type()

data_type type () const;

returns the concrete type of the entry

entry()

entry (list_type const&);
entry (integer_type const&);
entry (dictionary_type const&);
entry (string_type const&);

constructors directly from a specific type. The content of the argument is copied into the newly constructed entry

entry()

entry (data_type t);

construct an empty entry of the specified type. see data_type enum.

operator=()

void operator= (string_type const&);
void operator= (entry const&);
void operator= (integer_type const&);
void operator= (lazy_entry const&);
void operator= (dictionary_type const&);
void operator= (list_type const&);

copies the structure of the right hand side into this entry.

string() dict() integer() list()

const integer_type& integer () const;
const string_type& string () const;
const dictionary_type& dict () const;
string_type& string ();
list_type& list ();
dictionary_type& dict ();
integer_type& integer ();
const list_type& list () const;

The integer(), string(), list() and dict() functions are accessors that return the respective type. If the entry object isn't of the type you request, the accessor will throw libtorrent_exception (which derives from std::runtime_error). You can ask an entry for its type through the type() function.

If you want to create an entry you give it the type you want it to have in its constructor, and then use one of the non-const accessors to get a reference which you then can assign the value you want it to have.

The typical code to get info from a torrent file will then look like this:

entry torrent_file;
// ...

// throws if this is not a dictionary
entry::dictionary_type const& dict = torrent_file.dict();
entry::dictionary_type::const_iterator i;
i = dict.find("announce");
if (i != dict.end())
{
        std::string tracker_url = i->second.string();
        std::cout << tracker_url << "\n";
}

The following code is equivalent, but a little bit shorter:

entry torrent_file;
// ...

// throws if this is not a dictionary
if (entry* i = torrent_file.find_key("announce"))
{
        std::string tracker_url = i->string();
        std::cout << tracker_url << "\n";
}

To make it easier to extract information from a torrent file, the class torrent_info exists.

swap()

void swap (entry& e);

swaps the content of this with e.

operator[]()

entry& operator[] (std::string const& key);
const entry& operator[] (std::string const& key) const;
entry& operator[] (char const* key);
const entry& operator[] (char const* key) const;

All of these functions requires the entry to be a dictionary, if it isn't they will throw libtorrent::type_error.

The non-const versions of the operator[] will return a reference to either the existing element at the given key or, if there is no element with the given key, a reference to a newly inserted element at that key.

The const version of operator[] will only return a reference to an existing element at the given key. If the key is not found, it will throw libtorrent::type_error.

find_key()

entry const* find_key (char const* key) const;
entry* find_key (char const* key);
entry* find_key (std::string const& key);
entry const* find_key (std::string const& key) const;

These functions requires the entry to be a dictionary, if it isn't they will throw libtorrent::type_error.

They will look for an element at the given key in the dictionary, if the element cannot be found, they will return 0. If an element with the given key is found, the return a pointer to it.

to_string()

std::string to_string () const;

returns a pretty-printed string representation of the bencoded structure, with JSON-style syntax

enum data_type

Declared in "libtorrent/entry.hpp"

m_type_queried

in debug mode this is set to false by bdecode to indicate that the program has not yet queried the type of this entry, and sould not assume that it has a certain type. This is asserted in the accessor functions. This does not apply if exceptions are used.

pascal_string()

pascal_string (char const* p, int l);

construct a string pointing to the characters at p of length l characters. No NULL termination is required.

operator<()

bool operator< (pascal_string const& rhs) const;

lexicographical comparison of strings. Order is consisten with memcmp.

len

the number of characters in the string.

ptr

the pointer to the first character in the string. This is not NULL terminated, but instead consult the len field to know how many characters follow.

type()

entry_type_t type () const;

tells you which specific type this lazy entry has. See entry_type_t. The type determines which subset of member functions are valid to use.

construct_int()

void construct_int (char const* start, int length);

start points to the first decimal digit length is the number of digits

int_value()

boost::int64_t int_value () const;

requires the type to be an integer. return the integer value

string_ptr()

char const* string_ptr () const;

the string is not null-terminated! use string_length() to determine how many bytes are part of the string.

string_cstr()

char const* string_cstr () const;

this will return a null terminated string it will write to the source buffer!

string_pstr()

pascal_string string_pstr () const;

if this is a string, returns a pascal_string representing the string value.

string_value()

std::string string_value () const;

if this is a string, returns the string as a std::string. (which requires a copy)

string_length()

int string_length () const;

if the lazy_entry is a string, returns the length of the string, in bytes.

dict_find() dict_find_string()

lazy_entry const* dict_find_string (char const* name) const;
lazy_entry* dict_find (char const* name);
lazy_entry const* dict_find (char const* name) const;

if this is a dictionary, look for a key name, and return a pointer to its value, or NULL if there is none.

dict_find_pstr() dict_find_string_value()

pascal_string dict_find_pstr (char const* name) const;
std::string dict_find_string_value (char const* name) const;

if this is a dictionary, look for a key name whose value is a string. If such key exist, return a pointer to its value, otherwise NULL.

dict_find_int_value() dict_find_int()

boost::int64_t dict_find_int_value (char const* name, boost::int64_t default_val = 0) const;
lazy_entry const* dict_find_int (char const* name) const;

if this is a dictionary, look for a key name whose value is an int. If such key exist, return a pointer to its value, otherwise NULL.

dict_find_list() dict_find_dict()

lazy_entry const* dict_find_list (char const* name) const;
lazy_entry const* dict_find_dict (char const* name) const;

these functions require that this is a dictionary. (this->type() == dict_t). They look for an element with the specified name in the dictionary. dict_find_dict only finds dictionaries and dict_find_list only finds lists. if no key with the corresponding value of the right type is found, NULL is returned.

dict_at()

std::pair<std::string, lazy_entry const*> dict_at (int i) const;

if this is a dictionary, return the key value pair at position i from the dictionary.

dict_size()

int dict_size () const;

requires that this is a dictionary. return the number of items in it

list_at()

lazy_entry* list_at (int i);
lazy_entry const* list_at (int i) const;

requires that this is a list. return the item at index i.

list_string_value_at() list_pstr_at()

std::string list_string_value_at (int i) const;
pascal_string list_pstr_at (int i) const;

these functions require this to have the type list. (this->type() == list_t). list_string_value_at returns the string at index i. list_pstr_at returns a pascal_string of the string value at index i. if the element at i is not a string, an empty string is returned.

list_int_value_at()

boost::int64_t list_int_value_at (int i, boost::int64_t default_val = 0) const;

this function require this to have the type list. (this->type() == list_t). returns the integer value at index i. If the element at i is not an integer default_val is returned, which defaults to 0.

list_size()

int list_size () const;

if this is a list, return the number of items in it.

data_section()

std::pair<char const*, int> data_section () const;

returns pointers into the source buffer where this entry has its bencoded data

swap()

void swap (lazy_entry& e);

swap values of this and e.

enum entry_type_t

Declared in "libtorrent/lazy_entry.hpp"

bdecode() bencode()

Declared in "libtorrent/bencode.hpp"

template<class InIt> entry bdecode (InIt start, InIt end);
template<class OutIt> int bencode (OutIt out, const entry& e);
template<class InIt> entry bdecode (InIt start, InIt end, int& len);

These functions will encode data to bencoded or decode bencoded data.

If possible, lazy_bdecode() should be preferred over bdecode().

The entry class is the internal representation of the bencoded data and it can be used to retrieve information, an entry can also be build by the program and given to bencode() to encode it into the OutIt iterator.

The OutIt and InIt are iterators (InputIterator and OutputIterator respectively). They are templates and are usually instantiated as ostream_iterator, back_insert_iterator or istream_iterator. These functions will assume that the iterator refers to a character (char). So, if you want to encode entry e into a buffer in memory, you can do it like this:

std::vector<char> buffer;
bencode(std::back_inserter(buf), e);

If you want to decode a torrent file from a buffer in memory, you can do it like this:

std::vector<char> buffer;
// ...
entry e = bdecode(buf.begin(), buf.end());

Or, if you have a raw char buffer:

const char* buf;
// ...
entry e = bdecode(buf, buf + data_size);

Now we just need to know how to retrieve information from the entry.

If bdecode() encounters invalid encoded data in the range given to it it will throw libtorrent_exception.

operator<<()

Declared in "libtorrent/entry.hpp"

inline std::ostream& operator<< (std::ostream& os, const entry& e);

prints the bencoded structure to the ostream as a JSON-style structure.

lazy_bdecode()

Declared in "libtorrent/lazy_entry.hpp"

int lazy_bdecode (char const* start, char const* end
   , lazy_entry& ret, error_code& ec, int* error_pos = 0
   , int depth_limit = 1000, int item_limit = 1000000);

This function decodes bencoded data.

Whenever possible, lazy_bdecode() should be preferred over bdecode(). It is more efficient and more secure. It supports having constraints on the amount of memory is consumed by the parser.

lazy refers to the fact that it doesn't copy any actual data out of the bencoded buffer. It builds a tree of lazy_entry which has pointers into the bencoded buffer. This makes it very fast and efficient. On top of that, it is not recursive, which saves a lot of stack space when parsing deeply nested trees. However, in order to protect against potential attacks, the depth_limit and item_limit control how many levels deep the tree is allowed to get. With recursive parser, a few thousand levels would be enough to exhaust the threads stack and terminate the process. The item_limit protects against very large structures, not necessarily deep. Each bencoded item in the structure causes the parser to allocate some amount of memory, this memory is constant regardless of how much data actually is stored in the item. One potential attack is to create a bencoded list of hundreds of thousands empty strings, which would cause the parser to allocate a significant amount of memory, perhaps more than is available on the machine, and effectively provide a denial of service. The default item limit is set as a reasonable upper limit for desktop computers. Very few torrents have more items in them. The limit corresponds to about 25 MB, which might be a bit much for embedded systems.

start and end defines the bencoded buffer to be decoded. ret is the lazy_entry which is filled in with the whole decoded tree. ec is a reference to an error_code which is set to describe the error encountered in case the function fails. error_pos is an optional pointer to an int, which will be set to the byte offset into the buffer where an error occurred, in case the function fails.