1READAHEAD(2) Linux Programmer's Manual READAHEAD(2)
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6 readahead - initiate file readahead into page cache
7
9 #define _GNU_SOURCE /* See feature_test_macros(7) */
10 #include <fcntl.h>
11 ssize_t readahead(int fd, off64_t offset, size_t count);
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15 readahead() initiates readahead on a file so that subsequent reads from
16 that file will be satisfied from the cache, and not block on disk I/O
17 (assuming the readahead was initiated early enough and that other ac‐
18 tivity on the system did not in the meantime flush pages from the
19 cache).
20 The fd argument is a file descriptor identifying the file which is to
22 be read. The offset argument specifies the starting point from which
23 data is to be read and count specifies the number of bytes to be read.
24 I/O is performed in whole pages, so that offset is effectively rounded
25 down to a page boundary and bytes are read up to the next page boundary
26 greater than or equal to (offset+count). readahead() does not read be‐
27 yond the end of the file. The file offset of the open file description
28 referred to by the file descriptor fd is left unchanged.
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31 On success, readahead() returns 0; on failure, -1 is returned, with er‐
32 rno set to indicate the cause of the error.
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35 EBADF fd is not a valid file descriptor or is not open for reading.
36 EINVAL fd does not refer to a file type to which readahead() can be ap‐
38 plied.
39
41 The readahead() system call appeared in Linux 2.4.13; glibc support has
42 been provided since version 2.3.
43
45 The readahead() system call is Linux-specific, and its use should be
46 avoided in portable applications.
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49 On some 32-bit architectures, the calling signature for this system
50 call differs, for the reasons described in syscall(2).
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53 readahead() attempts to schedule the reads in the background and return
54 immediately. However, it may block while it reads the filesystem meta‐
55 data needed to locate the requested blocks. This occurs frequently
56 with ext[234] on large files using indirect blocks instead of extents,
57 giving the appearance that the call blocks until the requested data has
58 been read.
59
61 lseek(2), madvise(2), mmap(2), posix_fadvise(2), read(2)
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64 This page is part of release 5.10 of the Linux man-pages project. A
65 description of the project, information about reporting bugs, and the
66 latest version of this page, can be found at
67 https://www.kernel.org/doc/man-pages/.
68Linux 2019-03-06 READAHEAD(2)