llfio_v2_xxx::symlink_handle Class Reference

A handle to an inode which redirects to a different path. More...

#include "symlink_handle.hpp"

Inheritance diagram for llfio_v2_xxx::symlink_handle:

Classes
struct	buffers_type
struct	const_buffers_type
struct	io_request
	The i/o request type used by this handle. More...
struct	io_request< buffers_type, ____ >
	Specialisation for reading symlinks. More...
struct	io_request< const_buffers_type, ____ >
	Specialisation for writing symlinks. More...

Public Types
enum class	symlink_type { none , symbolic , win_wsl , win_junction }
	The type of symbolic link this is. More...
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	dev_t = fs_handle::dev_t
using	ino_t = fs_handle::ino_t
using	path_view_type = fs_handle::path_view_type
using	buffer_type = path_view
	The buffer type used by this handle, which is a path_view
using	const_buffer_type = path_view
	The const buffer type used by this handle, which is a path_view
using	unique_id_type = QUICKCPPLIB_NAMESPACE::integers128::uint128
	The unique identifier type used by this handle.
using	unique_id_type_hasher = QUICKCPPLIB_NAMESPACE::integers128::uint128_hasher
	A hasher for the unique identifier type used by this handle.

Public Member Functions
	symlink_handle ()
	Default constructor.
	symlink_handle (native_handle_type h, dev_t devid, ino_t inode, flag flags=flag::none)
	Construct a handle from a supplied native handle.
	symlink_handle (handle &&o) noexcept
	Explicit conversion from handle permitted.
	symlink_handle (symlink_handle &&)=default
	Move construction permitted.
	symlink_handle (const symlink_handle &)=delete
	No copy construction (use clone())
symlink_handle &	operator= (symlink_handle &&o) noexcept
	Move assignment permitted.
symlink_handle &	operator= (const symlink_handle &)=delete
	No copy assignment.
void	swap (symlink_handle &o) noexcept
	Swap with another instance.
virtual result< void >	close () noexcept override
	Immediately close the native handle type managed by this handle.
virtual result< symlink_handle >	reopen (mode mode_=mode::unchanged, deadline d=std::chrono::seconds(30)) const noexcept
template<class... Args>
bool	try_reopen (Args &&... args) noexcept
template<class... Args, class Rep , class Period >
bool	try_reopen_for (Args &&... args, const std::chrono::duration< Rep, Period > &duration) noexcept
template<class... Args, class Clock , class Duration >
bool	try_reopen_until (Args &&... args, const std::chrono::time_point< Clock, Duration > &timeout) noexcept
virtual result< path_type >	current_path () const noexcept override
virtual result< void >	relink (const path_handle &base, path_view_type path, bool atomic_replace=true, deadline d=std::chrono::seconds(30)) noexcept override
	Relinks the current path of this open handle to the new path specified. If atomic_replace is true, the relink atomically and silently replaces any item at the new path specified. This operation is both atomic and matching POSIX behaviour even on Microsoft Windows where no Win32 API can match POSIX semantics. More...
virtual result< void >	unlink (deadline d=std::chrono::seconds(30)) noexcept override
	Unlinks the current path of this open handle, causing its entry to immediately disappear from the filing system. More...
virtual result< buffers_type >	read (io_request< buffers_type > req={}) noexcept
virtual result< const_buffers_type >	write (io_request< const_buffers_type > req, deadline d=deadline()) noexcept
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.
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.
dev_t	st_dev () const noexcept
	Unless flag::disable_safety_unlinks is set, the device id of the file when opened.
ino_t	st_ino () const noexcept
	Unless flag::disable_safety_unlinks is set, the inode of the file when opened. When combined with st_dev(), forms a unique identifer on this system.
unique_id_type	unique_id () const noexcept
	A unique identifier for this handle across the entire system. Can be used in hash tables etc.
virtual result< path_handle >	parent_path_handle (deadline d=std::chrono::seconds(30)) const noexcept
	Obtain a handle to the path currently containing this handle's file entry. More...
template<class... Args>
bool	try_parent_path_handle (Args &&... args) noexcept
template<class... Args, class Rep , class Period >
bool	try_parent_path_handle_for (Args &&... args, const std::chrono::duration< Rep, Period > &duration) noexcept
template<class... Args, class Clock , class Duration >
bool	try_parent_path_handle_until (Args &&... args, const std::chrono::time_point< Clock, Duration > &timeout) noexcept
template<class... Args>
bool	try_relink (Args &&... args) noexcept
template<class... Args, class Rep , class Period >
bool	try_relink_for (Args &&... args, const std::chrono::duration< Rep, Period > &duration) noexcept
template<class... Args, class Clock , class Duration >
bool	try_relink_until (Args &&... args, const std::chrono::time_point< Clock, Duration > &timeout) noexcept
virtual result< void >	link (const path_handle &base, path_view_type path, deadline d=std::chrono::seconds(30)) noexcept
	Links the inode referred to by this open handle to the path specified. The current path of this open handle is not changed, unless it has no current path due to being unlinked. More...
template<class... Args>
bool	try_link (Args &&... args) noexcept
template<class... Args, class Rep , class Period >
bool	try_link_for (Args &&... args, const std::chrono::duration< Rep, Period > &duration) noexcept
template<class... Args, class Clock , class Duration >
bool	try_link_until (Args &&... args, const std::chrono::time_point< Clock, Duration > &timeout) noexcept
template<class... Args>
bool	try_unlink (Args &&... args) noexcept
template<class... Args, class Rep , class Period >
bool	try_unlink_for (Args &&... args, const std::chrono::duration< Rep, Period > &duration) noexcept
template<class... Args, class Clock , class Duration >
bool	try_unlink_until (Args &&... args, const std::chrono::time_point< Clock, Duration > &timeout) noexcept
virtual result< span< path_view_component > >	list_extended_attributes (span< byte > tofill) const noexcept
	Fill the supplied buffer with the names of all extended attributes set on this file or directory, returning a span of path view components. More...
virtual result< span< byte > >	get_extended_attribute (span< byte > tofill, path_view_component name) const noexcept
	Retrieve the value of an extended attribute set on this file or directory. More...
virtual result< void >	set_extended_attribute (path_view_component name, span< const byte > value) noexcept
	Sets the value of an extended attribute on this file or directory. More...
virtual result< void >	remove_extended_attribute (path_view_component) noexcept
	Removes the extended attribute set on this file or directory. More...
result< size_t >	copy_extended_attributes (const fs_handle &src, bool replace_all_local_attributes=false) noexcept
	Copies the extended attributes from one entity to another, optionally replacing all the extended attributes on the destination. More...

Static Public Member Functions
static result< symlink_handle >	symlink (const path_handle &base, path_view_type path, mode _mode=mode::read, creation _creation=creation::open_existing, flag flags=flag::none) noexcept
static result< symlink_handle >	uniquely_named_symlink (const path_handle &dirpath, mode _mode=mode::write, flag flags=flag::none) noexcept

Protected Member Functions
result< void >	_fetch_inode () const noexcept
	Fill in _devid and _inode from the handle via fstat()

Protected Attributes
union {
native_handle_type   _v
struct {
intptr_t   _padding0_
uint32_t   _padding1_
flag   flags
uint16_t   _padding2_
}   _
};
dev_t	_devid {0}
ino_t	_inode {0}

Detailed Description

A handle to an inode which redirects to a different path.

Microsoft Windows and Linux provide the ability to open the contents of a symbolic link directly, for those platforms this handle works exactly like any ordinary handle. For other POSIX platforms without proprietary extensions, it is not possible to get a valid file descriptor to the contents of a symlink, and in this situation the native handle returned will be -1 and the preprocessor macro LLFIO_SYMLINK_HANDLE_IS_FAKED will be non-zero.

If LLFIO_SYMLINK_HANDLE_IS_FAKED is on, the handle is race free up to the containing directory only. If a third party relocates the symbolic link into a different directory, and race free checking is enabled, this class will simply refuse to work with errc::no_such_file_or_directory as it no longer has any way of finding the symbolic link. You should take care that this does not become a denial of service attack.

On Microsoft Windows, there are many kinds of symbolic link: this implementation supports directory junctions, and NTFS symbolic links. Reads of any others will return an error code comparing equal to errc::protocol_not_supported. One should note that modifying symbolic links was not historically permitted by users with ordinary permissions on Microsoft Windows, however recent versions of Windows 10 do support symbolic links for ordinary users. All versions of Windows support directory symbolic links (junctions), these work for all users in any configuration.

(Note that to create a directory junction on Windows, first create a directory, open that empty directory for modification using symlink_handle, then write using symlink_type::win_junction)

Member Enumeration Documentation

symlink_type

enum llfio_v2_xxx::symlink_handle::symlink_type

strong

The type of symbolic link this is.

Enumerator
none	! No link
symbolic	Standard symbolic link.
win_wsl	WSL symbolic link (Windows only, not actually implemented currently)
win_junction	NTFS directory junction (Windows only, directories and volumes only)

  {
    none,      //!<! No link
    symbolic,  //!< Standard symbolic link
 
    win_wsl,      //!< WSL symbolic link (Windows only, not actually implemented currently)
    win_junction  //!< NTFS directory junction (Windows only, directories and volumes only)
  };

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.

copy_extended_attributes()

result<size_t> llfio_v2_xxx::fs_handle::copy_extended_attributes ( const fs_handle &  src, bool  replace_all_local_attributes = false  )

inlinenoexceptinherited

Copies the extended attributes from one entity to another, optionally replacing all the extended attributes on the destination.

This convenience function is implemented using the APIs above, and therefore is racy with respect to concurrent users. If you specifiy an invalid source with replace_all_local_attributes = true, then this is a convenient way to remove all extended attributes on the local inode.

Note

This function uses 130Kb of stack and cannot handle attribute values larger than 64Kb.

  {
    auto &h = _get_handle();
    (void) h;
    LLFIO_LOG_FUNCTION_CALL(&h);
    byte buffer[65536 + 4096];
    if(replace_all_local_attributes)
    {
      for(;;)
      {
        OUTCOME_TRY(auto &&attribs, list_extended_attributes(buffer));
        if(attribs.empty())
        {
          break;
        }
        for(auto &attrib : attribs)
        {
          OUTCOME_TRY(remove_extended_attribute(attrib));
        }
      }
    }
    if(src._get_handle().is_valid())
    {
      OUTCOME_TRY(auto &&attribs, src.list_extended_attributes(buffer));
      for(auto &attrib : attribs)
      {
        byte buffer2[65536];
        OUTCOME_TRY(auto &&value, src.get_extended_attribute(buffer2, attrib));
        OUTCOME_TRY(set_extended_attribute(attrib, value));
      }
    }
    return success();
  }

current_path()

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

inlineoverridevirtualnoexcept

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 from llfio_v2_xxx::handle.

get_extended_attribute()

virtual result<span<byte> > llfio_v2_xxx::fs_handle::get_extended_attribute ( span< byte >  tofill, path_view_component  name  ) const

inlinevirtualnoexceptinherited

Retrieve the value of an extended attribute set on this file or directory.

Note

On Windows, this is the list of alternate streams on a file, NOT NTFS extended attributes.

link()

virtual result<void> llfio_v2_xxx::fs_handle::link ( const path_handle &  base, path_view_type  path, deadline  d = std::chrono::seconds(30)  )

inlinevirtualnoexceptinherited

Links the inode referred to by this open handle to the path specified. The current path of this open handle is not changed, unless it has no current path due to being unlinked.

Warning

Some operating systems provide a race free syscall for linking an open handle to a new location (Linux, Windows). On all other operating systems this call is racy and can result in the wrong inode being linked. Note that unless flag::disable_safety_unlinks is set, this implementation opens a path_handle to the source containing directory first, then checks before linking that the item about to be hard linked has the same inode as the open file handle. It will retry this matching until success until the deadline given. This should prevent most unmalicious accidental loss of data.

Parameters

base	Base for any relative path.
path	The relative or absolute new path to hard link to.
d	The deadline by which the matching of the containing directory to the open handle's inode must succeed, else errc::timed_out will be returned.

Memory Allocations\n Except on platforms with race free syscalls for renaming open handles (Windows), calls

current_path() via parent_path_handle() and thus is both expensive and calls malloc many times.

list_extended_attributes()

virtual result<span<path_view_component> > llfio_v2_xxx::fs_handle::list_extended_attributes ( span< byte >  tofill ) const

inlinevirtualnoexceptinherited

Fill the supplied buffer with the names of all extended attributes set on this file or directory, returning a span of path view components.

Note that this routine is a very thin wrap of listxattr() on POSIX and NtQueryInformationFile() on Windows. If the supplied buffer is too small, the syscall typically returns failure rather than do a partial fill. Most implementations do not support more than 64Kb of extended attribute information per inode so maybe 70Kb is a safe default (to account for the return value storage), however properly written code will detect the buffer being too small and will auto-expand it until success.

Note

On Windows, this is the list of alternate streams on a file, NOT NTFS extended attributes.

Race Guarantees\n The list of extended attributes is fetched in a single syscall. This may be an

atomically consistent snapshot.

parent_path_handle()

virtual result<path_handle> llfio_v2_xxx::fs_handle::parent_path_handle ( deadline  d = std::chrono::seconds(30) ) const

inlinevirtualnoexceptinherited

Obtain a handle to the path currently containing this handle's file entry.

Warning

This call is racy and can result in the wrong path handle being returned. Note that unless flag::disable_safety_unlinks is set, this implementation opens a path_handle to the source containing directory, then checks if the file entry within has the same inode as the open file handle. It will retry this matching until success until the deadline given.

Memory Allocations\n Calls current_path() and thus is both expensive and calls malloc many times.

See also

algorithm::cached_parent_handle_adapter<T> which overrides this with a zero cost implementation, thus making unlinking and relinking very considerably quicker.

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)

read()

virtual result<buffers_type> llfio_v2_xxx::symlink_handle::read ( io_request< buffers_type >  req = {} )

inlinevirtualnoexcept

Read the contents of the symbolic link.

If supplying your own kernelbuffer, be aware that the length of the contents of the symbolic link may change at any time. You should therefore retry reading the symbolic link, expanding your kernelbuffer each time, until a successful read occurs.

Returns

Returns the buffers filled, with its path adjusted to the bytes filled.

Parameters

req	A buffer to fill with the contents of the symbolic link.

Errors returnable\n Any of the errors which readlinkat() or DeviceIoControl() might return, or failure

to allocate memory if the user did not supply a kernel buffer to use, or the user supplied buffer was too small.

Memory Allocations\n If the kernelbuffer parameter is set in the request, no memory allocations.

If unset, at least one memory allocation, possibly more is performed.

relink()

virtual result<void> llfio_v2_xxx::symlink_handle::relink ( const path_handle &  base, path_view_type  path, bool  atomic_replace = true, deadline  d = std::chrono::seconds(30)  )

inlineoverridevirtualnoexcept

Relinks the current path of this open handle to the new path specified. If atomic_replace is true, the relink atomically and silently replaces any item at the new path specified. This operation is both atomic and matching POSIX behaviour even on Microsoft Windows where no Win32 API can match POSIX semantics.

Note that if atomic_replace is false, the operation may be implemented as creating a hard link to the destination (which fails if the destination exists), opening a new file descriptor to the destination, closing the existing file descriptor, replacing the existing file descriptor with the new one (this is to ensure path tracking continues to work), then unlinking the previous link. Thus native_handle()'s value may change. This is not the case on Microsoft Windows nor Linux, both of which provide syscalls capable of refusing to rename if the destination exists.

If the handle refers to a pipe, on Microsoft Windows the base path handle is ignored as there is a single global named pipe namespace. Unless the path fragment begins with \, the string \??\ is prefixed to the name before passing it to the NT kernel API which performs the rename. This is because \\.\ in Win32 maps onto \??\ in the NT kernel.

Warning

Some operating systems provide a race free syscall for renaming an open handle (Windows). On all other operating systems this call is racy and can result in the wrong file entry being relinked. Note that unless flag::disable_safety_unlinks is set, this implementation opens a path_handle to the source containing directory first, then checks before relinking that the item about to be relinked has the same inode as the open file handle. It will retry this matching until success until the deadline given. This should prevent most unmalicious accidental loss of data.

Parameters

base	Base for any relative path.
path	The relative or absolute new path to relink to.
atomic_replace	Atomically replace the destination if a file entry already is present there. Choosing false for this will fail if a file entry is already present at the destination, and may not be an atomic operation on some platforms (i.e. both the old and new names may be linked to the same inode for a very short period of time). Windows and recent Linuxes are always atomic.
d	The deadline by which the matching of the containing directory to the open handle's inode must succeed, else errc::timed_out will be returned.

Memory Allocations\n Except on platforms with race free syscalls for renaming open handles (Windows), calls

current_path() via parent_path_handle() and thus is both expensive and calls malloc many times.

Reimplemented from llfio_v2_xxx::fs_handle.

remove_extended_attribute()

virtual result<void> llfio_v2_xxx::fs_handle::remove_extended_attribute ( path_view_component  )

inlinevirtualnoexceptinherited

Removes the extended attribute set on this file or directory.

Note

On Windows, this is the list of alternate streams on a file, NOT NTFS extended attributes. We do not prevent you trying to remove internal alternate streams, either.

reopen()

virtual result<symlink_handle> llfio_v2_xxx::symlink_handle::reopen ( mode  mode_ = mode::unchanged, deadline  d = std::chrono::seconds(30)  ) const

inlinevirtualnoexcept

Reopen this handle (copy constructor is disabled to avoid accidental copying), optionally race free reopening the handle with different access or caching.

Microsoft Windows provides a syscall for cloning an existing handle but with new access. On POSIX, we must loop calling current_path(), trying to open the path returned and making sure it is the same inode.

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

Memory Allocations\n On POSIX if changing the mode, we must loop calling current_path() and

trying to open the path returned. Thus many allocations may occur.

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.

set_extended_attribute()

virtual result<void> llfio_v2_xxx::fs_handle::set_extended_attribute ( path_view_component  name, span< const byte >  value  )

inlinevirtualnoexceptinherited

Sets the value of an extended attribute on this file or directory.

To prevent collision in a globally visible resource, there is a convention whereby you ought to namespace the names of your values as namespace.attribute e.g. appname.setting to prevent unintentional collision with other programs. Obviously, do choose a unique appname if there is any chance another program might use the same namespace name.

On POSIX, there are additional namespacing requirements: before your value name, you need to prefix one of user or system, so the actual name you might set would be user.appname.propname. Windows does not have the user/system prefix requirement, but it does no harm to do the exact same on Windows as on POSIX.

The host OS and target filing system choose the limits on value size, and will fail accordingly. Some impose a maximum of 64Kb for all names and values per inode, others have a 4Kb maximum value size, there are lots of combinations. You are probably safest not setting many names, and keep the values short.

Warning

Extended attributes are 'brittle' because they can get silently wiped at any moment. Never store anything in extended attributes which cannot be recalculated if missing. The ideal use case for extended attributes is as a cache of additional metadata about a file or directory e.g. "I last checked this directory at timestamp X", or "the MD5 hash at last modified timestamp X for this file was Y". Also remember that other processes can and do arbitrarily modify extended attributes concurrent to you.

Windows only

This API is implemented as file alternate data streams, rather than the Extended Attributes API as accessed via NtSetEaFile() and NtQueryEaFile() (which actually modify the file alternate data stream $EA in any case).

The reason why is that NtSetEaFile() can only append new records to EA storage. It cannot deallocate any existing EA records, if you try to do so you will get STATUS_EA_CORRUPT_ERROR. You can append setting the same name to a different value, which can include a null value which then appears as if the name is no longer there. But there is a cap of 64kB for the EA record, and once it is consumed, it is gone forever for that inode.

Obviously that doesn't map at all well onto POSIX extended attributes, where you can set the value of an attribute as frequently as you like. The closest equivalent on Windows is therefore file alternate data streams, even though the attribute's value is then worked with as a whole proper file with all the attendant performance consequences.

As a result, name must be a valid filename and not contain any characters not permitted in a filename. We use the NT API here, so the character restrictions are far fewer than for the Win32 API e.g. single character names do NOT cause misoperation like on Win32.

symlink()

static result<symlink_handle> llfio_v2_xxx::symlink_handle::symlink ( const path_handle &  base, path_view_type  path, mode  _mode = mode::read, creation  _creation = creation::open_existing, flag  flags = flag::none  )

inlinestaticnoexcept

Create a symlink handle opening access to a symbolic link.

For obvious reasons, one cannot append to a symbolic link, nor create with truncate. In this situation a failure comparing equal to errc::function_not_supported shall be returned.

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

Memory Allocations\n None, unless LLFIO_SYMLINK_HANDLE_IS_FAKED is on, in which case one.

uniquely_named_symlink()

static result<symlink_handle> llfio_v2_xxx::symlink_handle::uniquely_named_symlink ( const path_handle &  dirpath, mode  _mode = mode::write, flag  flags = flag::none  )

inlinestaticnoexcept

Create a symlink handle creating a uniquely named symlink on a path. The symlink is opened exclusively with creation::only_if_not_exist so it will never collide with nor overwrite any existing symlink.

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

or failure to allocate memory.

  {
    try
    {
      for(;;)
      {
        auto randomname = utils::random_string(32);
        randomname.append(".random");
        result<symlink_handle> ret = symlink(dirpath, randomname, _mode, creation::only_if_not_exist, flags);
        if(ret || (!ret && ret.error() != errc::file_exists))
        {
          return ret;
        }
      }
    }
    catch(...)
    {
      return error_from_exception();
    }
  }

unlink()

virtual result<void> llfio_v2_xxx::symlink_handle::unlink ( deadline  d = std::chrono::seconds(30) )

inlineoverridevirtualnoexcept

Unlinks the current path of this open handle, causing its entry to immediately disappear from the filing system.

On Windows before Windows 10 1709 unless flag::win_disable_unlink_emulation is set, this behaviour is simulated by renaming the file to something random and setting its delete-on-last-close flag. Note that Windows may prevent the renaming of a file in use by another process, if so it will NOT be renamed. After the next handle to that file closes, it will become permanently unopenable by anyone else until the last handle is closed, whereupon the entry will be eventually removed by the operating system.

Warning

Some operating systems provide a race free syscall for unlinking an open handle (Windows). On all other operating systems this call is racy and can result in the wrong file entry being unlinked. Note that unless flag::disable_safety_unlinks is set, this implementation opens a path_handle to the containing directory first, then checks that the item about to be unlinked has the same inode as the open file handle. It will retry this matching until success until the deadline given. This should prevent most unmalicious accidental loss of data.

Parameters

d	The deadline by which the matching of the containing directory to the open handle's inode must succeed, else errc::timed_out will be returned.

Memory Allocations\n Except on platforms with race free syscalls for unlinking open handles (Windows), calls

current_path() and thus is both expensive and calls malloc many times. On Windows, also calls current_path() if flag::disable_safety_unlinks is not set.

Reimplemented from llfio_v2_xxx::fs_handle.

write()

virtual result<const_buffers_type> llfio_v2_xxx::symlink_handle::write ( io_request< const_buffers_type >  req, deadline  d = deadline()  )

inlinevirtualnoexcept

Write the contents of the symbolic link.

Parameters

req	A buffer with which to replace the contents of the symbolic link.
d	An optional deadline by which the i/o must complete, else it is cancelled. Ignored on Windows.

Errors returnable\n Any of the errors which symlinkat() or DeviceIoControl() might return.

Memory Allocations\n On Windows, if the kernelbuffer parameter is set on entry, no memory allocations.

If unset, then at least one memory allocation, possibly more is performed. On POSIX, at least one memory allocation.

---

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

• include/llfio/v2.0/symlink_handle.hpp