Shared Memory Transport
[Built-in Transport Plugins]

Built-in transport plug-in for inter-process communications using shared memory (NDDS_TRANSPORT_CLASSID_SHMEM). More...


Classes
struct	NDDS_Transport_Shmem_Property_t
	Subclass of NDDS_Transport_Property_t allowing specification of parameters that are specific to the shared-memory transport. More...
Defines
#define	NDDS_TRANSPORT_CLASSID_SHMEM (2)
	Builtin Shared-Memory Transport-Plugin class ID.
#define	NDDS_TRANSPORT_SHMEM_ADDRESS_BIT_COUNT (0)
	Default value of NDDS_Transport_Property_t::address_bit_count.
#define	NDDS_TRANSPORT_SHMEM_PROPERTIES_BITMAP_DEFAULT ( NDDS_TRANSPORT_PROPERTY_BIT_BUFFER_ALWAYS_LOANED )
	Default value of NDDS_Transport_Property_t::properties_bitmap.
#define	NDDS_TRANSPORT_SHMEM_GATHER_SEND_BUFFER_COUNT_MAX_DEFAULT (1024)
	Default value of NDDS_Transport_Property_t::gather_send_buffer_count_max.
#define	NDDS_TRANSPORT_SHMEM_MESSAGE_SIZE_MAX_DEFAULT (9216)
	Default value of NDDS_Transport_Property_t::message_size_max.
#define	NDDS_TRANSPORT_SHMEM_RECEIVED_MESSAGE_COUNT_MAX_DEFAULT (32)
	Default value of NDDS_Transport_Shmem_Property_t::received_message_count_max.
#define	NDDS_TRANSPORT_SHMEM_RECEIVE_BUFFER_SIZE_DEFAULT
	Default value of NDDS_Transport_Shmem_Property_t::receive_buffer_size.
#define	NDDS_TRANSPORT_SHMEM_PROPERTY_DEFAULT
	Use this to initialize stack variable.
Functions
NDDS_Transport_Plugin *	NDDS_Transport_Shmem_new (const struct NDDS_Transport_Shmem_Property_t *property_in)
	Create a new shmem process transport.
NDDS_Transport_Plugin *	NDDS_Transport_Shmem_create (NDDS_Transport_Address_t default_network_address_out, const struct DDS_PropertyQosPolicy property_in)
	Create a new shmem process transport, using PropertyQosPolicy.

Detailed Description

Built-in transport plug-in for inter-process communications using shared memory (NDDS_TRANSPORT_CLASSID_SHMEM).

This plugin uses System Shared Memory to send messages between processes on the same node.

The transport plugin has exactly one "receive interface"; since the address_bit_count is 0, it can be assigned any address. Thus the interface is located by the "network address" associated with the transport plugin.

Compatibility of Sender and Receiver Transports

Opening a receiver "port" on shared memory corresponds to creating a shared memory segment using a name based on the port number. The transport plugin's properties are embedded in the shared memory segment.

When a sender tries to send to the shared memory port, it verifies that properties of the receiver's shared memory transport are compatible with those specified in its transport plugin. If not, the sender will fail to attach to the port and will output messages such as below (with numbers appropriate to the properties of the transport plugins involved).

  NDDS_Transport_Shmem_attachShmem:failed to initialize incompatible properties
  NDDS_Transport_Shmem_attachShmem:countMax 0 > -19417345 or max size -19416188 > 2147482624

In this scenario, the properties of the sender or receiver transport plugin instances should be adjusted, so that they are compatible.

Crashing and Restarting Programs

If a process using shared memory crashes (say because the user typed in ^C), resources associated with its shared memory ports may not be properly cleaned up. Later, if another RTI Data Distribution Service process needs to open the same ports (say, the crashed program is restarted), it will attempt to reuse the shared memory segment left behind by the crashed process.

The reuse is allowed iff the properties of transport plugin are compatible with those embedded in the shared memory segment (i.e., of the original creator). Otherwise, the process will fail to open the ports, and will output messages such as below (with numbers appropriate to the properties of the transport plugins involved).

  NDDS_Transport_Shmem_create_recvresource_rrEA:failed to initialize shared 
  	memory resource Cannot recycle existing shmem: size not compatible for key 0x1234

In this scenario, the shared memory segments must be cleaned up using appropriate platform specific commands. For details, please refer to the platform notes.

Shared Resource Keys

The transport uses the shared memory segment keys, given by the formula below.

                  0x400000 + port

The transport also uses signaling shared semaphore keys given by the formula below.

                  0x800000 + port

The transport also uses mutex shared semaphore keys given by the formula below.

                  0xb00000 + port

wher the port is a function of the domain_id and the participant_id, as described in DDS_WireProtocolQosPolicy::participant_id

See also:: DDS_WireProtocolQosPolicy::participant_id
NDDSTransportSupport::set_builtin_transport_property()

Creating and Registering Shared Memory Transport Plugin

RTI Data Distribution Service can implicitly create this plugin and register with the DDSDomainParticipant if this transport is specified in DDS_TransportBuiltinQosPolicy.

To specify the properties of the builtin shared memory transport that is implicitly registered, you can either:

call NDDSTransportSupport::set_builtin_transport_property or
specify the pre-defined property names in DDS_PropertyQosPolicy associated with the DDSDomainParticipant. (see Shared Memory Transport Property Names in Property QoS Policy of Domain Participant).

Builtin transport plugin properties specified in DDS_PropertyQosPolicy always overwrite the ones specified through NDDSTransportSupport::set_builtin_transport_property(). The default value is assumed on any unspecified property.

Note that all properties should be set before the transport is implicitly created and registered by RTI Data Distribution Service. See Built-in Transport Plugins for details on when a builtin transport is registered.

To explicitly create an instance of this plugin, NDDS_Transport_Shmem_new() should be called. The instance should be registered with RTI Data Distribution Service, see NDDSTransportSupport::register_transport. In some configurations, you may have to disable the builtin shared memory transport plugin instance (DDS_TransportBuiltinQosPolicy, DDS_TRANSPORTBUILTIN_SHMEM), to avoid port conflicts with the newly created plugin instance.

Shared Memory Transport Property Names in Property QoS Policy of Domain Participant

The following table lists the predefined property names that can be set in the DDS_PropertyQosPolicy of a DDSDomainParticipant to configure the builtin shared memory transport plugin.

*Property Strings for Shared Memory Transport*
Name	Descriptions
dds.transport.shmem.builtin.parent.address_bit_count	See NDDS_Transport_Property_t::address_bit_count
dds.transport.shmem.builtin.parent.properties_bitmap	See NDDS_Transport_Property_t::properties_bitmap
dds.transport.shmem.builtin.parent.gather_send_buffer_count_max	See NDDS_Transport_Property_t::gather_send_buffer_count_max
dds.transport.shmem.builtin.parent.message_size_max	See NDDS_Transport_Property_t::message_size_max
dds.transport.shmem.builtin.parent.allow_interfaces	See NDDS_Transport_Property_t::allow_interfaces_list and NDDS_Transport_Property_t::allow_interfaces_list_length. Interfaces should be specified as comma-separated strings, with each comma delimiting an interface. For example: 127.0.0.1,eth0
dds.transport.shmem.builtin.parent.deny_interfaces	See NDDS_Transport_Property_t::deny_interfaces_list and NDDS_Transport_Property_t::deny_interfaces_list_length. Interfaces should be specified as comma-separated strings, with each comma delimiting an interface. For example: 127.0.0.1,eth0
dds.transport.shmem.builtin.parent.allow_multicast_interfaces	See NDDS_Transport_Property_t::allow_multicast_interfaces_list and NDDS_Transport_Property_t::allow_multicast_interfaces_list_length. Interfaces should be specified as comma-separated strings, with each comma delimiting an interface. For example: 127.0.0.1,eth0
dds.transport.shmem.builtin.parent.deny_multicast_interfaces	See NDDS_Transport_Property_t::deny_multicast_interfaces_list and NDDS_Transport_Property_t::deny_multicast_interfaces_list_length. Interfaces should be specified as comma-separated strings, with each comma delimiting an interface. For example: 127.0.0.1,eth0
dds.transport.shmem.builtin.received_message_count_max	See NDDS_Transport_Shmem_Property_t::received_message_count_max
dds.transport.shmem.builtin.receive_buffer_size	See NDDS_Transport_Shmem_Property_t::receive_buffer_size

Define Documentation

#define NDDS_TRANSPORT_CLASSID_SHMEM (2)

Builtin Shared-Memory Transport-Plugin class ID.

#define NDDS_TRANSPORT_SHMEM_ADDRESS_BIT_COUNT (0)

Default value of NDDS_Transport_Property_t::address_bit_count.

#define NDDS_TRANSPORT_SHMEM_PROPERTIES_BITMAP_DEFAULT ( NDDS_TRANSPORT_PROPERTY_BIT_BUFFER_ALWAYS_LOANED )

Default value of NDDS_Transport_Property_t::properties_bitmap.

#define NDDS_TRANSPORT_SHMEM_GATHER_SEND_BUFFER_COUNT_MAX_DEFAULT (1024)

Default value of NDDS_Transport_Property_t::gather_send_buffer_count_max.

#define NDDS_TRANSPORT_SHMEM_MESSAGE_SIZE_MAX_DEFAULT (9216)

Default value of NDDS_Transport_Property_t::message_size_max.

#define NDDS_TRANSPORT_SHMEM_RECEIVED_MESSAGE_COUNT_MAX_DEFAULT (32)

Default value of NDDS_Transport_Shmem_Property_t::received_message_count_max.

#define NDDS_TRANSPORT_SHMEM_RECEIVE_BUFFER_SIZE_DEFAULT

Value:

(NDDS_TRANSPORT_SHMEM_RECEIVED_MESSAGE_COUNT_MAX_DEFAULT * \
         NDDS_TRANSPORT_SHMEM_MESSAGE_SIZE_MAX_DEFAULT / 4)

Default value of NDDS_Transport_Shmem_Property_t::receive_buffer_size.

#define NDDS_TRANSPORT_SHMEM_PROPERTY_DEFAULT

Use this to initialize stack variable.

Function Documentation

NDDS_Transport_Plugin* NDDS_Transport_Shmem_new ( const struct NDDS_Transport_Shmem_Property_t * property_in )

Create a new shmem process transport.

An application may create multiple transports, possibly for use in different domains.

Parameters:

property_in

<<in>> Desired behavior of this transport. May be NULL for default property. The transport plugin can only support one unicast receive interface; therefore the interface selection lists are ignored.

Returns:: handle to a Shmem inter-process Transport Plugin on success
NULL on failure.

NDDS_Transport_Plugin* NDDS_Transport_Shmem_create	(	NDDS_Transport_Address_t *	default_network_address_out,
		const struct DDS_PropertyQosPolicy *	property_in
	)

Create a new shmem process transport, using PropertyQosPolicy.