man2/posix_fadvise.2

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  24 .\" 2005-04-08 mtk, noted kernel version and added BUGS
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  26 .TH POSIX_FADVISE 2 2010-06-14 "Linux" "Linux Programmer's Manual"
  27 .SH NAME
  28 posix_fadvise \- predeclare an access pattern for file data
  29 .SH SYNOPSIS
  30 .nf
  31 .B #include <fcntl.h>
  32 .sp
  33 .BI "int posix_fadvise(int " fd ", off_t " offset ", off_t " len \
  34 ", int " advice ");"
  35 .fi
  36 .sp
  37 .ad l
  38 .in -4n
  39 Feature Test Macro Requirements for glibc (see
  40 .BR feature_test_macros (7)):
  41 .in
  42 .sp
  43 .BR posix_fadvise ():
  44 .RS 4
  45 _XOPEN_SOURCE\ >=\ 600 || _POSIX_C_SOURCE\ >=\ 200112L
  46 .RE
  47 .ad
  48 .SH DESCRIPTION
  49 Programs can use
  50 .BR posix_fadvise ()
  51 to announce an intention to access
  52 file data in a specific pattern in the future, thus allowing the kernel
  53 to perform appropriate optimizations.
  54
  55 The \fIadvice\fP applies to a (not necessarily existent) region starting
  56 at \fIoffset\fP and extending for \fIlen\fP bytes (or until the end of
  57 the file if \fIlen\fP is 0) within the file referred to by \fIfd\fP.
  58 The advice is not binding; it merely constitutes an expectation on behalf of
  59 the application.
  60
  61 Permissible values for \fIadvice\fP include:
  62 .TP
  63 .B POSIX_FADV_NORMAL
  64 Indicates that the application has no advice to give about its access
  65 pattern for the specified data.
  66 If no advice is given for an open file,
  67 this is the default assumption.
  68 .TP
  69 .B POSIX_FADV_SEQUENTIAL
  70 The application expects to access the specified data sequentially (with
  71 lower offsets read before higher ones).
  72 .TP
  73 .B POSIX_FADV_RANDOM
  74 The specified data will be accessed in random order.
  75 .TP
  76 .B POSIX_FADV_NOREUSE
  77 The specified data will be accessed only once.
  78 .TP
  79 .B POSIX_FADV_WILLNEED
  80 The specified data will be accessed in the near future.
  81 .TP
  82 .B POSIX_FADV_DONTNEED
  83 The specified data will not be accessed in the near future.
  84 .SH "RETURN VALUE"
  85 On success, zero is returned.
  86 On error, an error number is returned.
  87 .SH ERRORS
  88 .TP
  89 .B EBADF
  90 The \fIfd\fP argument was not a valid file descriptor.
  91 .TP
  92 .B EINVAL
  93 An invalid value was specified for \fIadvice\fP.
  94 .TP
  95 .B ESPIPE
  96 The specified file descriptor refers to a pipe or FIFO.
  97 (Linux actually
  98 returns
  99 .B EINVAL
 100 in this case.)
 101 .SH VERSIONS
 102 .BR posix_fadvise ()
 103 appeared in kernel 2.5.60.
 104 Glibc support has been provided since version 2.2.
 105 .\" Actually as fadvise64() -- MTK
 106 .SH "CONFORMING TO"
 107 POSIX.1-2001.
 108 Note that the type of the
 109 .I len
 110 argument was changed from
 111 .I size_t
 112 to
 113 .I off_t
 114 in POSIX.1-2003 TC1.
 115 .SH NOTES
 116 Under Linux, \fBPOSIX_FADV_NORMAL\fP sets the readahead window to the
 117 default size for the backing device; \fBPOSIX_FADV_SEQUENTIAL\fP doubles
 118 this size, and \fBPOSIX_FADV_RANDOM\fP disables file readahead entirely.
 119 These changes affect the entire file, not just the specified region
 120 (but other open file handles to the same file are unaffected).
 121
 122 \fBPOSIX_FADV_WILLNEED\fP initiates a
 123 nonblocking read of the specified region into the page cache.
 124 The amount of data read may be decreased by the kernel depending
 125 on virtual memory load.
 126 (A few megabytes will usually be fully satisfied,
 127 and more is rarely useful.)
 128
 129 In kernels before 2.6.18, \fBPOSIX_FADV_NOREUSE\fP had the
 130 same semantics as \fBPOSIX_FADV_WILLNEED\fP.
 131 This was probably a bug; since kernel 2.6.18, this flag is a no-op.
 132
 133 \fBPOSIX_FADV_DONTNEED\fP attempts to free cached pages associated with
 134 the specified region.
 135 This is useful, for example, while streaming large
 136 files.
 137 A program may periodically request the kernel to free cached data
 138 that has already been used, so that more useful cached pages are not
 139 discarded instead.
 140
 141 Pages that have not yet been written out will be unaffected, so if the
 142 application wishes to guarantee that pages will be released, it should
 143 call
 144 .BR fsync (2)
 145 or
 146 .BR fdatasync (2)
 147 first.
 148 .SH BUGS
 149 In kernels before 2.6.6, if
 150 .I len
 151 was specified as 0, then this was interpreted literally as "zero bytes",
 152 rather than as meaning "all bytes through to the end of the file".
 153 .SH "SEE ALSO"
 154 .BR readahead (2),
 155 .BR sync_file_range (2),
 156 .BR posix_fallocate (3),
 157 .BR posix_madvise (3),
 158 .\" FIXME . Write a posix_fadvise(3) page.
 159 .BR feature_test_macros (7)