POSIX_FADVISE(2)           Linux Programmer's Manual          POSIX_FADVISE(2)

       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  sequen-
              tially (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.  (ESPIPE
              is the error specified by POSIX, but before kernel version 2.16,
              Linux returned 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().

       Since  Linux  3.18, support for the underlying system call is optional,
       depending on the setting of  the  CONFIG_ADVISE_SYSCALLS  configuration

       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.

       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 architec-
       tures, 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 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
       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  applica-
       tions  by the glibc posix_fadvise() wrapper function, which invokes the
       appropriate architecture-specific system call.

       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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Linux                             2015-07-23                  POSIX_FADVISE(2)
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