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.\" 2005-04-08 mtk, noted kernel version and added BUGS
+.\" 2010-10-09, mtk, document arm_fadvise64_64()
.\"
-.TH POSIX_FADVISE 2 "14 Feb 2003" "Linux 2.5.60" "Linux Programmer's Manual"
+.TH POSIX_FADVISE 2 2019-03-06 "Linux" "Linux Programmer's Manual"
.SH NAME
posix_fadvise \- predeclare an access pattern for file data
.SH SYNOPSIS
.nf
-.B #define _XOPEN_SOURCE 600
.B #include <fcntl.h>
-.sp
-.BI "int posix_fadvise(int " fd ", off_t " offset ", off_t " len ", int " advice ");"
+.PP
+.BI "int posix_fadvise(int " fd ", off_t " offset ", off_t " len \
+", int " advice ");"
.fi
+.PP
+.ad l
+.in -4n
+Feature Test Macro Requirements for glibc (see
+.BR feature_test_macros (7)):
+.in
+.PP
+.BR posix_fadvise ():
+.RS 4
+_POSIX_C_SOURCE\ >=\ 200112L
+.RE
+.ad
.SH DESCRIPTION
-Programs can use \fBposix_fadvise\fP() to announce an intention to access
+Programs can use
+.BR posix_fadvise ()
+to announce an intention to access
file data in a specific pattern in the future, thus allowing the kernel
-to perform appropriate optimisations.
-
+to perform appropriate optimizations.
+.PP
The \fIadvice\fP applies to a (not necessarily existent) region starting
at \fIoffset\fP and extending for \fIlen\fP bytes (or until the end of
-the file if \fIlen\fP is 0) within the file referred to by \fIfd\fP. The
-advice is not binding; it merely constitutes an expectation on behalf of
+the file if \fIlen\fP is 0) within the file referred to by \fIfd\fP.
+The \fIadvice\fP is not binding;
+it merely constitutes an expectation on behalf of
the application.
-
+.PP
Permissible values for \fIadvice\fP include:
.TP
.B POSIX_FADV_NORMAL
Indicates that the application has no advice to give about its access
-pattern for the specified data. If no advice is given for an open file,
+pattern for the specified data.
+If no advice is given for an open file,
this is the default assumption.
.TP
.B POSIX_FADV_SEQUENTIAL
.TP
.B POSIX_FADV_NOREUSE
The specified data will be accessed only once.
+.IP
+In kernels before 2.6.18, \fBPOSIX_FADV_NOREUSE\fP had the
+same semantics as \fBPOSIX_FADV_WILLNEED\fP.
+This was probably a bug; since kernel 2.6.18, this flag is a no-op.
.TP
.B POSIX_FADV_WILLNEED
The specified data will be accessed in the near future.
+.IP
+\fBPOSIX_FADV_WILLNEED\fP initiates a
+nonblocking read of the specified region into the page cache.
+The amount of data read may be decreased by the kernel depending
+on virtual memory load.
+(A few megabytes will usually be fully satisfied,
+and more is rarely useful.)
.TP
.B POSIX_FADV_DONTNEED
The specified data will not be accessed in the near future.
-.SH "RETURN VALUE"
-On success, zero is returned. On error, \-1 is returned, and \fIerrno\fP
-is set appropriately.
+.IP
+\fBPOSIX_FADV_DONTNEED\fP attempts to free cached pages associated with
+the specified region.
+This is useful, for example, while streaming large
+files.
+A program may periodically request the kernel to free cached data
+that has already been used, so that more useful cached pages are not
+discarded instead.
+.IP
+Requests to discard partial pages are ignored.
+It is preferable to preserve needed data than discard unneeded data.
+If the application requires that data be considered for discarding, then
+.I offset
+and
+.I len
+must be page-aligned.
+.IP
+The implementation
+.I may
+attempt to write back dirty pages in the specified region,
+but this is not guaranteed.
+Any unwritten dirty pages will not be freed.
+If the application wishes to ensure that dirty pages will be released,
+it should call
+.BR fsync (2)
+or
+.BR fdatasync (2)
+first.
+.SH RETURN VALUE
+On success, zero is returned.
+On error, an error number is returned.
.SH ERRORS
.TP
.B EBADF
An invalid value was specified for \fIadvice\fP.
.TP
.B ESPIPE
-The specified file descriptor refers to a pipe or FIFO. (Linux actually
-returns EINVAL in this case.)
+The specified file descriptor refers to a pipe or FIFO.
+.RB ( ESPIPE
+is the error specified by POSIX,
+but before kernel version 2.6.16,
+.\" commit 87ba81dba431232548ce29d5d224115d0c2355ac
+Linux returned
+.B EINVAL
+in this case.)
+.SH VERSIONS
+Kernel support first appeared in Linux 2.5.60;
+the underlying system call is called
+.BR fadvise64 ().
+.\" of fadvise64_64()
+Library support has been provided since glibc version 2.2,
+via the wrapper function
+.BR posix_fadvise ().
+.PP
+Since Linux 3.18,
+.\" commit d3ac21cacc24790eb45d735769f35753f5b56ceb
+support for the underlying system call is optional,
+depending on the setting of the
+.B CONFIG_ADVISE_SYSCALLS
+configuration option.
+.SH CONFORMING TO
+POSIX.1-2001, POSIX.1-2008.
+Note that the type of the
+.I len
+argument was changed from
+.I size_t
+to
+.I off_t
+in POSIX.1-2003 TC1.
.SH NOTES
-.BR posix_fadvise ()
-appeared in kernel 2.5.60.
-.\" Actually as fadvise64() -- MTK
-
Under Linux, \fBPOSIX_FADV_NORMAL\fP sets the readahead window to the
default size for the backing device; \fBPOSIX_FADV_SEQUENTIAL\fP doubles
this size, and \fBPOSIX_FADV_RANDOM\fP disables file readahead entirely.
These changes affect the entire file, not just the specified region
(but other open file handles to the same file are unaffected).
-
-\fBPOSIX_FADV_WILLNEED\fP and \fBPOSIX_FADV_NOREUSE\fP both initiate a
-non-blocking read of the specified region into the page cache. The amount
-of data read may be decreased by the kernel depending on VM load. (A few
-megabytes will usually be fully satisfied, and more is rarely useful.)
-
-\fBPOSIX_FADV_DONTNEED\fP attempts to free cached pages associated with
-the specified region. This is useful, for example, while streaming large
-files. A program may periodically request the kernel to free cached data
-that has already been used, so that more useful cached pages are not
-discarded instead.
-
-Pages that have not yet been written out will be unaffected, so if the
-application wishes to guarantee that pages will be released, it should
-call \fBfsync\fP() or \fBfdatasync\fP() first.
-.SH "CONFORMING TO"
-SUSv3 (Advanced Realtime Option), POSIX 1003.1-2003.
-Note that the type of the
-.I len
-parameter was changed from size_t to off_t in POSIX 1003.1-2003 TC5.
+.PP
+The contents of the kernel buffer cache can be cleared via the
+.IR /proc/sys/vm/drop_caches
+interface described in
+.BR proc (5).
+.PP
+One can obtain a snapshot of which pages of a file are resident
+in the buffer cache by opening a file, mapping it with
+.BR mmap (2),
+and then applying
+.BR mincore (2)
+to the mapping.
+.SS C library/kernel differences
+The name of the wrapper function in the C library is
+.BR posix_fadvise ().
+The underlying system call is called
+.BR fadvise64 ()
+(or, on some architectures,
+.BR fadvise64_64 ());
+the difference between the two is that the former system call
+assumes that the type of the \fIlen\fP argument is \fIsize_t\fP,
+while the latter expects \fIloff_t\fP there.
+.SS Architecture-specific variants
+Some architectures require
+64-bit arguments to be aligned in a suitable pair of registers (see
+.BR syscall (2)
+for further detail).
+On such architectures, the call signature of
+.BR posix_fadvise ()
+shown in the SYNOPSIS would force
+a register to be wasted as padding between the
+.I fd
+and
+.I offset
+arguments.
+Therefore, these architectures define a version of the
+system call that orders the arguments suitably,
+but is otherwise exactly the same as
+.BR posix_fadvise ().
+.PP
+For example, since Linux 2.6.14, ARM has the following system call:
+.PP
+.in +4n
+.EX
+.BI "long arm_fadvise64_64(int " fd ", int " advice ,
+.BI " loff_t " offset ", loff_t " len );
+.EE
+.in
+.PP
+These architecture-specific details are generally
+hidden from applications by the glibc
+.BR posix_fadvise ()
+wrapper function,
+which invokes the appropriate architecture-specific system call.
.SH BUGS
-In kernels before 2.6.6, if
+In kernels before 2.6.6, if
.I len
was specified as 0, then this was interpreted literally as "zero bytes",
rather than as meaning "all bytes through to the end of the file".
-.SH "SEE ALSO"
-.BR posix_madvise (2),
-.BR posix_fallocate (3)
+.SH SEE ALSO
+.BR fincore (1),
+.BR mincore (2),
+.BR readahead (2),
+.BR sync_file_range (2),
+.BR posix_fallocate (3),
+.BR posix_madvise (3)
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