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_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

       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.

              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.

              The specified data will be accessed in the near future.

              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.)

              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 application 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 ensure that dirty pages will be released, it should
              call fsync(2) or fdatasync(2) first.

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.
              (ESPIPE is the error specified by POSIX, but before kernel
              version 2.6.16, Linux returned 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().

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

CONFORMING TO         top

       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         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).

       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
       applying 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
       architectures, 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 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 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

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

COLOPHON         top

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Linux                            2017-03-13                 POSIX_FADVISE(2)