AFIO  v2.00 late alpha
afio_v2_xxx::section_handle Class Reference

A handle to a source of mapped memory. More...

#include "map_handle.hpp"

Inheritance diagram for afio_v2_xxx::section_handle:
afio_v2_xxx::handle

Classes

struct  flag_base
 The behaviour of the memory section. More...
 

Public Types

using extent_type = handle::extent_type
 
using size_type = handle::size_type
 
using flag = quickcpplib::_xxx ::bitfield::bitfield< flag_base >
 
enum  mode : unsigned char {
  unchanged = 0, mode::none = 2, mode::attr_read = 4, mode::attr_write = 5,
  mode::read = 6, mode::write = 7, mode::append = 9
}
 The behaviour of the handle: does it read, read and write, or atomic append? More...
 
enum  creation : unsigned char { open_existing = 0, only_if_not_exist, if_needed, creation::truncate }
 On opening, do we also create a new file or truncate an existing one? More...
 
enum  caching : unsigned char {
  unchanged = 0, caching::none = 1, caching::only_metadata = 2, caching::reads = 3,
  caching::reads_and_metadata = 5, caching::all = 4, caching::safety_fsyncs = 7, caching::temporary = 6
}
 What i/o on the handle will complete immediately due to kernel caching. More...
 
using path_type = filesystem::path
 The path type used by this handle.
 

Public Member Functions

virtual result< void > close () noexcept override
 Immediately close the native handle type managed by this handle.
 
constexpr section_handle ()=default
 Default constructor.
 
 section_handle (native_handle_type sectionh, file_handle *backing, file_handle anonymous, flag __flag)
 Construct a section handle using the given native handle type for the section and the given i/o handle for the backing storage.
 
constexpr section_handle (section_handle &&o) noexcept
 Implicit move construction of section_handle permitted.
 
section_handleoperator= (section_handle &&o) noexcept
 Move assignment of section_handle permitted.
 
void swap (section_handle &o) noexcept
 Swap with another instance.
 
flag section_flags () const noexcept
 Returns the memory section's flags.
 
file_handlebacking () const noexcept
 Returns the borrowed handle backing this section, if any.
 
void set_backing (file_handle *fh) noexcept
 Sets the borrowed handle backing this section, if any.
 
native_handle_type backing_native_handle () const noexcept
 Returns the borrowed native handle backing this section.
 
result< extent_typelength () const noexcept
 Return the current maximum permitted extent of the memory section.
 
result< extent_typetruncate (extent_type newsize=0) noexcept
 
void swap (handle &o) noexcept
 Swap with another instance.
 
virtual result< path_typecurrent_path () const noexcept
 
result< handleclone () 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
 
bool is_overlapped () const noexcept
 True if overlapped.
 
bool is_seekable () const noexcept
 True if seekable.
 
bool requires_aligned_io () const noexcept
 True if requires aligned i/o.
 
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_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.
 
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_fsyncs_issued () const noexcept
 True if issuing safety fsyncs is on.
 
flag flags () const noexcept
 The flags this handle was opened with.
 
native_handle_type native_handle () const noexcept
 The native handle used by this handle.
 

Static Public Member Functions

static result< section_handlesection (file_handle &backing, extent_type bytes, flag _flag) noexcept
 Create a memory section backed by a file. More...
 
static result< section_handlesection (file_handle &backing, extent_type bytes=0) noexcept
 Create a memory section backed by a file. More...
 
static result< section_handlesection (extent_type bytes, const path_handle &dirh=path_discovery::storage_backed_temporary_files_directory(), flag _flag=flag::read|flag::write) noexcept
 Create a memory section backed by an anonymous, managed file. More...
 

Protected Attributes

file_handle_backing {nullptr}
 
file_handle _anonymous
 
flag _flag {flag::none}
 
caching _caching {caching::none}
 
flag _flags {flag::none}
 
native_handle_type _v
 

Detailed Description

A handle to a source of mapped memory.

There are two configurations of section handle, one where the user supplies the file backing for the section and the other where an internal file descriptor to an unnamed inode in a tmpfs or ramfs based temporary directory is kept and managed. The latter is merely a convenience for creating an anonymous source of memory which can be resized whilst preserving contents: see algorithm::trivial_vector<T>.

On Windows the native handle of this handle is that of the NT kernel section object. On POSIX it is a cloned file descriptor of the backing storage if there is backing storage, else it will be the aforementioned file descriptor to an unnamed inode.

Member Enumeration Documentation

◆ caching

enum afio_v2_xxx::handle::caching : unsigned char
stronginherited

What i/o on the handle will complete immediately due to kernel caching.

Enumerator
none 

No caching whatsoever, all reads and writes come from storage (i.e. O_DIRECT|O_SYNC). Align all i/o to 4Kb boundaries for this to work. flag_disable_safety_fsyncs can be used here.

only_metadata 

Cache reads and writes of metadata but avoid caching data (O_DIRECT), thus i/o here does not affect other cached data for other handles. Align all i/o to 4Kb boundaries for this to work.

reads 

Cache reads only. Writes of data and metadata do not complete until reaching storage (O_SYNC). flag_disable_safety_fsyncs can be used here.

reads_and_metadata 

Cache reads and writes of metadata, but writes of data do not complete until reaching storage (O_DSYNC). flag_disable_safety_fsyncs can be used here.

all 

Cache reads and writes of data and metadata so they complete immediately, sending writes to storage at some point when the kernel decides (this is the default file system caching on a system).

safety_fsyncs 

Cache reads and writes of data and metadata so they complete immediately, but issue safety fsyncs at certain points. See documentation for flag_disable_safety_fsyncs.

temporary 

Cache reads and writes of data and metadata so they complete immediately, only sending any updates to storage on last handle close in the system or if memory becomes tight as this file is expected to be temporary (Windows and FreeBSD only).

83  : unsigned char // bit 0 set means safety fsyncs enabled
84  {
85  unchanged = 0,
86  none = 1, //!< No caching whatsoever, all reads and writes come from storage (i.e. <tt>O_DIRECT|O_SYNC</tt>). Align all i/o to 4Kb boundaries for this to work. <tt>flag_disable_safety_fsyncs</tt> can be used here.
87  only_metadata = 2, //!< Cache reads and writes of metadata but avoid caching data (<tt>O_DIRECT</tt>), thus i/o here does not affect other cached data for other handles. Align all i/o to 4Kb boundaries for this to work.
88  reads = 3, //!< Cache reads only. Writes of data and metadata do not complete until reaching storage (<tt>O_SYNC</tt>). <tt>flag_disable_safety_fsyncs</tt> can be used here.
89  reads_and_metadata = 5, //!< Cache reads and writes of metadata, but writes of data do not complete until reaching storage (<tt>O_DSYNC</tt>). <tt>flag_disable_safety_fsyncs</tt> can be used here.
90  all = 4, //!< Cache reads and writes of data and metadata so they complete immediately, sending writes to storage at some point when the kernel decides (this is the default file system caching on a system).
91  safety_fsyncs = 7, //!< Cache reads and writes of data and metadata so they complete immediately, but issue safety fsyncs at certain points. See documentation for <tt>flag_disable_safety_fsyncs</tt>.
92  temporary = 6 //!< Cache reads and writes of data and metadata so they complete immediately, only sending any updates to storage on last handle close in the system or if memory becomes tight as this file is expected to be temporary (Windows and FreeBSD only).
93  // NOTE: IF UPDATING THIS UPDATE THE std::ostream PRINTER BELOW!!!
94  };

◆ creation

enum afio_v2_xxx::handle::creation : unsigned char
stronginherited

On opening, do we also create a new file or truncate an existing one?

Enumerator
truncate 

Atomically truncate on open, leaving creation date unmodified.

74  : unsigned char
75  {
76  open_existing = 0,
77  only_if_not_exist,
78  if_needed,
79  truncate //!< Atomically truncate on open, leaving creation date unmodified.
80  // NOTE: IF UPDATING THIS UPDATE THE std::ostream PRINTER BELOW!!!
81  };
result< file_handle::extent_type > truncate(file_handle &self, file_handle::extent_type newsize) noexcept
Definition: file_handle.hpp:373

◆ mode

enum afio_v2_xxx::handle::mode : unsigned char
stronginherited

The behaviour of the handle: does it read, read and write, or atomic append?

Enumerator
none 

No ability to read or write anything, but can synchronise (SYNCHRONIZE or 0)

attr_read 

Ability to read attributes (FILE_READ_ATTRIBUTES|SYNCHRONIZE or O_RDONLY)

attr_write 

Ability to read and write attributes (FILE_READ_ATTRIBUTES|FILE_WRITE_ATTRIBUTES|SYNCHRONIZE or O_RDONLY)

read 

Ability to read (READ_CONTROL|FILE_READ_DATA|FILE_READ_ATTRIBUTES|FILE_READ_EA|SYNCHRONISE or O_RDONLY)

write 

Ability to read and write (READ_CONTROL|FILE_READ_DATA|FILE_READ_ATTRIBUTES|FILE_READ_EA|FILE_WRITE_DATA|FILE_WRITE_ATTRIBUTES|FILE_WRITE_EA|FILE_APPEND_DATA|SYNCHRONISE or O_RDWR)

append 

All mainstream OSs and CIFS guarantee this is atomic with respect to all other appenders (FILE_APPEND_DATA|SYNCHRONISE or O_APPEND)

62  : unsigned char // bit 0 set means writable
63  {
64  unchanged = 0,
65  none = 2, //!< No ability to read or write anything, but can synchronise (SYNCHRONIZE or 0)
66  attr_read = 4, //!< Ability to read attributes (FILE_READ_ATTRIBUTES|SYNCHRONIZE or O_RDONLY)
67  attr_write = 5, //!< Ability to read and write attributes (FILE_READ_ATTRIBUTES|FILE_WRITE_ATTRIBUTES|SYNCHRONIZE or O_RDONLY)
68  read = 6, //!< Ability to read (READ_CONTROL|FILE_READ_DATA|FILE_READ_ATTRIBUTES|FILE_READ_EA|SYNCHRONISE or O_RDONLY)
69  write = 7, //!< Ability to read and write (READ_CONTROL|FILE_READ_DATA|FILE_READ_ATTRIBUTES|FILE_READ_EA|FILE_WRITE_DATA|FILE_WRITE_ATTRIBUTES|FILE_WRITE_EA|FILE_APPEND_DATA|SYNCHRONISE or O_RDWR)
70  append = 9 //!< All mainstream OSs and CIFS guarantee this is atomic with respect to all other appenders (FILE_APPEND_DATA|SYNCHRONISE or O_APPEND)
71  // NOTE: IF UPDATING THIS UPDATE THE std::ostream PRINTER BELOW!!!
72  };
io_handle::io_result< io_handle::buffers_type > read(io_handle &self, io_handle::io_request< io_handle::buffers_type > reqs, deadline d=deadline()) noexcept
Read data from the open handle.
Definition: io_handle.hpp:467
io_handle::io_result< io_handle::const_buffers_type > write(io_handle &self, io_handle::io_request< io_handle::const_buffers_type > reqs, deadline d=deadline()) noexcept
Write data to the open handle.
Definition: io_handle.hpp:493

Member Function Documentation

◆ clone()

result<handle> afio_v2_xxx::handle::clone ( ) const
inlinenoexceptinherited

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

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

◆ current_path()

virtual result<path_type> afio_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 AFIO 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, AFIO 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::stablized_path<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 AFIO 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 AFIO appropriately.

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
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.

◆ section() [1/3]

static result<section_handle> afio_v2_xxx::section_handle::section ( file_handle backing,
extent_type  bytes,
flag  _flag 
)
inlinestaticnoexcept

Create a memory section backed by a file.

Parameters
backingThe handle to use as backing storage.
bytesThe initial size of this section, which cannot be larger than any backing file. Zero means to use backing.length().
_flagHow to create the section.
Errors returnable
Any of the values POSIX dup(), open() or NtCreateSection() can return.

◆ section() [2/3]

static result<section_handle> afio_v2_xxx::section_handle::section ( file_handle backing,
extent_type  bytes = 0 
)
inlinestaticnoexcept

Create a memory section backed by a file.

Parameters
backingThe handle to use as backing storage.
bytesThe initial size of this section, which cannot be larger than any backing file. Zero means to use backing.length().

This convenience overload create a writable section if the backing file is writable, otherwise a read-only section.

Errors returnable
Any of the values POSIX dup(), open() or NtCreateSection() can return.
134 { return section(backing, bytes, backing.is_writable() ? (flag::readwrite) : (flag::read)); }
static result< section_handle > section(file_handle &backing, extent_type bytes, flag _flag) noexcept
Create a memory section backed by a file.
map_handle::io_result< map_handle::buffers_type > read(map_handle &self, map_handle::io_request< map_handle::buffers_type > reqs, deadline d=deadline()) noexcept
Read data from the mapped view.
Definition: map_handle.hpp:505
file_handle * backing() const noexcept
Returns the borrowed handle backing this section, if any.
Definition: map_handle.hpp:148
bool is_writable() const noexcept
True if the handle is writable.
Definition: handle.hpp:269

◆ section() [3/3]

static result<section_handle> afio_v2_xxx::section_handle::section ( extent_type  bytes,
const path_handle dirh = path_discovery::storage_backed_temporary_files_directory(),
flag  _flag = flag::read|flag::write 
)
inlinestaticnoexcept

Create a memory section backed by an anonymous, managed file.

Parameters
bytesThe initial size of this section. Cannot be zero.
dirhWhere to create the anonymous, managed file.
_flagHow to create the section.
Errors returnable
Any of the values POSIX dup(), open() or NtCreateSection() can return.

◆ set_append_only()

virtual result<void> afio_v2_xxx::handle::set_append_only ( bool  enable)
inlinevirtualnoexceptinherited

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
Whatever POSIX fcntl() returns. On Windows nothing is changed on the handle.
Memory Allocations
No memory allocation.

◆ truncate()

result<extent_type> afio_v2_xxx::section_handle::truncate ( extent_type  newsize = 0)
inlinenoexcept

Resize the current maximum permitted extent of the memory section to the given extent.

Parameters
newsizeThe new size of the memory section, which cannot be zero. Specify zero to use backing.length(). This cannot exceed the size of any backing file used if that file is not writable.
Errors returnable
Any of the values NtExtendSection() or ftruncate() can return.

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