EvmConnection(5) File Formats Manual EvmConnection(5)
NAME
EvmConnection - Connection to the Event Management daemon
DESCRIPTION
An EVM connection is the context through which data is passed to and from the EVM daemon. A connection has the following attributes: Con-
nection Type Response Mode Transport Type Callback, and associated Callback Argument Connection Context
These attributes are described in the following sections.
An EVM client is any program that processes events through the EVM daemon. EVM supports three distinct types of client: posting clients,
subscribing (listening) clients, and service clients. Regardless of type, all clients connect to the EVM daemon in the same way.
Connection Type
The type of connection established determines the client type. There are associated constants to be used when the connection is created.
The client uses this connection to post events to the daemon for distribution. The client uses this connection to listen for events dis-
tributed by the daemon. The client uses this connection to request that the daemon provide a service, such as retrieve events from a log.
A client may use all three types of connection, but must establish each connection separately.
Response Mode
The response mode associated with a connection determines the manner in which certain API functions will deal with the daemon's responses
to request messages. See the EvmConnCreate(3) reference page for full details of each mode. The modes are: The API functions will return
as soon as the request has been sent to the daemon, and the caller will not receive the daemon's response. The API functions will send the
request to the daemon, and wait until a response has been received before returning to the caller. The returned status code will reflect
the response. The API functions will return as soon as the request has been sent to the daemon, and the caller must monitor the connection
for a response. When the response is received, the connection's callback function will be invoked to handle it.
Transport type
This specifies the type of connection to be made to the daemon. Valid connection types are as follows: Connection is through a domain
socket to a daemon running on the local host. Connection is through a TCP/IP socket to a daemon running on a remote host.
Callback
This attribute specifies the function you want to handle any incoming responses resulting from activity on the connection. Callbacks are
discussed in more detail in the EvmCallback(5) reference page. This attribute is valid only when the Response Mode is EvmRESPONSE_CALL-
BACK.
Connection Context
This attribute is a handle to a connection returned when a connection is created. You must pass this handle to all other functions you call
which need to access the connection.
Connection Monitoring
Once you have established a connection, the API functions will take care of all communication activity with the daemon. However, you must
make sure that these functions get an opportunity to do their work when activity occurs. EVM provides several ways for you to do this -
the method you choose depends on the model of your program. If your program is I/O driven, waiting for I/O to occur on one or more file
descriptors, handling activity as it occurs and then returning to wait for more I/O, it probably spends most of its time in a select()
call. In this case, you should use EvmConnFdGet() to establish which file descriptor the EVM connection is using, and then include that fd
in your select() read mask. When you detect activity on the connection, invoke EvmConnDispatch() to handle the activity. If your program
is driven purely by activity on a single EVM connection, you can let the API handle the I/O entirely by using EvmConnWait() to wait for
activity to occur on the connection. When the function returns, dispatch the I/O using EvmConnDispatch(), and then return to EvmConnWait().
If your program is driven in some other way, and there are certain points (for example, at the end of some control loop) at which you want
to handle EVM activity, you can call EvmConnCheck() to check for any outstanding activity. If this function indicates that there is some-
thing to do, you can call EvmConnDispatch(); otherwise you can continue immediately with normal processing.
Destroying a connection
When you are finished with the connection, use EvmConnDestroy() to disconnect from the daemon. It is important to check the return status
for any failure each time you call a connection function and destroy the connection if a failure occurred. The file descriptor associated
with the connection remains open until you destroy the connection even if I/O errors have been detected on the connection.
SEE ALSO
Functions: select(2)
Routines: EvmConnControl(3), EvmConnCreate(3), EvmConnFdGet(3), EvmConnCheck(3), EvmConnWait(3), EvmConnDispatch(3), EvmConnDestroy(3)
Event Management: EVM(5)
Event Callback: EvmCallback(5)
EVM Events: EvmEvent(5)
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EvmConnection(5)