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POSIX_FADVISE(2) Linux Programmer's Manual POSIX_FADVISE(2)
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)):
posix_fadvise():
_XOPEN_SOURCE >= 600 || _POSIX_C_SOURCE >= 200112L
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:
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, 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.)
Kernel support first appeared in Linux 2.5.60; the underlying system call is
called fadvise64(). Library support has been provided since glibc version
2.2, via the wrapper function posix_fadvise().
POSIX.1-2001. Note that the type of the len argument 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 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.)
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.
The ARM architecture needs 64-bit arguments to be aligned in a suitable pair
of registers. On this architecture, the call signature of posix_fadvise() is
flawed, since it forces a register to be wasted as padding between the fd and
len arguments. Therefore, since Linux 2.6.14, ARM defines a different system
call that orders the arguments suitably:
long arm_fadvise64_64(int fd, int advice,
loff_t offset, loff_t len);
The behavior of this system call is otherwise exactly the same as
posix_fadvise(). No library support is provided for this system call in
glibc.
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), sync_file_range(2), posix_fallocate(3), posix_madvise(3)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2010-10-09 POSIX_FADVISE(2)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface