
\%http://www.kernel.org/doc/man\-pages/.
|
