A handle to somewhere originally identified by a path on the filing system. Typically used as the lightest weight handle to some location on the filing system which may unpredictably relocate over time. This handle is thus an anchor to a subset island of the filing system, free of any race conditions introduced by third party changes to any part of the path leading to that island. More...

#include "path_handle.hpp"

Inheritance diagram for llfio_v2_xxx::path_handle:

Public Types
using	path_type = handle::path_type

using	extent_type = handle::extent_type

using	size_type = handle::size_type

using	mode = handle::mode

using	creation = handle::creation

using	caching = handle::caching

using	flag = handle::flag

using	path_view_type = path_view
	The path view type used by this handle.

Public Member Functions
constexpr	path_handle ()
	Default constructor.

constexpr	path_handle (native_handle_type h, flag flags)
	Construct a handle from a supplied native handle.

constexpr	path_handle (handle &&o) noexcept
	Explicit conversion from handle permitted.

	path_handle (path_handle &&)=default
	Move construction permitted.

	path_handle (const path_handle &)=delete
	No copy construction (use `clone()`)

path_handle &	operator= (path_handle &&o) noexcept
	Move assignment permitted.

path_handle &	operator= (const path_handle &)=delete
	No copy assignment.

void	swap (path_handle &o) noexcept
	Swap with another instance.

result< path_handle >	clone_to_path_handle () const noexcept
	A `path_handle` returning edition of `handle::clone()`

result< bool >	exists (path_view_type path) const noexcept

virtual result< void >	close () noexcept override
	Immediately close the native handle type managed by this handle.

flag	flags () const noexcept
	The flags this handle was opened with.

	QUICKCPPLIB_BITFIELD_BEGIN_T (flag, uint16_t)
	Bitwise flags which can be specified. More...

void	swap (handle &o) noexcept
	Swap with another instance.

virtual result< path_type >	current_path () const noexcept

result< handle >	clone () const noexcept

virtual native_handle_type	release () noexcept
	Release the native handle type managed by this handle.

bool	is_valid () const noexcept
	True if the handle is valid (and usually open)

bool	is_readable () const noexcept
	True if the handle is readable.

bool	is_writable () const noexcept
	True if the handle is writable.

bool	is_append_only () const noexcept
	True if the handle is append only.

virtual result< void >	set_append_only (bool enable) noexcept
	EXTENSION: Changes whether this handle is append only or not. More...

bool	is_multiplexable () const noexcept
	True if multiplexable.

bool	is_nonblocking () const noexcept
	True if nonblocking.

bool	is_seekable () const noexcept
	True if seekable.

bool	requires_aligned_io () const noexcept
	True if requires aligned i/o.

bool	is_kernel_handle () const noexcept
	True if `native_handle()` is a valid kernel handle.

bool	is_regular () const noexcept
	True if a regular file or device.

bool	is_directory () const noexcept
	True if a directory.

bool	is_symlink () const noexcept
	True if a symlink.

bool	is_pipe () const noexcept
	True if a pipe.

bool	is_socket () const noexcept
	True if a socket.

bool	is_multiplexer () const noexcept
	True if a multiplexer like BSD kqueues, Linux epoll or Windows IOCP.

bool	is_process () const noexcept
	True if a process.

bool	is_section () const noexcept
	True if a memory section.

bool	is_allocation () const noexcept
	True if a memory allocation.

bool	is_path () const noexcept
	True if a path or a directory.

bool	is_tls_socket () const noexcept
	True if a TLS socket.

bool	is_http_socket () const noexcept
	True if a HTTP socket.

caching	kernel_caching () const noexcept
	Kernel cache strategy used by this handle.

bool	are_reads_from_cache () const noexcept
	True if the handle uses the kernel page cache for reads.

bool	are_writes_durable () const noexcept
	True if writes are safely on storage on completion.

bool	are_safety_barriers_issued () const noexcept
	True if issuing safety fsyncs is on.

native_handle_type	native_handle () const noexcept
	The native handle used by this handle.

Static Public Member Functions
static result< bool >	exists (const path_handle &base, path_view_type path) noexcept
	This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

static result< path_handle >	path (const path_handle &base, path_view_type path) noexcept

static result< path_handle >	path (path_view_type _path) noexcept
	This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

Protected Attributes
union {
native_handle_type _v

struct {
intptr_t _padding0_

uint32_t _padding1_

flag flags

uint16_t _padding2_

} _

};

Friends
class	directory_handle

Detailed Description

A handle to somewhere originally identified by a path on the filing system. Typically used as the lightest weight handle to some location on the filing system which may unpredictably relocate over time. This handle is thus an anchor to a subset island of the filing system, free of any race conditions introduced by third party changes to any part of the path leading to that island.

Member Function Documentation

◆ clone()

result<handle> llfio_v2_xxx::handle::clone ( ) const

inlinenoexceptinherited

Clone this handle (copy constructor is disabled to avoid accidental copying)

Errors returnable\n Any of the values POSIX dup() or DuplicateHandle() can return.

◆ current_path()

virtual result<path_type> llfio_v2_xxx::handle::current_path ( ) const

inlinevirtualnoexceptinherited

Returns the current path of the open handle as said by the operating system. Note that you are NOT guaranteed that any path refreshed bears any resemblance to the original, some operating systems will return some different path which still reaches the same inode via some other route e.g. hardlinks, dereferenced symbolic links, etc. Windows and Linux correctly track changes to the specific path the handle was opened with, not getting confused by other hard links. MacOS nearly gets it right, but under some circumstances e.g. renaming may switch to a different hard link's path which is almost certainly a bug.

If LLFIO was not able to determine the current path for this open handle e.g. the inode has been unlinked, it returns an empty path. Be aware that FreeBSD can return an empty (deleted) path for file inodes no longer cached by the kernel path cache, LLFIO cannot detect the difference. FreeBSD will also return any path leading to the inode if it is hard linked. FreeBSD does implement path retrieval for directory inodes correctly however, and see algorithm::cached_parent_handle_adapter<T> for a handle adapter which makes use of that.

On Linux if /proc is not mounted, this call fails with an error. All APIs in LLFIO which require the use of current_path() can be told to not use it e.g. flag::disable_safety_unlinks. It is up to you to detect if current_path() is not working, and to change how you call LLFIO appropriately.

On Windows, you will almost certainly get back a path of the form \!!\Device\HarddiskVolume10\Users\ned\.... See path_view for what all the path prefix sequences mean, but to summarise the \!!\ prefix is LLFIO-only and will not be accepted by other Windows APIs. Pass LLFIO derived paths through the function to_win32_path() to Win32-ise them. This function is also available on Linux where it does nothing, so you can use it in portable code.

Warning: This call is expensive, it always asks the kernel for the current path, and no checking is done to ensure what the kernel returns is accurate or even sensible. Be aware that despite these precautions, paths are unstable and can change randomly at any moment. Most code written to use absolute file systems paths is racy, so don't do it, use path_handle to fix a base location on the file system and work from that anchor instead!

Memory Allocations\n At least one malloc for the path_type, likely several more.

See also: algorithm::cached_parent_handle_adapter<T> which overrides this with an implementation based on retrieving the current path of a cached handle to the parent directory. On platforms with instability or failure to retrieve the correct current path for regular files, the cached parent handle adapter works around the problem by taking advantage of directory inodes not having the same instability problems on any platform.

Reimplemented in llfio_v2_xxx::symlink_handle, and llfio_v2_xxx::process_handle.

◆ exists()

result<bool> llfio_v2_xxx::path_handle::exists ( path_view_type path ) const

inlinenoexcept

Returns whether a file entry exists more efficiently that opening and closing a file_handle. Note that this can be a rich source of TOCTOU security attacks! Be aware that symbolic links are NOT dereferenced, so a subsequent file handle open may fail.

◆ path()

static result<path_handle> llfio_v2_xxx::path_handle::path	(	const path_handle &	base,
		path_view_type	path
	)

inlinestaticnoexcept

Create a path handle opening access to some location on the filing system. Some operating systems provide a particularly lightweight method of doing this (Linux: O_PATH, Windows: no access perms) which is much faster than opening a directory. For other systems, we open a directory with read only permissions.

Errors returnable\n Any of the values POSIX open() or CreateFile() can return.

◆ QUICKCPPLIB_BITFIELD_BEGIN_T()

llfio_v2_xxx::handle::QUICKCPPLIB_BITFIELD_BEGIN_T	(	flag	,
		uint16_t
	)

inlineinherited

Bitwise flags which can be specified.

< No flags

Unlinks the file on handle close. On POSIX, this simply unlinks whatever is pointed to by path() upon the call of close() if and only if the inode matches. On Windows, if you are on Windows 10 1709 or later, exactly the same thing occurs. If on previous editions of Windows, the file entry does not disappears but becomes unavailable for anyone else to open with an errc::resource_unavailable_try_again error return. Because this is confusing, unless the win_disable_unlink_emulation flag is also specified, this POSIX behaviour is somewhat emulated by LLFIO on older Windows by renaming the file to a random name on close() causing it to appear to have been unlinked immediately.

Some kernel caching modes have unhelpfully inconsistent behaviours in getting your data onto storage, so by default unless this flag is specified LLFIO adds extra fsyncs to the following operations for the caching modes specified below: truncation of file length either explicitly or during file open. closing of the handle either explicitly or in the destructor.

Additionally on Linux only to prevent loss of file metadata: On the parent directory whenever a file might have been created. On the parent directory on file close.

This only occurs for these kernel caching modes: caching::none caching::reads caching::reads_and_metadata caching::safety_barriers

file_handle::unlink() could accidentally delete the wrong file if someone has renamed the open file handle since the time it was opened. To prevent this occuring, where the OS doesn't provide race free unlink-by-open-handle we compare the inode of the path we are about to unlink with that of the open handle before unlinking.

Warning: This does not prevent races where in between the time of checking the inode and executing the unlink a third party changes the item about to be unlinked. Only operating systems with a true race-free unlink syscall are race free.

Ask the OS to disable prefetching of data. This can improve random i/o performance.

Ask the OS to maximise prefetching of data, possibly prefetching the entire file into kernel cache. This can improve sequential i/o performance.

< See the documentation for unlink_on_first_close

Microsoft Windows NTFS, having been created in the late 1980s, did not originally implement extents-based storage and thus could only represent sparse files via efficient compression of intermediate zeros. With NTFS v3.0 (Microsoft Windows 2000), a proper extents-based on-storage representation was added, thus allowing only 64Kb extent chunks written to be stored irrespective of whatever the maximum file extent was set to.

For various historical reasons, extents-based storage is disabled by default in newly created files on NTFS, unlike in almost every other major filing system. You have to explicitly "opt in" to extents-based storage.

As extents-based storage is nearly cost free on NTFS, LLFIO by default opts in to extents-based storage for any empty file it creates. If you don't want this, you can specify this flag to prevent that happening.

Filesystems tend to be embarrassingly parallel for operations performed to different inodes. Where LLFIO performs i/o to multiple inodes at a time, it will use OpenMP or the Parallelism or Concurrency standard library extensions to usually complete the operation in constant rather than linear time. If you don't want this default, you can disable default using this flag.

Microsoft Windows NTFS has the option, when creating a directory, to set whether leafname lookup will be case sensitive. This is the only way of getting exact POSIX semantics on Windows without resorting to editing the system registry, however it also affects all code doing lookups within that directory, so we must default it to off.

Create the handle in a way where i/o upon it can be multiplexed with other i/o on the same initiating thread of execution i.e. you can perform more than one read concurrently, without using threads. The blocking operations .read() and .write() may have to use a less efficient, but cancellable, blocking implementation for handles created in this way. On Microsoft Windows, this creates handles with OVERLAPPED semantics. On POSIX, this creates handles with nonblocking semantics for non-file handles such as pipes and sockets, however for file, directory and symlink handles it does not set nonblocking, as it is non-portable.

< Using insane POSIX byte range locks

< This is an inode created with no representation on the filing system

                                               {
   none = uint16_t(0),  //!< No flags
   /*! Unlinks the file on handle close. On POSIX, this simply unlinks whatever is pointed
   to by `path()` upon the call of `close()` if and only if the inode matches. On Windows,
   if you are on Windows 10 1709 or later, exactly the same thing occurs. If on previous
   editions of Windows, the file entry does not disappears but becomes unavailable for
   anyone else to open with an `errc::resource_unavailable_try_again` error return. Because this is confusing, unless the
   `win_disable_unlink_emulation` flag is also specified, this POSIX behaviour is
   somewhat emulated by LLFIO on older Windows by renaming the file to a random name on `close()`
   causing it to appear to have been unlinked immediately.
   */
   unlink_on_first_close = uint16_t(1U << 0U),
  
   /*! Some kernel caching modes have unhelpfully inconsistent behaviours
   in getting your data onto storage, so by default unless this flag is
   specified LLFIO adds extra fsyncs to the following operations for the
   caching modes specified below:
   * truncation of file length either explicitly or during file open.
   * closing of the handle either explicitly or in the destructor.
  
   Additionally on Linux only to prevent loss of file metadata:
   * On the parent directory whenever a file might have been created.
   * On the parent directory on file close.
  
   This only occurs for these kernel caching modes:
   * caching::none
   * caching::reads
   * caching::reads_and_metadata
   * caching::safety_barriers
   */
   disable_safety_barriers = uint16_t(1U << 2U),
   /*! `file_handle::unlink()` could accidentally delete the wrong file if someone has
   renamed the open file handle since the time it was opened. To prevent this occuring,
   where the OS doesn't provide race free unlink-by-open-handle we compare the inode of
   the path we are about to unlink with that of the open handle before unlinking.
   \warning This does not prevent races where in between the time of checking the inode
   and executing the unlink a third party changes the item about to be unlinked. Only
   operating systems with a true race-free unlink syscall are race free.
   */
   disable_safety_unlinks = uint16_t(1U << 3U),
   /*! Ask the OS to disable prefetching of data. This can improve random
   i/o performance.
   */
   disable_prefetching = uint16_t(1U << 4U),
   /*! Ask the OS to maximise prefetching of data, possibly prefetching the entire file
   into kernel cache. This can improve sequential i/o performance.
   */
   maximum_prefetching = uint16_t(1U << 5U),
  
   win_disable_unlink_emulation = uint16_t(1U << 9U),  //!< See the documentation for `unlink_on_first_close`
   /*! Microsoft Windows NTFS, having been created in the late 1980s, did not originally
   implement extents-based storage and thus could only represent sparse files via
   efficient compression of intermediate zeros. With NTFS v3.0 (Microsoft Windows 2000),
   a proper extents-based on-storage representation was added, thus allowing only 64Kb
   extent chunks written to be stored irrespective of whatever the maximum file extent
   was set to.
  
   For various historical reasons, extents-based storage is disabled by default in newly
   created files on NTFS, unlike in almost every other major filing system. You have to
   explicitly "opt in" to extents-based storage.
  
   As extents-based storage is nearly cost free on NTFS, LLFIO by default opts in to
   extents-based storage for any empty file it creates. If you don't want this, you
   can specify this flag to prevent that happening.
   */
   win_disable_sparse_file_creation = uint16_t(1U << 10U),
   /*! Filesystems tend to be embarrassingly parallel for operations performed to different
   inodes. Where LLFIO performs i/o to multiple inodes at a time, it will use OpenMP or
   the Parallelism or Concurrency standard library extensions to usually complete the
   operation in constant rather than linear time. If you don't want this default, you can
   disable default using this flag.
   */
   disable_parallelism = uint16_t(1U << 11U),
   /*! Microsoft Windows NTFS has the option, when creating a directory, to set whether
   leafname lookup will be case sensitive. This is the only way of getting exact POSIX
   semantics on Windows without resorting to editing the system registry, however it also
   affects all code doing lookups within that directory, so we must default it to off.
   */
   win_create_case_sensitive_directory = uint16_t(1U << 12U),
  
   /*! Create the handle in a way where i/o upon it can be multiplexed with other i/o
   on the same initiating thread of execution i.e. you can perform more than one read
   concurrently, without using threads. The blocking operations `.read()` and `.write()`
   may have to use a less efficient, but cancellable, blocking implementation for handles created
   in this way. On Microsoft Windows, this creates handles with `OVERLAPPED` semantics.
   On POSIX, this creates handles with nonblocking semantics for non-file handles such
   as pipes and sockets, however for file, directory and symlink handles it does not set
   nonblocking, as it is non-portable.
   */
   multiplexable = uint16_t(1U << 13U),
  
   // NOTE: IF UPDATING THIS UPDATE THE std::ostream PRINTER BELOW!!!
  
   byte_lock_insanity = uint16_t(1U << 14U),  //!< Using insane POSIX byte range locks
   anonymous_inode = uint16_t(1U << 15U)      //!< This is an inode created with no representation on the filing system
   } QUICKCPPLIB_BITFIELD_END(flag)

◆ set_append_only()

virtual result<void> llfio_v2_xxx::handle::set_append_only ( bool enable )

inlinevirtualnoexceptinherited

EXTENSION: Changes whether this handle is append only or not.

Warning: On Windows this is implemented as a bit of a hack to make it fast like on POSIX, so make sure you open the handle for read/write originally. Note unlike on POSIX the append_only disposition will be the only one toggled, seekable and readable will remain turned on.

Errors returnable\n Whatever POSIX fcntl() returns. On Windows nothing is changed on the handle.

Memory Allocations\n No memory allocation.

Reimplemented in llfio_v2_xxx::process_handle.

The documentation for this class was generated from the following file:

include/llfio/v2.0/path_handle.hpp

Public Types

Public Member Functions

Static Public Member Functions

Protected Attributes

Friends