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)):

           _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.

              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

              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.

       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 2.6.16,
              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.1-2001 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).

       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()); the difference between the two is that
       the former 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
       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.

       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".

       fincore(1), mincore(2), readahead(2), sync_file_range(2),
       posix_fallocate(3), posix_madvise(3)

       This page is part of release 5.11 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

Linux                              2021-03-22                   POSIX_FADVISE(2)