Originální popis anglicky:
posix_fadvise - predeclare an access pattern for file data
Návod, kniha: Linux Programmer's Manual
#include <fcntl.h>
int posix_fadvise(int fd, off_t offset, off_t len, int advice);
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 optimisations.
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, -1 is returned, and
errno is set
appropriately.
- 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.
(Linux actually returns EINVAL in this case.)
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 the entire file, not just the specified region (but other open file
handles to the same file are unaffected).
POSIX_FADV_WILLNEED and
POSIX_FADV_NOREUSE both initiate a
non-blocking read of the specified region into the page cache. The amount of
data read may be decreased by the kernel depending on VM load. (A few
megabytes will usually be fully satisfied, and more is rarely useful.)
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.
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 or
fdatasync first.
SUSv3 (Advanced Realtime Option), POSIX 1003.1-2003. Note that the type of the
len parameter was changed from size_t to off_t in POSIX 1003.1-2003
TC5.
posix_fallocate(2),
posix_madvise(2).
