man2/posix_fadvise.2

   1 .\" Copyright 2003 Abhijit Menon-Sen <ams@wiw.org>
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  25 .\" 2005-04-08 mtk, noted kernel version and added BUGS
  26 .\" 2010-10-09, mtk, document arm_fadvise64_64()
  27 .\"
  28 .TH POSIX_FADVISE 2 2016-03-15 "Linux" "Linux Programmer's Manual"
  29 .SH NAME
  30 posix_fadvise \- predeclare an access pattern for file data
  31 .SH SYNOPSIS
  32 .nf
  33 .B #include <fcntl.h>
  34 .sp
  35 .BI "int posix_fadvise(int " fd ", off_t " offset ", off_t " len \
  36 ", int " advice ");"
  37 .fi
  38 .sp
  39 .ad l
  40 .in -4n
  41 Feature Test Macro Requirements for glibc (see
  42 .BR feature_test_macros (7)):
  43 .in
  44 .sp
  45 .BR posix_fadvise ():
  46 .RS 4
  47 _POSIX_C_SOURCE\ >=\ 200112L
  48 .RE
  49 .ad
  50 .SH DESCRIPTION
  51 Programs can use
  52 .BR posix_fadvise ()
  53 to announce an intention to access
  54 file data in a specific pattern in the future, thus allowing the kernel
  55 to perform appropriate optimizations.
  56
  57 The \fIadvice\fP applies to a (not necessarily existent) region starting
  58 at \fIoffset\fP and extending for \fIlen\fP bytes (or until the end of
  59 the file if \fIlen\fP is 0) within the file referred to by \fIfd\fP.
  60 The \fIadvice\fP is not binding;
  61 it merely constitutes an expectation on behalf of
  62 the application.
  63
  64 Permissible values for \fIadvice\fP include:
  65 .TP
  66 .B POSIX_FADV_NORMAL
  67 Indicates that the application has no advice to give about its access
  68 pattern for the specified data.
  69 If no advice is given for an open file,
  70 this is the default assumption.
  71 .TP
  72 .B POSIX_FADV_SEQUENTIAL
  73 The application expects to access the specified data sequentially (with
  74 lower offsets read before higher ones).
  75 .TP
  76 .B POSIX_FADV_RANDOM
  77 The specified data will be accessed in random order.
  78 .TP
  79 .B POSIX_FADV_NOREUSE
  80 The specified data will be accessed only once.
  81
  82 In kernels before 2.6.18, \fBPOSIX_FADV_NOREUSE\fP had the
  83 same semantics as \fBPOSIX_FADV_WILLNEED\fP.
  84 This was probably a bug; since kernel 2.6.18, this flag is a no-op.
  85 .TP
  86 .B POSIX_FADV_WILLNEED
  87 The specified data will be accessed in the near future.
  88
  89 \fBPOSIX_FADV_WILLNEED\fP initiates a
  90 nonblocking read of the specified region into the page cache.
  91 The amount of data read may be decreased by the kernel depending
  92 on virtual memory load.
  93 (A few megabytes will usually be fully satisfied,
  94 and more is rarely useful.)
  95 .TP
  96 .B POSIX_FADV_DONTNEED
  97 The specified data will not be accessed in the near future.
  98
  99 \fBPOSIX_FADV_DONTNEED\fP attempts to free cached pages associated with
 100 the specified region.
 101 This is useful, for example, while streaming large
 102 files.
 103 A program may periodically request the kernel to free cached data
 104 that has already been used, so that more useful cached pages are not
 105 discarded instead.
 106
 107 Requests to discard partial pages are ignored.
 108 It is preferable to preserve needed data than discard unneeded data.
 109 If the application requires that data be considered for discarding, then
 110 .I offset
 111 and
 112 .I len
 113 must be page-aligned.
 114
 115 The implementation
 116 .I may
 117 attempt to write back dirty pages in the specified region,
 118 but this is not guaranteed.
 119 Any unwritten dirty pages will not be freed.
 120 If the application wishes to ensure that dirty pages will be released,
 121 it should call
 122 .BR fsync (2)
 123 or
 124 .BR fdatasync (2)
 125 first.
 126 .SH RETURN VALUE
 127 On success, zero is returned.
 128 On error, an error number is returned.
 129 .SH ERRORS
 130 .TP
 131 .B EBADF
 132 The \fIfd\fP argument was not a valid file descriptor.
 133 .TP
 134 .B EINVAL
 135 An invalid value was specified for \fIadvice\fP.
 136 .TP
 137 .B ESPIPE
 138 The specified file descriptor refers to a pipe or FIFO.
 139 .RB ( ESPIPE
 140 is the error specified by POSIX,
 141 but before kernel version 2.6.16,
 142 .\" commit 87ba81dba431232548ce29d5d224115d0c2355ac
 143 Linux returned
 144 .B EINVAL
 145 in this case.)
 146 .SH VERSIONS
 147 Kernel support first appeared in Linux 2.5.60;
 148 the underlying system call is called
 149 .BR fadvise64 ().
 150 .\" of fadvise64_64()
 151 Library support has been provided since glibc version 2.2,
 152 via the wrapper function
 153 .BR posix_fadvise ().
 154
 155 Since Linux 3.18,
 156 .\" commit d3ac21cacc24790eb45d735769f35753f5b56ceb
 157 support for the underlying system call is optional,
 158 depending on the setting of the
 159 .B CONFIG_ADVISE_SYSCALLS
 160 configuration option.
 161 .SH CONFORMING TO
 162 POSIX.1-2001, POSIX.1-2008.
 163 Note that the type of the
 164 .I len
 165 argument was changed from
 166 .I size_t
 167 to
 168 .I off_t
 169 in POSIX.1-2003 TC1.
 170 .SH NOTES
 171 Under Linux, \fBPOSIX_FADV_NORMAL\fP sets the readahead window to the
 172 default size for the backing device; \fBPOSIX_FADV_SEQUENTIAL\fP doubles
 173 this size, and \fBPOSIX_FADV_RANDOM\fP disables file readahead entirely.
 174 These changes affect the entire file, not just the specified region
 175 (but other open file handles to the same file are unaffected).
 176
 177 The contents of the kernel buffer cache can be cleared via the
 178 .IR /proc/sys/vm/drop_caches
 179 interface described in
 180 .BR proc (5).
 181 .SS C library/kernel differences
 182 The name of the wrapper function in the C library is
 183 .BR posix_fadvise ().
 184 The underlying system call is called
 185 .BR fadvise64 ()
 186 (or, on some architectures,
 187 .BR fadvise64_64 ()).
 188 .SS Architecture-specific variants
 189 Some architectures require
 190 64-bit arguments to be aligned in a suitable pair of registers (see
 191 .BR syscall (2)
 192 for further detail).
 193 On such architectures, the call signature of
 194 .BR posix_fadvise ()
 195 shown in the SYNOPSIS would force
 196 a register to be wasted as padding between the
 197 .I fd
 198 and
 199 .I offset
 200 arguments.
 201 Therefore, these architectures define a version of the
 202 system call that orders the arguments suitably,
 203 but is otherwise exactly the same as
 204 .BR posix_fadvise ().
 205
 206 For example, since Linux 2.6.14, ARM has the following system call:
 207 .PP
 208 .in +4n
 209 .nf
 210 .BI "long arm_fadvise64_64(int " fd ", int " advice ,
 211 .BI "                      loff_t " offset ", loff_t " len );
 212 .fi
 213 .in
 214 .PP
 215 These architecture-specific details are generally
 216 hidden from applications by the glibc
 217 .BR posix_fadvise ()
 218 wrapper function,
 219 which invokes the appropriate architecture-specific system call.
 220 .SH BUGS
 221 In kernels before 2.6.6, if
 222 .I len
 223 was specified as 0, then this was interpreted literally as "zero bytes",
 224 rather than as meaning "all bytes through to the end of the file".
 225 .SH SEE ALSO
 226 .BR mincore (2),
 227 .BR readahead (2),
 228 .BR sync_file_range (2),
 229 .BR posix_fallocate (3),
 230 .BR posix_madvise (3)