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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 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.
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, an error number is returned.
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.)
posix_fadvise() appeared in kernel 2.5.60. Glibc support has been
provided since version 2.2.
POSIX.1-2001. Note that the type of the len parameter was changed from
size_t to off_t in POSIX.1-2003 TC1.
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 initiates 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 virtual memory load. (A few megabytes will
usually be fully satisfied, and more is rarely useful.)
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_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(2) or fdatasync(2) first.
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".
readahead(2), posix_fallocate(3), posix_madvise(3),
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