POSIX_FADVISE(2) manual page
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posix_fadvise - predeclare an access pattern
for file data
#include <fcntl.h>
int posix_fadvise(int fd, off_t offset, off_t lenint " advice ");"
Feature Test Macro Requirements for glibc (see feature_test_macros(7)
):
posix_fadvise():
_XOPEN_SOURCE >= 600 || _POSIX_C_SOURCE >= 200112L
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.
- POSIX_FADV_WILLNEED
- The specified data
will be accessed in the near future.
- POSIX_FADV_DONTNEED
- The specified data
will not be accessed in the near future.
On success, zero is
returned. On error, an error number is returned.
- 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 2.16, Linux
returned EINVAL in this case.)
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.
POSIX.1-2001. Note that the type of the len argument was changed
from size_t to off_t in POSIX.1-2003 TC1.
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).
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.)
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_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.
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 fsync(2)
or fdatasync(2)
first.
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 otherwise 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.
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".
readahead(2)
, sync_file_range(2)
, posix_fallocate(3)
, posix_madvise(3)
This page is part of release 3.78 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 http://www.kernel.org/doc/man-pages/.
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