POSIX_FADVISE
Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
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NAME
posix_fadvise - predeclare an access pattern for file data
SYNOPSIS
#include <fcntl.h>
int posix_fadvise(int fd, off_t offset, off_t len, int advice);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
posix_fadvise():
-
_POSIX_C_SOURCE >= 200112L
DESCRIPTION
Programs can use
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 advice applies to a (not necessarily existent) region starting
at offset and extending for len bytes (or until the end of
the file if len is 0) within the file referred to by fd.
The advice is not binding;
it merely constitutes an expectation on behalf of
the application.
Permissible values for advice include:
- 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.
- POSIX_FADV_SEQUENTIAL
-
The application expects to access the specified data sequentially (with
lower offsets read before higher ones).
- POSIX_FADV_RANDOM
-
The specified data will be accessed in random order.
- POSIX_FADV_NOREUSE
-
The specified data will be accessed only once.
-
In kernels before 2.6.18, POSIX_FADV_NOREUSE had the
same semantics as POSIX_FADV_WILLNEED.
This was probably a bug; since kernel 2.6.18, this flag is a no-op.
- POSIX_FADV_WILLNEED
-
The specified data will be accessed in the near future.
-
POSIX_FADV_WILLNEED 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.)
- POSIX_FADV_DONTNEED
-
The specified data will not be accessed in the near future.
-
POSIX_FADV_DONTNEED 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
offset
and
len
must be page-aligned.
-
The implementation
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
fsync(2)
or
fdatasync(2)
first.
RETURN VALUE
On success, zero is returned.
On error, an error number is returned.
ERRORS
- EBADF
-
The fd argument was not a valid file descriptor.
- EINVAL
-
An invalid value was specified for advice.
- ESPIPE
-
The specified file descriptor refers to a pipe or FIFO.
(ESPIPE
is the error specified by POSIX,
but before kernel version 2.6.16,
Linux returned
EINVAL
in this case.)
VERSIONS
Kernel support first appeared in Linux 2.5.60;
the underlying system call is called
fadvise64().
Library support has been provided since glibc version 2.2,
via the wrapper function
posix_fadvise().
Since Linux 3.18,
support for the underlying system call is optional,
depending on the setting of the
CONFIG_ADVISE_SYSCALLS
configuration option.
CONFORMING TO
POSIX.1-2001, POSIX.1-2008.
Note that the type of the
len
argument was changed from
size_t
to
off_t
in POSIX.1-2003 TC1.
NOTES
Under Linux,
POSIX_FADV_NORMAL sets the readahead window to the
default size for the backing device;
POSIX_FADV_SEQUENTIAL doubles
this size, and
POSIX_FADV_RANDOM 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).
The contents of the kernel buffer cache can be cleared via the
/proc/sys/vm/drop_caches
interface described in
proc(5).
One can obtain a snapshot of which pages of a file are resident
in the buffer cache by opening a file, mapping it with
mmap(2),
and then applying
mincore(2)
to the mapping.
C library/kernel differences
The name of the wrapper function in the C library is
posix_fadvise().
The underlying system call is called
fadvise64()
(or, on some architectures,
fadvise64_64()).
Architecture-specific variants
Some architectures require
64-bit arguments to be aligned in a suitable pair of registers (see
syscall(2)
for further detail).
On such architectures, the call signature of
posix_fadvise()
shown in the SYNOPSIS would force
a register to be wasted as padding between the
fd
and
offset
arguments.
Therefore, these architectures define a version of the
system call that orders the arguments suitably,
but is otherwise exactly the same as
posix_fadvise().
For example, since Linux 2.6.14, ARM has the following system call:
long arm_fadvise64_64(int fd, int advice,
loff_t offset, loff_t len);
These architecture-specific details are generally
hidden from applications by the glibc
posix_fadvise()
wrapper function,
which invokes the appropriate architecture-specific system call.
BUGS
In kernels before 2.6.6, if
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".
SEE ALSO
fincore(1),
mincore(2),
readahead(2),
sync_file_range(2),
posix_fallocate(3),
posix_madvise(3)
COLOPHON
This page is part of release 4.13 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- CONFORMING TO
-
- NOTES
-
- C library/kernel differences
-
- Architecture-specific variants
-
- BUGS
-
- SEE ALSO
-
- COLOPHON
-
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