posix_fadvise

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():
           _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  sequen-
              tially (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 seman-
              tics 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 speci-
              fied 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  ap-
              plication  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 en-
              sure  that dirty pages will be released, it should call fsync(2)
              or fdatasync(2) first.

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
              2.6.16, 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).

       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 ap-
       plying 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 architec-
       tures, fadvise64_64()); the difference between the two is that the for-
       mer  system  call  assumes that the type of the len argument is size_t,
       while the latter expects loff_t there.

   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 archi-
       tectures, the call signature of posix_fadvise() shown in  the  SYNOPSIS
       would  force a register to be wasted as padding between the fd and off-
       set arguments.  Therefore, these architectures define a version of  the
       system  call  that  orders the arguments suitably, but is otherwise ex-
       actly 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  applica-
       tions  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  in-
       terpreted  literally as "zero bytes", rather than as meaning "all bytes
       through to the end of the file".

SEE ALSO
       fincore(1), mincore(2), readahead(2), sync_file_range(2),  posix_fallo-
       cate(3), posix_madvise(3)

COLOPHON
       This  page  is  part of release 5.05 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                             2019-03-06                  POSIX_FADVISE(2)