man2/posix_fadvise.2

   1 .\" Copyright 2003 Abhijit Menon-Sen <ams@wiw.org>
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  20 .\" the source, must acknowledge the copyright and authors of this work.
  21 .\"
  22 .\" 2005-04-08 mtk, noted kernel version and added BUGS
  23 .\" 2010-10-09, mtk, document arm_fadvise64_64()
  24 .\"
  25 .TH POSIX_FADVISE 2 2010-10-09 "Linux" "Linux Programmer's Manual"
  26 .SH NAME
  27 posix_fadvise \- predeclare an access pattern for file data
  28 .SH SYNOPSIS
  29 .nf
  30 .B #include <fcntl.h>
  31 .sp
  32 .BI "int posix_fadvise(int " fd ", off_t " offset ", off_t " len \
  33 ", int " advice ");"
  34 .fi
  35 .sp
  36 .ad l
  37 .in -4n
  38 Feature Test Macro Requirements for glibc (see
  39 .BR feature_test_macros (7)):
  40 .in
  41 .sp
  42 .BR posix_fadvise ():
  43 .RS 4
  44 _XOPEN_SOURCE\ >=\ 600 || _POSIX_C_SOURCE\ >=\ 200112L
  45 .RE
  46 .ad
  47 .SH DESCRIPTION
  48 Programs can use
  49 .BR posix_fadvise ()
  50 to announce an intention to access
  51 file data in a specific pattern in the future, thus allowing the kernel
  52 to perform appropriate optimizations.
  53
  54 The \fIadvice\fP applies to a (not necessarily existent) region starting
  55 at \fIoffset\fP and extending for \fIlen\fP bytes (or until the end of
  56 the file if \fIlen\fP is 0) within the file referred to by \fIfd\fP.
  57 The \fIadvice\fP is not binding;
  58 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 Kernel support first appeared in Linux 2.5.60;
 103 the underlying system call is called
 104 .BR fadvise64 ().
 105 .\" of fadvise64_64()
 106 Library support has been provided since glibc version 2.2,
 107 via the wrapper function
 108 .BR posix_fadvise ().
 109 .SH CONFORMING TO
 110 POSIX.1-2001.
 111 Note that the type of the
 112 .I len
 113 argument was changed from
 114 .I size_t
 115 to
 116 .I off_t
 117 in POSIX.1-2003 TC1.
 118 .SH NOTES
 119 Under Linux, \fBPOSIX_FADV_NORMAL\fP sets the readahead window to the
 120 default size for the backing device; \fBPOSIX_FADV_SEQUENTIAL\fP doubles
 121 this size, and \fBPOSIX_FADV_RANDOM\fP disables file readahead entirely.
 122 These changes affect the entire file, not just the specified region
 123 (but other open file handles to the same file are unaffected).
 124
 125 \fBPOSIX_FADV_WILLNEED\fP initiates a
 126 nonblocking read of the specified region into the page cache.
 127 The amount of data read may be decreased by the kernel depending
 128 on virtual memory load.
 129 (A few megabytes will usually be fully satisfied,
 130 and more is rarely useful.)
 131
 132 In kernels before 2.6.18, \fBPOSIX_FADV_NOREUSE\fP had the
 133 same semantics as \fBPOSIX_FADV_WILLNEED\fP.
 134 This was probably a bug; since kernel 2.6.18, this flag is a no-op.
 135
 136 \fBPOSIX_FADV_DONTNEED\fP attempts to free cached pages associated with
 137 the specified region.
 138 This is useful, for example, while streaming large
 139 files.
 140 A program may periodically request the kernel to free cached data
 141 that has already been used, so that more useful cached pages are not
 142 discarded instead.
 143
 144 Pages that have not yet been written out will be unaffected, so if the
 145 application wishes to guarantee that pages will be released, it should
 146 call
 147 .BR fsync (2)
 148 or
 149 .BR fdatasync (2)
 150 first.
 151 .SS arm_fadvise()
 152 The ARM architecture
 153 needs 64-bit arguments to be aligned in a suitable pair of registers.
 154 On this architecture, the call signature of
 155 .BR posix_fadvise ()
 156 is flawed, since it forces a register to be wasted as padding between the
 157 .I fd
 158 and
 159 .I len
 160 arguments.
 161 Therefore, since Linux 2.6.14, ARM defines a different
 162 system call that orders the arguments suitably:
 163 .PP
 164 .in +4n
 165 .nf
 166 .BI "long arm_fadvise64_64(int " fd ", int " advice ,
 167 .BI "                      loff_t " offset ", loff_t " len );
 168 .fi
 169 .in
 170 .PP
 171 The behavior of this system call is otherwise exactly the same as
 172 .BR posix_fadvise ().
 173 No library support is provided for this system call in glibc.
 174 .\" No ARM support in glibc.
 175 .SH BUGS
 176 In kernels before 2.6.6, if
 177 .I len
 178 was specified as 0, then this was interpreted literally as "zero bytes",
 179 rather than as meaning "all bytes through to the end of the file".
 180 .SH SEE ALSO
 181 .BR readahead (2),
 182 .BR sync_file_range (2),
 183 .BR posix_fallocate (3),
 184 .BR posix_madvise (3)
 185 .\" FIXME . Write a posix_fadvise(3) page.