readahead(2)

1readahead(2)                  System Calls Manual                 readahead(2)
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NAME

6       readahead - initiate file readahead into page cache
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LIBRARY

9       Standard C library (libc, -lc)
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SYNOPSIS

12       #define _GNU_SOURCE             /* See feature_test_macros(7) */
13       #define _FILE_OFFSET_BITS 64
14       #include <fcntl.h>
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16       ssize_t readahead(int fd, off_t offset, size_t count);
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DESCRIPTION

19       readahead() initiates readahead on a file so that subsequent reads from
20       that file will be satisfied from the cache, and not block on  disk  I/O
21       (assuming  the  readahead was initiated early enough and that other ac‐
22       tivity on the system did not in  the  meantime  flush  pages  from  the
23       cache).
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25       The  fd  argument is a file descriptor identifying the file which is to
26       be read.  The offset argument specifies the starting point  from  which
27       data  is to be read and count specifies the number of bytes to be read.
28       I/O is performed in whole pages, so that offset is effectively  rounded
29       down to a page boundary and bytes are read up to the next page boundary
30       greater than or equal to (offset+count).  readahead() does not read be‐
31       yond the end of the file.  The file offset of the open file description
32       referred to by the file descriptor fd is left unchanged.
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RETURN VALUE

35       On success, readahead() returns 0; on failure, -1 is returned, with er‐
36       rno set to indicate the error.
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ERRORS

39       EBADF  fd is not a valid file descriptor or is not open for reading.
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41       EINVAL fd does not refer to a file type to which readahead() can be ap‐
42              plied.
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VERSIONS

45       On some 32-bit architectures, the calling  signature  for  this  system
46       call differs, for the reasons described in syscall(2).
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STANDARDS

49       Linux.
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HISTORY

52       Linux 2.4.13, glibc 2.3.
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NOTES

55       _FILE_OFFSET_BITS  should  be  defined  to  be  64  in code that uses a
56       pointer to readahead, if the code is intended to be portable to  tradi‐
57       tional  32-bit x86 and ARM platforms where off_t's width defaults to 32
58       bits.
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BUGS

61       readahead() attempts to schedule the reads in the background and return
62       immediately.  However, it may block while it reads the filesystem meta‐
63       data needed to locate the requested  blocks.   This  occurs  frequently
64       with  ext[234] on large files using indirect blocks instead of extents,
65       giving the appearance that the call blocks until the requested data has
66       been read.
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