posix_fadvise - predeclare an access pattern for file data
#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)):
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:
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.
The application expects to access the specified data sequentially (with lower offsets read before higher ones).
The specified data will be accessed in random order.
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; since Linux 2.6.18, this flag is a no-op.
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.)
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.
On success, zero is returned. On error, an error number is returned.
fincore(1), mincore(2), readahead(2), sync_file_range(2), posix_fallocate(3), posix_madvise(3)