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

NAME

       posix_fadvise - predeclare an access pattern for file data

LIBRARY

       Standard C library (libc, -lc)

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.

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.

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 Linux 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 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.

STANDARDS

       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

       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".