This chapter presents a program that reads NSCLDAQ ring items from a data source. The full source code for the program is in $DAQROOT/share/recipies/readrings a Makefile is included. Background provides some background. If you just want to dive into the source code, by all means, just skip ahead to The code where the code is presented and deescribed.
This program introduces severa class families that are used throughout the NSCLDAQ software. The CRingItem class is the
base class for objects that are data items created by the data acquisition system.
Since we're reading ring items from a data source, the classes that are important to that process are:
CDataSourceThe base class for sources of data in the NSCLDAQ. These are objects that provide a source of data. This class is an abstract base class.
CFileDataSourceIs a concrete data source that provides data from a file.
CRingDataSourceIs a concrete data source that provides data from a ring buffer. Note that the ringbufer could be a local ringbuffer or a remote ring buffer. If necessary, the class will setup the proxy ring and feeder processes needed to transfer data from a remote ringbuffer to the proxy ringbuffer in the local host.
In the NSCLDAQ, data sources are described via Uniform Resource Identifiers (URIs). File data sources have URIs that look like file:///path/to/the/file. Ring buffer URIs on the other hand, look like tcp://hostname/ringbuffername.
The CDataSourceFactory class accepts an NSCLDAQ data source URI and returns a pointer to the appropriate type of
CDataSource.
Once your program is finished using a data source, it should be deleted. In the case of a ring data source, the destruction of that object also disconnects as a consumer of the ringbuffer.
So we have a mechanism to get something from an NSCLDAQ file or ring buffer: Use the data source factory to create a data source. But what do we get from data sources?
Data sources return pointers to CRingItem objects.
CRingItem is the base class of a hierarchy of classes that exist for each of the supported ring item classes. in
Peforming type independent processing we'll look in greater detail at this
class hierarchy and how to use it.