posix_fadvise.2: Reorganize some text

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.\" Copyright 2003 Abhijit Menon-Sen <ams@wiw.org>
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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 2016-03-15 "Linux" "Linux Programmer's Manual"
.SH NAME
posix_fadvise \- predeclare an access pattern for file data
.SH SYNOPSIS
.nf
.B #include <fcntl.h>
.sp
.BI "int posix_fadvise(int " fd ", off_t " offset ", off_t " len \
", int " advice ");"
.fi
.sp
.ad l
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.BR posix_fadvise ():
.RS 4
_POSIX_C_SOURCE\ >=\ 200112L
.RE
.ad
.SH DESCRIPTION
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 optimizations.

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 \fIadvice\fP is not binding;
it merely constitutes an expectation on behalf of
the application.

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,
this is the default assumption.
.TP
.B POSIX_FADV_SEQUENTIAL
The application expects to access the specified data sequentially (with
lower offsets read before higher ones).
.TP
.B POSIX_FADV_RANDOM
The specified data will be accessed in random order.
.TP
.B POSIX_FADV_NOREUSE
The specified data will be accessed only once.

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.

\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.

\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.

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.

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
.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
The \fIfd\fP argument was not a valid file descriptor.
.TP
.B EINVAL
An invalid value was specified for \fIadvice\fP.
.TP
.B ESPIPE
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 ().

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
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).
.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 ()).
.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 ().

For example, since Linux 2.6.14, ARM has the following system call:
.PP
.in +4n
.nf
.BI "long arm_fadvise64_64(int " fd ", int " advice ,
.BI "                      loff_t " offset ", loff_t " len );
.fi
.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
.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 readahead (2),