POSIX_FADVISE(2) BSD System Calls Manual POSIX_FADVISE(2)NAME
posix_fadvise -- give advice about use of file data
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <fcntl.h>
int
posix_fadvise(int fd, off_t offset, off_t len, int advice);
DESCRIPTION
The posix_fadvise() system call allows a process to describe to the system its data access behavior for an open file descriptor fd. The
advice covers the data starting at offset offset and continuing for len bytes. If len is zero, all data from offset to the end of the file
is covered.
The behavior is specified by the advice parameter and may be one of:
POSIX_FADV_NORMAL Tells the system to revert to the default data access behavior.
POSIX_FADV_RANDOM Is a hint that file data will be accessed randomly, and prefetching is likely not advantageous.
POSIX_FADV_SEQUENTIAL Tells the system that file data will be accessed sequentially. This currently does nothing as the default behavior
uses heuristics to detect sequential behavior.
POSIX_FADV_WILLNEED Tells the system that the specified data will be accessed in the near future. The system may initiate an asynchronous
read of the data if it is not already present in memory.
POSIX_FADV_DONTNEED Tells the system that the specified data will not be accessed in the near future. The system may decrease the in-mem-
ory priority of clean data within the specified range and future access to this data may require a read operation.
POSIX_FADV_NOREUSE Tells the system that the specified data will only be accessed once and then not reused. The system may decrease the
in-memory priority of data once it has been read or written. Future access to this data may require a read operation.
RETURN VALUES
The posix_fadvise() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to
indicate the error.
ERRORS
The posix_fadvise() system call returns zero on success, and an error on failure:
[EBADF] The fd argument is not a valid file descriptor.
[EINVAL] The advice argument is not valid.
[EINVAL] The offset or len arguments are negative, or offset + len is greater than the maximum file size.
[ENODEV] The fd argument does not refer to a regular file.
[ESPIPE] The fd argument is associated with a pipe or FIFO.
SEE ALSO madvise(2)STANDARDS
The posix_fadvise() interface conforms to IEEE Std 1003.1-2001 (``POSIX.1'').
HISTORY
The posix_fadvise() system call first appeared in FreeBSD 9.1.
BSD January 30, 2014 BSD
Check Out this Related Man Page
POSIX_FADVISE(2) Linux Programmer's Manual POSIX_FADVISE(2)NAME
posix_fadvise - predeclare an access pattern for file data
SYNOPSIS
#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
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 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.
RETURN VALUE
On success, zero is returned. On error, an error number is returned.
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.)
VERSIONS
posix_fadvise() appeared in kernel 2.5.60. Glibc support has been provided since version 2.2.
CONFORMING TO
POSIX.1-2001. Note that the type of the len argument was changed from size_t to off_t in POSIX.1-2003 TC1.
NOTES
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.
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 ALSO readahead(2), sync_file_range(2), posix_fallocate(3), posix_madvise(3), feature_test_macros(7)COLOPHON
This page is part of release 3.27 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 2010-09-20 POSIX_FADVISE(2)