int posix_fadvise(int fd, off_t offset, off_t len, int advice);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
_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:
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 sequen-
tially (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. (ESPIPE
is the error specified by POSIX, but before kernel version 2.16,
Linux returned EINVAL in this case.)
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.
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.
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.
C library/kernel differences
The name of the wrapper function in the C library is posix_fadvise().
The underlying system call is called fadvise64() (or, on some architec-
Some architectures require 64-bit arguments to be aligned in a suitable
pair of registers (see syscall(2) for further detail). On such archi-
tectures, the call signature of posix_fadvise() shown in the SYNOPSIS
would force a register to be wasted as padding between the fd and off-
set arguments. Therefore, these architectures define a version of the
system call that orders the arguments suitably, but is otherwise
exactly the same as posix_fadvise().
For example, since Linux 2.6.14, ARM has the following system call:
long arm_fadvise64_64(int fd, int advice,
loff_t offset, loff_t len);
These architecture-specific details are generally hidden from applica-
tions by the glibc posix_fadvise() wrapper function, which invokes the
appropriate architecture-specific system call.
latest version of this page, can be found at
Linux 2015-07-23 POSIX_FADVISE(2)
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