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 spe‐
       cific pattern in the future, thus allowing the kernel  to  perform  appropriate  optimiza‐
       tions.

       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  off‐
              sets 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
       Kernel support first appeared in Linux 2.5.60; the underlying system call is  called  fad‐
       vise64().   Library  support  has  been  provided since glibc version 2.2, via the wrapper
       function posix_fadvise().

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 back‐
       ing 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_WILL‐
       NEED.  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.

   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 otherwise 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 sys‐
       tem 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".

SEE ALSO
       readahead(2), sync_file_range(2), posix_fallocate(3), posix_madvise(3)

COLOPHON
       This  page  is  part of release 3.74 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/.



Linux                                       2014-05-03                           POSIX_FADVISE(2)