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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);
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
optimisations.
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 pat-
tern 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, -1 is returned, and errno is set appropri-
ately.
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.)
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 the entire file, not
just the specified region (but other open file handles to the same file are unaf-
fected).
POSIX_FADV_WILLNEED and POSIX_FADV_NOREUSE both initiate 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 VM load. (A few megabytes will usually be fully satisfied,
and more is rarely useful.)
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 or fdatasync
first.
CONFORMING TO
SUSv3 (Advanced Realtime Option), POSIX 1003.1-2003. Note that the type of the len
parameter was changed from size_t to off_t in POSIX 1003.1-2003 TC5.
SEE ALSO
posix_fallocate(2), posix_madvise(2).
Linux 2.5.60 14 Feb 2003 POSIX_FADVISE(2)
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