This chapter contains the following sections:
A User’s Perspective¶
To use a vconnection, a user must first get a handle to one. This
is usually accomplished by having it handed to the user; the user
may also simply issue a call that creates a vconnection (such as
c:func:TSNetConnect). In the case of transform plugins, the plugin
creates a transformation vconnection viav
and then accesses the output vconnection using
After getting a handle to a vconnection, the user can then issue a read or write call. It’s important to note that not all vconnections support both reading and writing - as of yet, there has not been a need to query a vconnection about whether it can perform a read or write operation. That ability should be obvious from context.
To issue a read or write operation, a user calls
TSVConnWrite(). These two operations both return
VIO (TSVIO). The
VIO describes the operation being performed and how much progress has
been made. Transform plugins initiate output to the downstream
vconnection by calling
A vconnection read or write operation is different from a normal UNIX
read(2) or write(2) operation. Specifically, the vconnection
operation can specify more data to be read or written than exists in the
buffer handed to the operation. For example, it’s typical to issue a
INT64_MAX (9 quintillion) bytes from a network vconnection
in order to read all the data from the network connection until the end
of stream is reached. This contrasts with the usual UNIX fashion of
issuing repeated calls to read(2) until one of the calls finally
0 to indicate the end of stream was reached (indeed, the
underlying implementation of vconnections on UNIX still does issue those
calls to read(2), but the interface does not expose that detail).
At most, a given vconnection can have one read operation and one write operation being performed on it. This is restricted both by design and common sense: if two write operations were performed on a single vconnection, then the user would not be able to specify which should occur first and the output would occur in an intermingled fashion. Note that both a read operation and a write operation can happen on a single vconnection at the same time; the restriction is for more than one operation of the same type.
One obvious issue is that the buffer passed to
TSVConnWrite() won’t be large enough - there is no reasonable way to
make a buffer that can hold
INT64_MAX (9 quintillion) bytes! The
secret is that vconnections engage in a protocol whereby they signal to
the user (via the continuation passed to
TSVConnWrite()) that they have emptied the buffers passed to them and
are ready for more data. When this occurs, it is up to the user to add
more data to the buffers (or wait for more data to be added) and then
wake up the vconnection by calling
TSVIOReenable() on the VIO
describing the operation.
TSVIOReenable() specifies that the buffer
for the operation has been modified and that the vconnection should
reexamine it to see if it can make further progress.
The null transform plugin provides an example of how this is done. Below
is a prototype for
TSVIO TSVConnWrite (TSVConn connp, TSCont contp, TSIOBufferReader readerp, int nbytes)
connp is the vconnection the user is writing to and
the “user” - i.e., the continuation that
connp calls back when it
has emptied its buffer and is ready for more data.
The call made in the null transform plugin is:
TSVConnWrite (output_conn, contp, data->output_reader, TSVIONBytesGet (input_vio));
In the example above,
contp is the transformation vconnection that
is writing to the output vconnection. The number of bytes to be written
is obtained from
When a vconnection calls back its user to indicate that it wants more
data (or when some other condition has occurred), it issues a call to
TSContCall(). It passes the
TSVIO describing the operation as the
data parameter, and one of the values below as the event parameter.
- Indicates an error has occurred on the vconnection. This will happen for network IO if the underlying read(2) or write(2) call returns an error.
- The vconnection has placed data in the buffer passed to an
TSVConnRead()operation and it would like to do more IO, but the buffer is now full. When the user consumes the data from the buffer, this should re-enable the VIO so it indicates to the vconnection that the buffer has been modified.
- The vconnection has removed data from the buffer passed to an
TSVConnWrite()operation and it would like to do more IO, but the buffer does not have enough data in it. When placing more data in the buffer, the user should re-enable the VIO so it indicates to the vconnection that the buffer has been modified.
- The vconnection has read all the bytes specified by an
TSVConnRead()operation. The vconnection can now be used to initiate a new IO operation.
- The vconnection has written all the bytes specified by an
TSVConnWrite()operation and can now be used to initiate a new IO operation.
- An attempt was made to read past the end of the stream of bytes
during the handling of an
TSVConnRead()operation. This event occurs when the number of bytes available for reading from a vconnection is less than the number of bytes the user specifies should be read from the vconnection in a call to
TSVConnRead(). A common case where this occurs is when the user specifies that
INT64_MAXbytes are to be read from a network connection.
For example: the null transform plugin’s transformation receives
events from the downstream vconnection as a result of the call to
After using a vconnection, the user must call
TSVConnAbort(). While both calls indicate that the vconnection can
TSVConnAbort() should be used when the connection is
being closed abnormally. After a call to
TSVConnAbort(), the user will not be called back by the vconnection
Sometimes it’s desirable to simply close down the write portion of a
connection while keeping the read portion open. This can be accomplished
TSVConnShutdown() function, which shuts down either the read
or write portion of a vconnection. Shutdown means that the vconnection
will no longer call back the user with events for the portion of the
connection that was shut down. For example: if the user shuts down the
write portion of a connection, then the
TS_EVENT_VCONN_WRITE_COMPLETE events will not be produced. In the
null transform plugin, the write operation is shut down with a call to
TSVConnShutdown(). To learn how vconnections are used in
transformation plugins, see Writing Content Transform
The vconnection functions are listed below: