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1 | .\" Hey Emacs! This file is -*- nroff -*- source. |
2 | .\" | |
3 | .\" Copyright 2003 Abhijit Menon-Sen <ams@wiw.org> | |
4 | .\" Permission is granted to make and distribute verbatim copies of this | |
5 | .\" manual provided the copyright notice and this permission notice are | |
6 | .\" preserved on all copies. | |
7 | .\" | |
8 | .\" Permission is granted to copy and distribute modified versions of this | |
9 | .\" manual under the conditions for verbatim copying, provided that the | |
10 | .\" entire resulting derived work is distributed under the terms of a | |
11 | .\" permission notice identical to this one. | |
c13182ef | 12 | .\" |
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13 | .\" Since the Linux kernel and libraries are constantly changing, this |
14 | .\" manual page may be incorrect or out-of-date. The author(s) assume no | |
15 | .\" responsibility for errors or omissions, or for damages resulting from | |
16 | .\" the use of the information contained herein. The author(s) may not | |
17 | .\" have taken the same level of care in the production of this manual, | |
18 | .\" which is licensed free of charge, as they might when working | |
19 | .\" professionally. | |
c13182ef | 20 | .\" |
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21 | .\" Formatted or processed versions of this manual, if unaccompanied by |
22 | .\" the source, must acknowledge the copyright and authors of this work. | |
23 | .\" | |
f21a10c8 | 24 | .\" 2005-04-08 mtk, noted kernel version and added BUGS |
dc30fdc6 | 25 | .\" 2010-10-09, mtk, document arm_fadvise64_64() |
f21a10c8 | 26 | .\" |
e049eee8 | 27 | .TH POSIX_FADVISE 2 2010-10-09 "Linux" "Linux Programmer's Manual" |
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28 | .SH NAME |
29 | posix_fadvise \- predeclare an access pattern for file data | |
30 | .SH SYNOPSIS | |
31 | .nf | |
32 | .B #include <fcntl.h> | |
33 | .sp | |
34e8ac03 MK |
34 | .BI "int posix_fadvise(int " fd ", off_t " offset ", off_t " len \ |
35 | ", int " advice ");" | |
fea681da | 36 | .fi |
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37 | .sp |
38 | .ad l | |
39 | .in -4n | |
40 | Feature Test Macro Requirements for glibc (see | |
41 | .BR feature_test_macros (7)): | |
42 | .in | |
43 | .sp | |
44 | .BR posix_fadvise (): | |
45 | .RS 4 | |
46 | _XOPEN_SOURCE\ >=\ 600 || _POSIX_C_SOURCE\ >=\ 200112L | |
47 | .RE | |
48 | .ad | |
fea681da | 49 | .SH DESCRIPTION |
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50 | Programs can use |
51 | .BR posix_fadvise () | |
52 | to announce an intention to access | |
fea681da | 53 | file data in a specific pattern in the future, thus allowing the kernel |
d9bfdb9c | 54 | to perform appropriate optimizations. |
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55 | |
56 | The \fIadvice\fP applies to a (not necessarily existent) region starting | |
57 | at \fIoffset\fP and extending for \fIlen\fP bytes (or until the end of | |
c13182ef | 58 | the file if \fIlen\fP is 0) within the file referred to by \fIfd\fP. |
b265f7bb YK |
59 | The \fIadvice\fP is not binding; |
60 | it merely constitutes an expectation on behalf of | |
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61 | the application. |
62 | ||
63 | Permissible values for \fIadvice\fP include: | |
64 | .TP | |
65 | .B POSIX_FADV_NORMAL | |
66 | Indicates that the application has no advice to give about its access | |
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67 | pattern for the specified data. |
68 | If no advice is given for an open file, | |
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69 | this is the default assumption. |
70 | .TP | |
71 | .B POSIX_FADV_SEQUENTIAL | |
72 | The application expects to access the specified data sequentially (with | |
73 | lower offsets read before higher ones). | |
74 | .TP | |
75 | .B POSIX_FADV_RANDOM | |
76 | The specified data will be accessed in random order. | |
77 | .TP | |
78 | .B POSIX_FADV_NOREUSE | |
79 | The specified data will be accessed only once. | |
80 | .TP | |
81 | .B POSIX_FADV_WILLNEED | |
82 | The specified data will be accessed in the near future. | |
83 | .TP | |
84 | .B POSIX_FADV_DONTNEED | |
85 | The specified data will not be accessed in the near future. | |
86 | .SH "RETURN VALUE" | |
c13182ef | 87 | On success, zero is returned. |
b857d3da | 88 | On error, an error number is returned. |
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89 | .SH ERRORS |
90 | .TP | |
91 | .B EBADF | |
92 | The \fIfd\fP argument was not a valid file descriptor. | |
93 | .TP | |
94 | .B EINVAL | |
95 | An invalid value was specified for \fIadvice\fP. | |
96 | .TP | |
97 | .B ESPIPE | |
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98 | The specified file descriptor refers to a pipe or FIFO. |
99 | (Linux actually | |
100 | returns | |
101 | .B EINVAL | |
102 | in this case.) | |
a1d5f77c | 103 | .SH VERSIONS |
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104 | Kernel support first appeared in Linux 2.5.60; |
105 | the underlying system call is called | |
106 | .BR fadvise64 (). | |
107 | .\" of fadvise64_64() | |
108 | Library support has been provided since glibc version 2.2, | |
109 | via the wrapper function | |
110 | .BR posix_fadvise (). | |
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111 | .SH "CONFORMING TO" |
112 | POSIX.1-2001. | |
113 | Note that the type of the | |
114 | .I len | |
c4bb193f | 115 | argument was changed from |
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116 | .I size_t |
117 | to | |
118 | .I off_t | |
119 | in POSIX.1-2003 TC1. | |
120 | .SH NOTES | |
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121 | Under Linux, \fBPOSIX_FADV_NORMAL\fP sets the readahead window to the |
122 | default size for the backing device; \fBPOSIX_FADV_SEQUENTIAL\fP doubles | |
123 | this size, and \fBPOSIX_FADV_RANDOM\fP disables file readahead entirely. | |
8c450534 | 124 | These changes affect the entire file, not just the specified region |
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125 | (but other open file handles to the same file are unaffected). |
126 | ||
5e6055f4 | 127 | \fBPOSIX_FADV_WILLNEED\fP initiates a |
ff40dbb3 | 128 | nonblocking read of the specified region into the page cache. |
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129 | The amount of data read may be decreased by the kernel depending |
130 | on virtual memory load. | |
131 | (A few megabytes will usually be fully satisfied, | |
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132 | and more is rarely useful.) |
133 | ||
c13182ef | 134 | In kernels before 2.6.18, \fBPOSIX_FADV_NOREUSE\fP had the |
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135 | same semantics as \fBPOSIX_FADV_WILLNEED\fP. |
136 | This was probably a bug; since kernel 2.6.18, this flag is a no-op. | |
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137 | |
138 | \fBPOSIX_FADV_DONTNEED\fP attempts to free cached pages associated with | |
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139 | the specified region. |
140 | This is useful, for example, while streaming large | |
141 | files. | |
142 | A program may periodically request the kernel to free cached data | |
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143 | that has already been used, so that more useful cached pages are not |
144 | discarded instead. | |
145 | ||
146 | Pages that have not yet been written out will be unaffected, so if the | |
147 | application wishes to guarantee that pages will be released, it should | |
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148 | call |
149 | .BR fsync (2) | |
150 | or | |
151 | .BR fdatasync (2) | |
152 | first. | |
dc30fdc6 MK |
153 | .SS arm_fadvise() |
154 | The ARM architecture | |
155 | needs 64-bit arguments to be aligned in a suitable pair of registers. | |
156 | On this architecture, the call signature of | |
157 | .BR posix_fadvise () | |
158 | is flawed, since it forces a register to be wasted as padding between the | |
159 | .I fd | |
160 | and | |
161 | .I len | |
162 | arguments. | |
163 | Therefore, since Linux 2.6.14, ARM defines a different | |
164 | system call that orders the arguments suitably: | |
165 | .PP | |
166 | .in +4n | |
167 | .nf | |
168 | .BI "long arm_fadvise64_64(int " fd ", int " advice , | |
503979fa | 169 | .BI " loff_t " offset ", loff_t " len ); |
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170 | .fi |
171 | .in | |
172 | .PP | |
173 | The behavior of this system call is otherwise exactly the same as | |
62fc472f | 174 | .BR posix_fadvise (). |
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175 | No library support is provided for this system call in glibc. |
176 | .\" No ARM support in glibc. | |
f21a10c8 | 177 | .SH BUGS |
c13182ef | 178 | In kernels before 2.6.6, if |
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179 | .I len |
180 | was specified as 0, then this was interpreted literally as "zero bytes", | |
181 | rather than as meaning "all bytes through to the end of the file". | |
fea681da | 182 | .SH "SEE ALSO" |
ef276d2f | 183 | .BR readahead (2), |