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       posix_fadvise - predeclare an access pattern for file data


       #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)):

           _XOPEN_SOURCE >= 600 || _POSIX_C_SOURCE >= 200112L


       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:

              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.

              The  application  expects   to   access   the   specified   data
              sequentially (with lower offsets read before higher ones).

              The specified data will be accessed in random order.

              The specified data will be accessed only once.

              The specified data will be accessed in the near future.

              The specified data will not be accessed in the near future.


       On success, zero is returned.  On error, an error number is returned.


       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.)


       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().


       POSIX.1-2001.  Note that the type of the len argument was changed  from
       size_t to off_t in POSIX.1-2003 TC1.


       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.

       The ARM architecture needs 64-bit arguments to be aligned in a suitable
       pair  of  registers.   On  this  architecture,  the  call  signature of
       posix_fadvise() is flawed, since it forces a register to be  wasted  as
       padding  between  the  fd  and  len  arguments.  Therefore, since Linux
       2.6.14, ARM defines a different system call that orders  the  arguments

           long arm_fadvise64_64(int fd, int advice,
                                 loff_t offset, loff_t len);

       The  behavior  of  this  system  call  is otherwise exactly the same as
       posix_fadvise().  No library support is provided for this  system  call
       in glibc.


       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".


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


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