DD Configuration

By default the DD system is initialised to a rather large number of fifos and buffers (as defined in the header files db.h and fifo.h. These values are maximal allowed values. If they turn out to be too small, then the code has to be recompiled.

On the other hand one often wants to have less semaphores and smaller shared memory segments. This can be achieved by pointing the environment variable DD_CONFIGURATION to a file specifying the configuration. In this case only the specified number of semaphores and shared memory segments are created. The following variables have to be defined:

 
 max_fifo_nb 		 maximum number of fifos

 fifo_depth 		 fifo depth (number of entries per fifo)

 db_maxnbev 		 maximum number of event buffers

 db_maxevsize 		 maximum length of the event buffers

 db_max_buf_vol 		 maximum number of volatile buffers.

• The maximum event size db_maxevsize is the event size for the static buffers. If an event is of bigger size than db_maxevsize, additional memory is alloced (but not more segments than db_max_buf_vol. The system should be set up in a way, such that volatile buffers are only rarely needed.
• The fifo depth should be bigger than the number of buffers (static and volatile).
• The configuration file $DD_CONFIGURATION is read as the system is set up. If at later times processes with other configuration values are attaching to the DD system, they will not change the configuration values (an error message is printed).