posix_fadvise(2) - Linux manual page (original) (raw)
posixfadvise(2) System Calls Manual posixfadvise(2)
NAME top
posix_fadvise - predeclare an access pattern for file dataLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <fcntl.h>**
**int posix_fadvise(int** _fd_**, off_t** _offset_**, off_t** _size_**, int** _advice_**);**Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
**posix_fadvise**():
_POSIX_C_SOURCE >= 200112LDESCRIPTION top
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.
Before Linux 2.6.18, **POSIX_FADV_NOREUSE** had the same
semantics as **POSIX_FADV_WILLNEED**. This was probably a bug;
from Linux 2.6.18 until Linux 6.2 this flag was a no-op.
Since Linux 6.3, **POSIX_FADV_NOREUSE** signals that the kernel
page replacement algorithm can ignore access to mapped page
cache marked by this flag. This is useful, for example,
while streaming large files.
**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 _size_ 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)](../man2/fsync.2.html) or [fdatasync(2)](../man2/fdatasync.2.html) first.RETURN VALUE top
On success, zero is returned. On error, an error number is
returned.ERRORS top
**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 Linux
2.6.16, Linux returned **EINVAL** in this case.)VERSIONS top
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.
**POSIX_FADV_NOREUSE** does not modify the readahead window size.
These changes affect the entire file, not just the specified
region (but other open file handles to the same file are
unaffected).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()); the difference between the two is that the former system call assumes that the type of the len argument is sizet, while the latter expects lofft there.
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** _size_**);**
These architecture-specific details are generally hidden from
applications by the glibc **posix_fadvise**() wrapper function, which
invokes the appropriate architecture-specific system call.STANDARDS top
POSIX.1-2008.HISTORY top
POSIX.1-2001.
Kernel support first appeared in Linux 2.5.60; the underlying
system call is called **fadvise64**(). Library support has been
provided since glibc 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.
The type of the _size_ argument was changed from _sizet_ to _offt_ in
POSIX.1-2001 TC1.NOTES top
The contents of the kernel buffer cache can be cleared via the
_/proc/sys/vm/dropcaches_ interface described in [proc(5)](../man5/proc.5.html).
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)](../man2/mmap.2.html), and
then applying [mincore(2)](../man2/mincore.2.html) to the mapping.BUGS top
Before Linux 2.6.6, if _size_ 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 top
[fincore(1)](../man1/fincore.1.html), [mincore(2)](../man2/mincore.2.html), [readahead(2)](../man2/readahead.2.html), [sync_file_range(2)](../man2/sync%5Ffile%5Frange.2.html),
[posix_fallocate(3)](../man3/posix%5Ffallocate.3.html), [posix_madvise(3)](../man3/posix%5Fmadvise.3.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-11-20 posixfadvise(2)
Pages that refer to this page:fadvise(1), strace(1), fsync(2), io_uring_enter2(2), io_uring_enter(2), mincore(2), readahead(2), syscall(2), syscalls(2), io_uring_prep_fadvise(3), io_uring_prep_fadvise64(3), off_t(3type), posix_fallocate(3), posix_madvise(3)