man2/posix_fadvise.2

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   3 .\" Copyright 2003 Abhijit Menon-Sen <ams@wiw.org>
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  23 .\"
  24 .\" 2005-04-08 mtk, noted kernel version and added BUGS
  25 .\" 2010-10-09, mtk, document arm_fadvise64_64()
  26 .\"
  27 .TH POSIX_FADVISE 2 2010-10-09 "Linux" "Linux Programmer's Manual"
  28 .SH NAME
  29 posix_fadvise \- predeclare an access pattern for file data
  30 .SH SYNOPSIS
  31 .nf
  32 .B #include <fcntl.h>
  33 .sp
  34 .BI "int posix_fadvise(int " fd ", off_t " offset ", off_t " len \
  35 ", int " advice ");"
  36 .fi
  37 .sp
  38 .ad l
  39 .in -4n
  40 Feature Test Macro Requirements for glibc (see
  41 .BR feature_test_macros (7)):
  42 .in
  43 .sp
  44 .BR posix_fadvise ():
  45 .RS 4
  46 _XOPEN_SOURCE\ >=\ 600 || _POSIX_C_SOURCE\ >=\ 200112L
  47 .RE
  48 .ad
  49 .SH DESCRIPTION
  50 Programs can use
  51 .BR posix_fadvise ()
  52 to announce an intention to access
  53 file data in a specific pattern in the future, thus allowing the kernel
  54 to perform appropriate optimizations.
  55
  56 The \fIadvice\fP applies to a (not necessarily existent) region starting
  57 at \fIoffset\fP and extending for \fIlen\fP bytes (or until the end of
  58 the file if \fIlen\fP is 0) within the file referred to by \fIfd\fP.
  59 The advice is not binding; it merely constitutes an expectation on behalf of
  60 the application.
  61
  62 Permissible values for \fIadvice\fP include:
  63 .TP
  64 .B POSIX_FADV_NORMAL
  65 Indicates that the application has no advice to give about its access
  66 pattern for the specified data.
  67 If no advice is given for an open file,
  68 this is the default assumption.
  69 .TP
  70 .B POSIX_FADV_SEQUENTIAL
  71 The application expects to access the specified data sequentially (with
  72 lower offsets read before higher ones).
  73 .TP
  74 .B POSIX_FADV_RANDOM
  75 The specified data will be accessed in random order.
  76 .TP
  77 .B POSIX_FADV_NOREUSE
  78 The specified data will be accessed only once.
  79 .TP
  80 .B POSIX_FADV_WILLNEED
  81 The specified data will be accessed in the near future.
  82 .TP
  83 .B POSIX_FADV_DONTNEED
  84 The specified data will not be accessed in the near future.
  85 .SH "RETURN VALUE"
  86 On success, zero is returned.
  87 On error, an error number is returned.
  88 .SH ERRORS
  89 .TP
  90 .B EBADF
  91 The \fIfd\fP argument was not a valid file descriptor.
  92 .TP
  93 .B EINVAL
  94 An invalid value was specified for \fIadvice\fP.
  95 .TP
  96 .B ESPIPE
  97 The specified file descriptor refers to a pipe or FIFO.
  98 (Linux actually
  99 returns
 100 .B EINVAL
 101 in this case.)
 102 .SH VERSIONS
 103 Kernel support first appeared in Linux 2.5.60;
 104 the underlying system call is called
 105 .BR fadvise64 ().
 106 .\" of fadvise64_64()
 107 Library support has been provided since glibc version 2.2,
 108 via the wrapper function
 109 .BR posix_fadvise ().
 110 .SH "CONFORMING TO"
 111 POSIX.1-2001.
 112 Note that the type of the
 113 .I len
 114 argument was changed from
 115 .I size_t
 116 to
 117 .I off_t
 118 in POSIX.1-2003 TC1.
 119 .SH NOTES
 120 Under Linux, \fBPOSIX_FADV_NORMAL\fP sets the readahead window to the
 121 default size for the backing device; \fBPOSIX_FADV_SEQUENTIAL\fP doubles
 122 this size, and \fBPOSIX_FADV_RANDOM\fP disables file readahead entirely.
 123 These changes affect the entire file, not just the specified region
 124 (but other open file handles to the same file are unaffected).
 125
 126 \fBPOSIX_FADV_WILLNEED\fP initiates a
 127 nonblocking read of the specified region into the page cache.
 128 The amount of data read may be decreased by the kernel depending
 129 on virtual memory load.
 130 (A few megabytes will usually be fully satisfied,
 131 and more is rarely useful.)
 132
 133 In kernels before 2.6.18, \fBPOSIX_FADV_NOREUSE\fP had the
 134 same semantics as \fBPOSIX_FADV_WILLNEED\fP.
 135 This was probably a bug; since kernel 2.6.18, this flag is a no-op.
 136
 137 \fBPOSIX_FADV_DONTNEED\fP attempts to free cached pages associated with
 138 the specified region.
 139 This is useful, for example, while streaming large
 140 files.
 141 A program may periodically request the kernel to free cached data
 142 that has already been used, so that more useful cached pages are not
 143 discarded instead.
 144
 145 Pages that have not yet been written out will be unaffected, so if the
 146 application wishes to guarantee that pages will be released, it should
 147 call
 148 .BR fsync (2)
 149 or
 150 .BR fdatasync (2)
 151 first.
 152 .SS arm_fadvise()
 153 The ARM architecture
 154 needs 64-bit arguments to be aligned in a suitable pair of registers.
 155 On this architecture, the call signature of
 156 .BR posix_fadvise ()
 157 is flawed, since it forces a register to be wasted as padding between the
 158 .I fd
 159 and
 160 .I len
 161 arguments.
 162 Therefore, since Linux 2.6.14, ARM defines a different
 163 system call that orders the arguments suitably:
 164 .PP
 165 .in +4n
 166 .nf
 167 .BI "long arm_fadvise64_64(int " fd ", int " advice ,
 168 .BI "                     loff_t " offset ", loff_t " len );
 169 .fi
 170 .in
 171 .PP
 172 The behavior of this system call is otherwise exactly the same as
 173 .BR posix_fadvise().
 174 No library support is provided for this system call in glibc.
 175 .\" No ARM support in glibc.
 176 .SH BUGS
 177 In kernels before 2.6.6, if
 178 .I len
 179 was specified as 0, then this was interpreted literally as "zero bytes",
 180 rather than as meaning "all bytes through to the end of the file".
 181 .SH "SEE ALSO"
 182 .BR readahead (2),
 183 .BR sync_file_range (2),
 184 .BR posix_fallocate (3),
 185 .BR posix_madvise (3)
 186 .\" FIXME . Write a posix_fadvise(3) page.