Ubuntu Manpage: posix_fadvise - predeclare an access pattern for file data

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.  (ESPIPE is the error
              specified by POSIX, but before kernel version 2.16, Linux returned EINVAL  in  this
              case.)

VERSIONS

       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.

CONFORMING TO

       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.

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.

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 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 applications by the glibc
posix_fadvise() wrapper function, which invokes the appropriate architecture-specific
system call.

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".

COLOPHON

       This page is part of release 4.04 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 http://www.kernel.org/doc/man-pages/.