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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON |
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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():
_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.
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_WILLNEED
The specified data will be accessed in the near future.
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.)
POSIX_FADV_DONTNEED
The specified data will not be accessed in the near future.
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.
The implementation may attempt to write back dirty pages in
the specified region, but this is not guaranteed. Any
unwritten dirty pages will not be freed. If the application
wishes to ensure that dirty pages will be released, it should
call fsync(2) or fdatasync(2) first.
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 2.6.16, Linux returned 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().
Since Linux 3.18, support for the underlying system call is optional,
depending on the setting of the CONFIG_ADVISE_SYSCALLS configuration
option.
POSIX.1-2001, POSIX.1-2008. 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).
The contents of the kernel buffer cache can be cleared via the
/proc/sys/vm/drop_caches interface described in proc(5).
One can obtain a snapshot of which pages of a file are resident in
the buffer cache by opening a file, mapping it with mmap(2), and then
applying mincore(2) to the mapping.
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
architectures, fadvise64_64()).
Architecture-specific variants
Some architectures require 64-bit arguments to be aligned in a
suitable pair of registers (see syscall(2) for further detail). On
such architectures, the call signature of posix_fadvise() shown in
the SYNOPSIS would force a register to be wasted as padding between
the fd and offset 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 appli‐
cations by the glibc posix_fadvise() wrapper function, which invokes
the appropriate architecture-specific system call.
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".
fincore(1), mincore(2), readahead(2), sync_file_range(2),
posix_fallocate(3), posix_madvise(3)
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 POSIX_FADVISE(2)
Pages that refer to this page: strace(1), fsync(2), mincore(2), readahead(2), syscall(2), syscalls(2), posix_fallocate(3), posix_madvise(3)
Copyright and license for this manual page
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