Streams, Short Reads and Short Writes

Many I/O objects in Asio are stream-oriented. This means that:

There are no message boundaries. The data being transferred is a continuous sequence of bytes.
Read or write operations may transfer fewer bytes than requested. This is referred to as a short read or short write.

Objects that provide stream-oriented I/O model one or more of the following type requirements:

SyncReadStream, where synchronous read operations are performed using a member function called read_some().
AsyncReadStream, where asynchronous read operations are performed using a member function called async_read_some().
SyncWriteStream, where synchronous write operations are performed using a member function called write_some().
AsyncWriteStream, where asynchronous write operations are performed using a member function called async_write_some().

Examples of stream-oriented I/O objects include ip::tcp::socket, ssl::stream<>, posix::stream_descriptor, windows::stream_handle, etc.

Programs typically want to transfer an exact number of bytes. When a short read or short write occurs the program must restart the operation, and continue to do so until the required number of bytes has been transferred. Asio provides generic functions that do this automatically: read(), async_read(), write() and async_write().

Why EOF is an Error

The end of a stream can cause read, async_read, read_until or async_read_until functions to violate their contract. E.g. a read of N bytes may finish early due to EOF.
An EOF error may be used to distinguish the end of a stream from a successful read of size 0.

Streams, Short Reads and Short Writes

Why EOF is an Error

See Also