posix_fadvise() - Unix, Linux System Call
posix_fadvise - predeclare an access pattern for file data
#define _XOPEN_SOURCE 600
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
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.
The specified data will be accessed in the near future.
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.
The fd argument was not a valid file descriptor.
An invalid value was specified for advice.
The specified file descriptor refers to a pipe or FIFO. (Linux actually
returns EINVAL in this case.)
posix_fadvise() appeared in kernel 2.5.60.
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 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
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.
Note that the type of the
len parameter was changed from
off_t in POSIX.1-2003 TC5.
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".