man2/posix_fadvise.2

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  26 .\" 2005-04-08 mtk, noted kernel version and added BUGS
  27 .\" 2010-10-09, mtk, document arm_fadvise64_64()
  28 .\"
  29 .TH POSIX_FADVISE 2 2019-03-06 "Linux" "Linux Programmer's Manual"
  30 .SH NAME
  31 posix_fadvise \- predeclare an access pattern for file data
  32 .SH SYNOPSIS
  33 .nf
  34 .B #include <fcntl.h>
  35 .PP
  36 .BI "int posix_fadvise(int " fd ", off_t " offset ", off_t " len \
  37 ", int " advice ");"
  38 .fi
  39 .PP
  40 .ad l
  41 .in -4n
  42 Feature Test Macro Requirements for glibc (see
  43 .BR feature_test_macros (7)):
  44 .in
  45 .PP
  46 .BR posix_fadvise ():
  47 .RS 4
  48 _POSIX_C_SOURCE\ >=\ 200112L
  49 .RE
  50 .ad
  51 .SH DESCRIPTION
  52 Programs can use
  53 .BR posix_fadvise ()
  54 to announce an intention to access
  55 file data in a specific pattern in the future, thus allowing the kernel
  56 to perform appropriate optimizations.
  57 .PP
  58 The \fIadvice\fP applies to a (not necessarily existent) region starting
  59 at \fIoffset\fP and extending for \fIlen\fP bytes (or until the end of
  60 the file if \fIlen\fP is 0) within the file referred to by \fIfd\fP.
  61 The \fIadvice\fP is not binding;
  62 it merely constitutes an expectation on behalf of
  63 the application.
  64 .PP
  65 Permissible values for \fIadvice\fP include:
  66 .TP
  67 .B POSIX_FADV_NORMAL
  68 Indicates that the application has no advice to give about its access
  69 pattern for the specified data.
  70 If no advice is given for an open file,
  71 this is the default assumption.
  72 .TP
  73 .B POSIX_FADV_SEQUENTIAL
  74 The application expects to access the specified data sequentially (with
  75 lower offsets read before higher ones).
  76 .TP
  77 .B POSIX_FADV_RANDOM
  78 The specified data will be accessed in random order.
  79 .TP
  80 .B POSIX_FADV_NOREUSE
  81 The specified data will be accessed only once.
  82 .IP
  83 In kernels before 2.6.18, \fBPOSIX_FADV_NOREUSE\fP had the
  84 same semantics as \fBPOSIX_FADV_WILLNEED\fP.
  85 This was probably a bug; since kernel 2.6.18, this flag is a no-op.
  86 .TP
  87 .B POSIX_FADV_WILLNEED
  88 The specified data will be accessed in the near future.
  89 .IP
  90 \fBPOSIX_FADV_WILLNEED\fP initiates a
  91 nonblocking read of the specified region into the page cache.
  92 The amount of data read may be decreased by the kernel depending
  93 on virtual memory load.
  94 (A few megabytes will usually be fully satisfied,
  95 and more is rarely useful.)
  96 .TP
  97 .B POSIX_FADV_DONTNEED
  98 The specified data will not be accessed in the near future.
  99 .IP
 100 \fBPOSIX_FADV_DONTNEED\fP attempts to free cached pages associated with
 101 the specified region.
 102 This is useful, for example, while streaming large
 103 files.
 104 A program may periodically request the kernel to free cached data
 105 that has already been used, so that more useful cached pages are not
 106 discarded instead.
 107 .IP
 108 Requests to discard partial pages are ignored.
 109 It is preferable to preserve needed data than discard unneeded data.
 110 If the application requires that data be considered for discarding, then
 111 .I offset
 112 and
 113 .I len
 114 must be page-aligned.
 115 .IP
 116 The implementation
 117 .I may
 118 attempt to write back dirty pages in the specified region,
 119 but this is not guaranteed.
 120 Any unwritten dirty pages will not be freed.
 121 If the application wishes to ensure that dirty pages will be released,
 122 it should call
 123 .BR fsync (2)
 124 or
 125 .BR fdatasync (2)
 126 first.
 127 .SH RETURN VALUE
 128 On success, zero is returned.
 129 On error, an error number is returned.
 130 .SH ERRORS
 131 .TP
 132 .B EBADF
 133 The \fIfd\fP argument was not a valid file descriptor.
 134 .TP
 135 .B EINVAL
 136 An invalid value was specified for \fIadvice\fP.
 137 .TP
 138 .B ESPIPE
 139 The specified file descriptor refers to a pipe or FIFO.
 140 .RB ( ESPIPE
 141 is the error specified by POSIX,
 142 but before kernel version 2.6.16,
 143 .\" commit 87ba81dba431232548ce29d5d224115d0c2355ac
 144 Linux returned
 145 .B EINVAL
 146 in this case.)
 147 .SH VERSIONS
 148 Kernel support first appeared in Linux 2.5.60;
 149 the underlying system call is called
 150 .BR fadvise64 ().
 151 .\" of fadvise64_64()
 152 Library support has been provided since glibc version 2.2,
 153 via the wrapper function
 154 .BR posix_fadvise ().
 155 .PP
 156 Since Linux 3.18,
 157 .\" commit d3ac21cacc24790eb45d735769f35753f5b56ceb
 158 support for the underlying system call is optional,
 159 depending on the setting of the
 160 .B CONFIG_ADVISE_SYSCALLS
 161 configuration option.
 162 .SH CONFORMING TO
 163 POSIX.1-2001, POSIX.1-2008.
 164 Note that the type of the
 165 .I len
 166 argument was changed from
 167 .I size_t
 168 to
 169 .I off_t
 170 in POSIX.1-2003 TC1.
 171 .SH NOTES
 172 Under Linux, \fBPOSIX_FADV_NORMAL\fP sets the readahead window to the
 173 default size for the backing device; \fBPOSIX_FADV_SEQUENTIAL\fP doubles
 174 this size, and \fBPOSIX_FADV_RANDOM\fP disables file readahead entirely.
 175 These changes affect the entire file, not just the specified region
 176 (but other open file handles to the same file are unaffected).
 177 .PP
 178 The contents of the kernel buffer cache can be cleared via the
 179 .IR /proc/sys/vm/drop_caches
 180 interface described in
 181 .BR proc (5).
 182 .PP
 183 One can obtain a snapshot of which pages of a file are resident
 184 in the buffer cache by opening a file, mapping it with
 185 .BR mmap (2),
 186 and then applying
 187 .BR mincore (2)
 188 to the mapping.
 189 .SS C library/kernel differences
 190 The name of the wrapper function in the C library is
 191 .BR posix_fadvise ().
 192 The underlying system call is called
 193 .BR fadvise64 ()
 194 (or, on some architectures,
 195 .BR fadvise64_64 ());
 196 the difference between the two is that the former system call
 197 assumes that the type of the \fIlen\fP argument is \fIsize_t\fP,
 198 while the latter expects \fIloff_t\fP there.
 199 .SS Architecture-specific variants
 200 Some architectures require
 201 64-bit arguments to be aligned in a suitable pair of registers (see
 202 .BR syscall (2)
 203 for further detail).
 204 On such architectures, the call signature of
 205 .BR posix_fadvise ()
 206 shown in the SYNOPSIS would force
 207 a register to be wasted as padding between the
 208 .I fd
 209 and
 210 .I offset
 211 arguments.
 212 Therefore, these architectures define a version of the
 213 system call that orders the arguments suitably,
 214 but is otherwise exactly the same as
 215 .BR posix_fadvise ().
 216 .PP
 217 For example, since Linux 2.6.14, ARM has the following system call:
 218 .PP
 219 .in +4n
 220 .EX
 221 .BI "long arm_fadvise64_64(int " fd ", int " advice ,
 222 .BI "                      loff_t " offset ", loff_t " len );
 223 .EE
 224 .in
 225 .PP
 226 These architecture-specific details are generally
 227 hidden from applications by the glibc
 228 .BR posix_fadvise ()
 229 wrapper function,
 230 which invokes the appropriate architecture-specific system call.
 231 .SH BUGS
 232 In kernels before 2.6.6, if
 233 .I len
 234 was specified as 0, then this was interpreted literally as "zero bytes",
 235 rather than as meaning "all bytes through to the end of the file".
 236 .SH SEE ALSO
 237 .BR fincore (1),
 238 .BR mincore (2),
 239 .BR readahead (2),
 240 .BR sync_file_range (2),
 241 .BR posix_fallocate (3),
 242 .BR posix_madvise (3)