POSIX_FADVISE(2) Linux Programmer's Manual POSIX_FADVISE(2)NAMEposix_fadvise - predeclare an access pattern for file data
SYNOPSIS
#define _XOPEN_SOURCE 600
#include <fcntl.h>
int posix_fadvise(int fd, off_t offset, off_t len, int advice);
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 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 sequen‐
tially (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.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is
set appropriately.
ERRORS
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.)
NOTESposix_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 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.
CONFORMING TO
POSIX.1-2001. Note that the type of the len parameter was changed from
size_t to off_t in POSIX.1-2003 TC5.
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".
SEE ALSOposix_madvise(2), readahead(2), posix_fallocate(3)Linux 2.5.60 14 Feb 2003 POSIX_FADVISE(2)
