man2/posix_fadvise.2

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   3 .\" Copyright 2003 Abhijit Menon-Sen <ams@wiw.org>
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  24 .\" 2005-04-08 mtk, noted kernel version and added BUGS
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  26 .TH POSIX_FADVISE 2 "14 Feb 2003" "Linux 2.5.60" "Linux Programmer's Manual"
  27 .SH NAME
  28 posix_fadvise \- predeclare an access pattern for file data
  29 .SH SYNOPSIS
  30 .nf
  31 .B #define _XOPEN_SOURCE 600
  32 .B #include <fcntl.h>
  33 .sp
  34 .BI "int posix_fadvise(int " fd ", off_t " offset ", off_t " len ", int " advice ");"
  35 .fi
  36 .SH DESCRIPTION
  37 Programs can use \fBposix_fadvise\fP() to announce an intention to access
  38 file data in a specific pattern in the future, thus allowing the kernel
  39 to perform appropriate optimisations.
  40
  41 The \fIadvice\fP applies to a (not necessarily existent) region starting
  42 at \fIoffset\fP and extending for \fIlen\fP bytes (or until the end of
  43 the file if \fIlen\fP is 0) within the file referred to by \fIfd\fP.
  44 The advice is not binding; it merely constitutes an expectation on behalf of
  45 the application.
  46
  47 Permissible values for \fIadvice\fP include:
  48 .TP
  49 .B POSIX_FADV_NORMAL
  50 Indicates that the application has no advice to give about its access
  51 pattern for the specified data.
  52 If no advice is given for an open file,
  53 this is the default assumption.
  54 .TP
  55 .B POSIX_FADV_SEQUENTIAL
  56 The application expects to access the specified data sequentially (with
  57 lower offsets read before higher ones).
  58 .TP
  59 .B POSIX_FADV_RANDOM
  60 The specified data will be accessed in random order.
  61 .TP
  62 .B POSIX_FADV_NOREUSE
  63 The specified data will be accessed only once.
  64 .TP
  65 .B POSIX_FADV_WILLNEED
  66 The specified data will be accessed in the near future.
  67 .TP
  68 .B POSIX_FADV_DONTNEED
  69 The specified data will not be accessed in the near future.
  70 .SH "RETURN VALUE"
  71 On success, zero is returned.
  72 On error, an error number is returned.
  73 .SH ERRORS
  74 .TP
  75 .B EBADF
  76 The \fIfd\fP argument was not a valid file descriptor.
  77 .TP
  78 .B EINVAL
  79 An invalid value was specified for \fIadvice\fP.
  80 .TP
  81 .B ESPIPE
  82 The specified file descriptor refers to a pipe or FIFO. (Linux actually
  83 returns EINVAL in this case.)
  84 .SH NOTES
  85 .BR posix_fadvise ()
  86 appeared in kernel 2.5.60.
  87 .\" Actually as fadvise64() -- MTK
  88
  89 Under Linux, \fBPOSIX_FADV_NORMAL\fP sets the readahead window to the
  90 default size for the backing device; \fBPOSIX_FADV_SEQUENTIAL\fP doubles
  91 this size, and \fBPOSIX_FADV_RANDOM\fP disables file readahead entirely.
  92 These changes affect the entire file, not just the specified region
  93 (but other open file handles to the same file are unaffected).
  94
  95 \fBPOSIX_FADV_WILLNEED\fP initiates a
  96 non-blocking read of the specified region into the page cache.
  97 The amount of data read may be decreased by the kernel depending
  98 on virtual memory load.
  99 (A few megabytes will usually be fully satisfied,
 100 and more is rarely useful.)
 101
 102 In kernels before 2.6.18, \fBPOSIX_FADV_NOREUSE\fP had the
 103 same semantics as \fBPOSIX_FADV_WILLNEED\fP.
 104 This was probably a bug; since kernel 2.6.18, this flag is a no-op.
 105
 106 \fBPOSIX_FADV_DONTNEED\fP attempts to free cached pages associated with
 107 the specified region.
 108 This is useful, for example, while streaming large
 109 files.
 110 A program may periodically request the kernel to free cached data
 111 that has already been used, so that more useful cached pages are not
 112 discarded instead.
 113
 114 Pages that have not yet been written out will be unaffected, so if the
 115 application wishes to guarantee that pages will be released, it should
 116 call \fBfsync\fP() or \fBfdatasync\fP() first.
 117 .SH "CONFORMING TO"
 118 POSIX.1-2001.
 119 Note that the type of the
 120 .I len
 121 parameter was changed from
 122 .I size_t
 123 to
 124 .I off_t
 125 in POSIX.1-2003 TC1.
 126 .SH BUGS
 127 In kernels before 2.6.6, if
 128 .I len
 129 was specified as 0, then this was interpreted literally as "zero bytes",
 130 rather than as meaning "all bytes through to the end of the file".
 131 .SH "SEE ALSO"
 132 .BR posix_madvise (2),
 133 .BR readahead (2),
 134 .BR posix_fallocate (3) ,
 135 .BR feature_test_macros (7)