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

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():
           _XOPEN_SOURCE >= 600 || _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.

       POSIX_FADV_WILLNEED
              The specified data will be accessed in the near future.

       POSIX_FADV_DONTNEED
              The specified data will not be accessed in the near future.

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 2.16, 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.1-2003 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).

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.)

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_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.

Pages that have not yet been written out will be unaffected, so if the application wishes to guarantee
that pages will be released, it should call fsync(2) or fdatasync(2) first.

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

       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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       information   about   reporting   bugs,   and   the  latest  version  of  this  page,  can  be  found  at
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