предварительно объявить шаблон доступа для файловых данных (predeclare an access pattern for file data)
Имя (Name)
posix_fadvise - predeclare an access pattern for file data
Синопсис (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
Описание (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.
Возвращаемое значение (Return value)
On success, zero is returned. On error, an error number is
returned.
Ошибки (Error)
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.)
Версии (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.
Стандарты (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.1-2001 TC1.
Примечание (Note)
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()); the
difference between the two is that the former system call assumes
that the type of the len argument is size_t, while the latter
expects loff_t 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 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.
Ошибки (баги) (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".
Смотри также (See also)
fincore(1), mincore(2), readahead(2), sync_file_range(2),
posix_fallocate(3), posix_madvise(3)
