NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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

NAME         top

       posix_fadvise - predeclare an access pattern for file data

SYNOPSIS         top

       #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         top

       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         top

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

ERRORS         top

       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         top

       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().

CONFORMING TO         top

       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         top

       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.

   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 len 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 system call.

BUGS         top

       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         top

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

COLOPHON         top

       This page is part of release 3.65 of the Linux man-pages project.  A
       description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux                            2013-04-01                 POSIX_FADVISE(2)