Ubuntu Manpage: posix_fadvise - predeclare an access pattern for file data

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NAME

       posix_fadvise - predeclare an access pattern for file data

SYNOPSIS

       #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():
           _POSIX_C_SOURCE >= 200112L

DESCRIPTION

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.

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_WILLNEED
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.)

POSIX_FADV_DONTNEED
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

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

ERRORS

       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

       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 option.

CONFORMING TO

       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.

NOTES

       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.

BUGS

       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".

COLOPHON

       This  page  is  part of release 5.10 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 https://www.kernel.org/doc/man-pages/.