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

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