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 2003-02-14 "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 #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
  38 .BR posix_fadvise ()
  39 to announce an intention to access
  40 file data in a specific pattern in the future, thus allowing the kernel
  41 to perform appropriate optimisations.
  42
  43 The \fIadvice\fP applies to a (not necessarily existent) region starting
  44 at \fIoffset\fP and extending for \fIlen\fP bytes (or until the end of
  45 the file if \fIlen\fP is 0) within the file referred to by \fIfd\fP.
  46 The advice is not binding; it merely constitutes an expectation on behalf of
  47 the application.
  48
  49 Permissible values for \fIadvice\fP include:
  50 .TP
  51 .B POSIX_FADV_NORMAL
  52 Indicates that the application has no advice to give about its access
  53 pattern for the specified data.
  54 If no advice is given for an open file,
  55 this is the default assumption.
  56 .TP
  57 .B POSIX_FADV_SEQUENTIAL
  58 The application expects to access the specified data sequentially (with
  59 lower offsets read before higher ones).
  60 .TP
  61 .B POSIX_FADV_RANDOM
  62 The specified data will be accessed in random order.
  63 .TP
  64 .B POSIX_FADV_NOREUSE
  65 The specified data will be accessed only once.
  66 .TP
  67 .B POSIX_FADV_WILLNEED
  68 The specified data will be accessed in the near future.
  69 .TP
  70 .B POSIX_FADV_DONTNEED
  71 The specified data will not be accessed in the near future.
  72 .SH "RETURN VALUE"
  73 On success, zero is returned.
  74 On error, an error number is returned.
  75 .SH ERRORS
  76 .TP
  77 .B EBADF
  78 The \fIfd\fP argument was not a valid file descriptor.
  79 .TP
  80 .B EINVAL
  81 An invalid value was specified for \fIadvice\fP.
  82 .TP
  83 .B ESPIPE
  84 The specified file descriptor refers to a pipe or FIFO. (Linux actually
  85 returns EINVAL in this case.)
  86 .SH VERSIONS
  87 .BR posix_fadvise ()
  88 appeared in kernel 2.5.60.
  89 .\" Actually as fadvise64() -- MTK
  90 .SH "CONFORMING TO"
  91 POSIX.1-2001.
  92 Note that the type of the
  93 .I len
  94 parameter was changed from
  95 .I size_t
  96 to
  97 .I off_t
  98 in POSIX.1-2003 TC1.
  99 .SH NOTES
 100 VERSIONS
 101 Under Linux, \fBPOSIX_FADV_NORMAL\fP sets the readahead window to the
 102 default size for the backing device; \fBPOSIX_FADV_SEQUENTIAL\fP doubles
 103 this size, and \fBPOSIX_FADV_RANDOM\fP disables file readahead entirely.
 104 These changes affect the entire file, not just the specified region
 105 (but other open file handles to the same file are unaffected).
 106
 107 \fBPOSIX_FADV_WILLNEED\fP initiates a
 108 non-blocking read of the specified region into the page cache.
 109 The amount of data read may be decreased by the kernel depending
 110 on virtual memory load.
 111 (A few megabytes will usually be fully satisfied,
 112 and more is rarely useful.)
 113
 114 In kernels before 2.6.18, \fBPOSIX_FADV_NOREUSE\fP had the
 115 same semantics as \fBPOSIX_FADV_WILLNEED\fP.
 116 This was probably a bug; since kernel 2.6.18, this flag is a no-op.
 117
 118 \fBPOSIX_FADV_DONTNEED\fP attempts to free cached pages associated with
 119 the specified region.
 120 This is useful, for example, while streaming large
 121 files.
 122 A program may periodically request the kernel to free cached data
 123 that has already been used, so that more useful cached pages are not
 124 discarded instead.
 125
 126 Pages that have not yet been written out will be unaffected, so if the
 127 application wishes to guarantee that pages will be released, it should
 128 call
 129 .BR fsync (2)
 130 or
 131 .BR fdatasync (2)
 132 first.
 133 .SH BUGS
 134 In kernels before 2.6.6, if
 135 .I len
 136 was specified as 0, then this was interpreted literally as "zero bytes",
 137 rather than as meaning "all bytes through to the end of the file".
 138 .SH "SEE ALSO"
 139 .BR posix_madvise (2),
 140 .BR readahead (2),
 141 .BR posix_fallocate (3) ,
 142 .BR feature_test_macros (7)