Most experiments will record event data. Event data files are record the data taken from the experiment event by event. The format of event data files is identical to the format of data in ring buffers. See the chapter: Format of EventData In Ring Buffers for a description of what to expect in an event file. The $DAQROOT/include/DataFormat.h header is also helpful as it provides C/C++ structure definitions for data in ring buffers and event files.
The event logger takes data from any ring (local or remote via the networked proxy ring subsystem). When a begin run item is seen, a new event file is started. Event files are broken into segments. This is done to ensure that files do not exceed maximum sizes set by some filesystems and NFS. By default, the segment size is a bit less than 2Gbytes. When replaying data it is important to keep this in mind, and replay all segments of a run.
The Event logger places output files in a specified directory. By default, this directory is the working directory at the time the application was started. The default ring from which the program accepts data is tcp://localhost/username where username is the name of the user running the program.
The event log program includes a
--oneshot option that
records a single run and then exits. The
option also creates synchronization files in the event file directory that
allow a controlling program to know when the event logger is
running and when it is about to exit.
The control files are:
Created when the program is ready to record data. The controlling program can then start a run.
Created when the program has closed the event file and is about to exit. The controlling program can move any event files to a final directory if desired.
Example 20-1. Taking data from a remote ring
eventlog --source=tcp://spdaq22/fox --path=/user/0400x/stagearea/current --oneshot
The full reference documentation for the program is in the event log reference page.