posix_fadvise(2) — Linux manual page


posix_fadvise(2)           System Calls Manual          posix_fadvise(2)

NAME         top

       posix_fadvise - predeclare an access pattern for file data

LIBRARY         top

       Standard C library (libc, -lc)

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

           _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:

              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.

              Before Linux 2.6.18, POSIX_FADV_NOREUSE had the same
              semantics as POSIX_FADV_WILLNEED.  This was probably a
              bug; since Linux 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

              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

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 Linux
              2.6.16, Linux returned EINVAL in this case.)

VERSIONS         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).

   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

       For example, since Linux 2.6.14, ARM has the following system

           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.

STANDARDS         top


HISTORY         top


       Kernel support first appeared in Linux 2.5.60; the underlying
       system call is called fadvise64().  Library support has been
       provided since glibc 2.2, via the wrapper function

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

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

NOTES         top

       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.

BUGS         top

       Before Linux 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

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

COLOPHON         top

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Linux man-pages 6.9.1          2024-05-02               posix_fadvise(2)

Pages that refer to this page: fadvise(1)strace(1)fsync(2)io_uring_enter2(2)io_uring_enter(2)mincore(2)readahead(2)syscall(2)syscalls(2)io_uring_prep_fadvise(3)off_t(3type)posix_fallocate(3)posix_madvise(3)