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The asio::buffer function is used to create a buffer
object to represent raw memory, an array of POD elements, a vector of POD
elements, or a std::string.
mutable_buffers_1 buffer( const mutable_buffer & b); » more... mutable_buffers_1 buffer( const mutable_buffer & b, std::size_t max_size_in_bytes); » more... const_buffers_1 buffer( const const_buffer & b); » more... const_buffers_1 buffer( const const_buffer & b, std::size_t max_size_in_bytes); » more... mutable_buffers_1 buffer( void * data, std::size_t size_in_bytes); » more... const_buffers_1 buffer( const void * data, std::size_t size_in_bytes); » more... template< typename PodType, std::size_t N> mutable_buffers_1 buffer( PodType (&data)[N]); » more... template< typename PodType, std::size_t N> mutable_buffers_1 buffer( PodType (&data)[N], std::size_t max_size_in_bytes); » more... template< typename PodType, std::size_t N> const_buffers_1 buffer( const PodType (&data)[N]); » more... template< typename PodType, std::size_t N> const_buffers_1 buffer( const PodType (&data)[N], std::size_t max_size_in_bytes); » more... template< typename PodType, std::size_t N> mutable_buffers_1 buffer( boost::array< PodType, N > & data); » more... template< typename PodType, std::size_t N> mutable_buffers_1 buffer( boost::array< PodType, N > & data, std::size_t max_size_in_bytes); » more... template< typename PodType, std::size_t N> const_buffers_1 buffer( boost::array< const PodType, N > & data); » more... template< typename PodType, std::size_t N> const_buffers_1 buffer( boost::array< const PodType, N > & data, std::size_t max_size_in_bytes); » more... template< typename PodType, std::size_t N> const_buffers_1 buffer( const boost::array< PodType, N > & data); » more... template< typename PodType, std::size_t N> const_buffers_1 buffer( const boost::array< PodType, N > & data, std::size_t max_size_in_bytes); » more... template< typename PodType, std::size_t N> mutable_buffers_1 buffer( std::array< PodType, N > & data); » more... template< typename PodType, std::size_t N> mutable_buffers_1 buffer( std::array< PodType, N > & data, std::size_t max_size_in_bytes); » more... template< typename PodType, std::size_t N> const_buffers_1 buffer( std::array< const PodType, N > & data); » more... template< typename PodType, std::size_t N> const_buffers_1 buffer( std::array< const PodType, N > & data, std::size_t max_size_in_bytes); » more... template< typename PodType, std::size_t N> const_buffers_1 buffer( const std::array< PodType, N > & data); » more... template< typename PodType, std::size_t N> const_buffers_1 buffer( const std::array< PodType, N > & data, std::size_t max_size_in_bytes); » more... template< typename PodType, typename Allocator> mutable_buffers_1 buffer( std::vector< PodType, Allocator > & data); » more... template< typename PodType, typename Allocator> mutable_buffers_1 buffer( std::vector< PodType, Allocator > & data, std::size_t max_size_in_bytes); » more... template< typename PodType, typename Allocator> const_buffers_1 buffer( const std::vector< PodType, Allocator > & data); » more... template< typename PodType, typename Allocator> const_buffers_1 buffer( const std::vector< PodType, Allocator > & data, std::size_t max_size_in_bytes); » more... template< typename Elem, typename Traits, typename Allocator> const_buffers_1 buffer( const std::basic_string< Elem, Traits, Allocator > & data); » more... template< typename Elem, typename Traits, typename Allocator> const_buffers_1 buffer( const std::basic_string< Elem, Traits, Allocator > & data, std::size_t max_size_in_bytes); » more...
A buffer object represents a contiguous region of memory as a 2-tuple consisting
of a pointer and size in bytes. A tuple of the form {void*, size_t} specifies a mutable (modifiable) region
of memory. Similarly, a tuple of the form {const void*, size_t} specifies a const (non-modifiable) region
of memory. These two forms correspond to the classes mutable_buffer and const_buffer, respectively. To mirror
C++'s conversion rules, a mutable_buffer is implicitly convertible
to a const_buffer,
and the opposite conversion is not permitted.
The simplest use case involves reading or writing a single buffer of a specified size:
sock.send(asio::buffer(data, size));
In the above example, the return value of asio::buffer
meets the requirements of the ConstBufferSequence concept so that it may
be directly passed to the socket's write function. A buffer created for modifiable
memory also meets the requirements of the MutableBufferSequence concept.
An individual buffer may be created from a builtin array, std::vector, std::array or boost::array of POD elements. This helps prevent buffer overruns by automatically determining the size of the buffer:
char d1[128]; size_t bytes_transferred = sock.receive(asio::buffer(d1)); std::vector<char> d2(128); bytes_transferred = sock.receive(asio::buffer(d2)); std::array<char, 128> d3; bytes_transferred = sock.receive(asio::buffer(d3)); boost::array<char, 128> d4; bytes_transferred = sock.receive(asio::buffer(d4));
In all three cases above, the buffers created are exactly 128 bytes long.
Note that a vector is never automatically resized when
creating or using a buffer. The buffer size is determined using the vector's
size()
member function, and not its capacity.
The contents of a buffer may be accessed using the buffer_size and buffer_cast functions:
asio::mutable_buffer b1 = ...; std::size_t s1 = asio::buffer_size(b1); unsigned char* p1 = asio::buffer_cast<unsigned char*>(b1); asio::const_buffer b2 = ...; std::size_t s2 = asio::buffer_size(b2); const void* p2 = asio::buffer_cast<const void*>(b2);
The asio::buffer_cast function permits violations
of type safety, so uses of it in application code should be carefully considered.
For convenience, the buffer_size function also works
on buffer sequences (that is, types meeting the ConstBufferSequence or MutableBufferSequence
type requirements). In this case, the function returns the total size of
all buffers in the sequence.
The buffer_copy
function may be used to copy raw bytes between individual buffers and buffer
sequences.
In particular, when used with the buffer_size , the buffer_copy function can be used
to linearise a sequence of buffers. For example:
vector<const_buffer> buffers = ...; vector<unsigned char> data(asio::buffer_size(buffers)); asio::buffer_copy(asio::buffer(data), buffers);
Note that buffer_copy
is implemented in terms of memcpy,
and consequently it cannot be used to copy between overlapping memory regions.
A buffer object does not have any ownership of the memory it refers to. It is the responsibility of the application to ensure the memory region remains valid until it is no longer required for an I/O operation. When the memory is no longer available, the buffer is said to have been invalidated.
For the asio::buffer overloads that accept an argument
of type std::vector, the buffer objects returned are invalidated by any vector
operation that also invalidates all references, pointers and iterators referring
to the elements in the sequence (C++ Std, 23.2.4)
For the asio::buffer overloads that accept an argument
of type std::basic_string, the buffer objects returned are invalidated according
to the rules defined for invalidation of references, pointers and iterators
referring to elements of the sequence (C++ Std, 21.3).
Buffer objects may be manipulated using simple arithmetic in a safe way which helps prevent buffer overruns. Consider an array initialised as follows:
boost::array<char, 6> a = { 'a', 'b', 'c', 'd', 'e' };
A buffer object b1 created
using:
b1 = asio::buffer(a);
represents the entire array, { 'a', 'b', 'c',
'd', 'e' }. An optional
second argument to the asio::buffer
function may be used to limit the size, in bytes, of the buffer:
b2 = asio::buffer(a, 3);
such that b2 represents the
data { 'a', 'b',
'c' }.
Even if the size argument exceeds the actual size of the array, the size
of the buffer object created will be limited to the array size.
An offset may be applied to an existing buffer to create a new one:
b3 = b1 + 2;
where b3 will set to represent
{ 'c', 'd',
'e' }.
If the offset exceeds the size of the existing buffer, the newly created
buffer will be empty.
Both an offset and size may be specified to create a buffer that corresponds to a specific range of bytes within an existing buffer:
b4 = asio::buffer(b1 + 1, 3);
so that b4 will refer to
the bytes { 'b', 'c',
'd' }.
To read or write using multiple buffers (i.e. scatter-gather I/O), multiple buffer objects may be assigned into a container that supports the MutableBufferSequence (for read) or ConstBufferSequence (for write) concepts:
char d1[128]; std::vector<char> d2(128); boost::array<char, 128> d3; boost::array<mutable_buffer, 3> bufs1 = { asio::buffer(d1), asio::buffer(d2), asio::buffer(d3) }; bytes_transferred = sock.receive(bufs1); std::vector<const_buffer> bufs2; bufs2.push_back(asio::buffer(d1)); bufs2.push_back(asio::buffer(d2)); bufs2.push_back(asio::buffer(d3)); bytes_transferred = sock.send(bufs2);
Header: asio/buffer.hpp
Convenience header: asio.hpp