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1 | .\" This manpage is Copyright (C) 2004, Michael Kerrisk |
2 | .\" | |
93015253 | 3 | .\" %%%LICENSE_START(VERBATIM) |
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4 | .\" Permission is granted to make and distribute verbatim copies of this |
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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14 | .\" manual page may be incorrect or out-of-date.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 | |
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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. | |
4b72fb64 | 23 | .\" %%%LICENSE_END |
fea681da | 24 | .\" |
c11b1abf | 25 | .\" 2004-05-40 Created by Michael Kerrisk <mtk.manpages@gmail.com> |
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26 | .\" 2004-10-05 aeb, minor correction |
27 | .\" | |
9ba01802 | 28 | .TH READAHEAD 2 2019-03-06 "Linux" "Linux Programmer's Manual" |
fea681da | 29 | .SH NAME |
afc9c3a0 | 30 | readahead \- initiate file readahead into page cache |
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31 | .SH SYNOPSIS |
32 | .nf | |
86b91fdf | 33 | .BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */" |
fea681da | 34 | .B #include <fcntl.h> |
68e4db0a | 35 | .PP |
37cd82ad | 36 | .BI "ssize_t readahead(int " fd ", off64_t " offset ", size_t " count ); |
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37 | .fi |
38 | .SH DESCRIPTION | |
63aa9df0 | 39 | .BR readahead () |
0226bea1 | 40 | initiates readahead on a file so that subsequent reads from that file will |
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41 | be satisfied from the cache, and not block on disk I/O |
42 | (assuming the readahead was initiated early enough and that other activity | |
43 | on the system did not in the meantime flush pages from the cache). | |
efeece04 | 44 | .PP |
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45 | The |
46 | .I fd | |
47 | argument is a file descriptor identifying the file which is | |
48 | to be read. | |
49 | The | |
50 | .I offset | |
51 | argument specifies the starting point from which data is to be read | |
52 | and | |
53 | .I count | |
54 | specifies the number of bytes to be read. | |
c13182ef | 55 | I/O is performed in whole pages, so that |
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56 | .I offset |
57 | is effectively rounded down to a page boundary | |
58 | and bytes are read up to the next page boundary greater than or | |
59 | equal to | |
60 | .IR "(offset+count)" . | |
63aa9df0 | 61 | .BR readahead () |
fea681da | 62 | does not read beyond the end of the file. |
7f11e32c | 63 | The file offset of the open file description referred to by the file descriptor |
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64 | .I fd |
65 | is left unchanged. | |
47297adb | 66 | .SH RETURN VALUE |
c13182ef | 67 | On success, |
63aa9df0 | 68 | .BR readahead () |
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69 | returns 0; on failure, \-1 is returned, with |
70 | .I errno | |
71 | set to indicate the cause of the error. | |
72 | .SH ERRORS | |
73 | .TP | |
74 | .B EBADF | |
75 | .I fd | |
76 | is not a valid file descriptor or is not open for reading. | |
77 | .TP | |
78 | .B EINVAL | |
79 | .I fd | |
80 | does not refer to a file type to which | |
e511ffb6 | 81 | .BR readahead () |
fea681da | 82 | can be applied. |
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83 | .SH VERSIONS |
84 | The | |
85 | .BR readahead () | |
86 | system call appeared in Linux 2.4.13; | |
87 | glibc support has been provided since version 2.3. | |
47297adb | 88 | .SH CONFORMING TO |
c13182ef | 89 | The |
63aa9df0 | 90 | .BR readahead () |
8382f16d | 91 | system call is Linux-specific, and its use should be avoided |
fea681da | 92 | in portable applications. |
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93 | .SH NOTES |
94 | On some 32-bit architectures, | |
95 | the calling signature for this system call differs, | |
96 | for the reasons described in | |
97 | .BR syscall (2). | |
c41881de | 98 | .SH BUGS |
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99 | .BR readahead () |
100 | attempts to schedule the reads in the background and return immediately. | |
101 | However, it may block while it reads the filesystem metadata needed | |
102 | to locate the requested blocks. | |
103 | This occurs frequently with ext[234] on large files | |
104 | using indirect blocks instead of extents, | |
b3a695fc | 105 | giving the appearance that the call blocks until the requested data has |
afc9c3a0 | 106 | been read. |
47297adb | 107 | .SH SEE ALSO |
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108 | .BR lseek (2), |
109 | .BR madvise (2), | |
110 | .BR mmap (2), | |
cedfddcd | 111 | .BR posix_fadvise (2), |
fea681da | 112 | .BR read (2) |