+
+
+As mentioned above, a GDD can describe an n-dimension array. The user +can describe the bounds of the n-dimensional array. To facilitate the +use of large, and perhaps shared data arrays, a GDD allows a user to +store a reference to an array of data. In addition, a destructor function +can be registed in the GDD to inform the owner of the data array when +the GDD referencing the data goes away. The purpose of the destructor +function is to delete the array of data, since the GDD does not know +what to do with referenced data. +
+To manage GDDs in a multi-tasking system or a system that +uses many layers of software, GDDs implement reference counting. With +reference counting, only one copy of GDD can be shared by many subsystems. +The GDD creator function can pass it to many other functions without +worrying about deleting it, when the last function using the GDD requests +that it be destroyed does it really go away. +
+As menitioned above, a GDD allows the user to describe the data in terms +of the application. This is done by the user by assigning an arbitrary +integer identifier to a GDD. The user places a meaning on the identifiers +such as 1=high-alarm-limit, 2=low-alarm-limit. This identifier is termed +application type. A second component of the GDD library known as the +Application Type Table is used to manage the application type identifiers. +Application type values are registered in the table along with a text +string and optionally a prototype GDD. The prototype GDD can be a +container GDD. The table allows users to retreive GDDs of a specific +application type. +
+A GDD describe and manages a piece of data using the following information: +
+
+The GDD library is a C++ class library and therefore requires using the +C++ compiler. + +
+All GDDs must be created dynamically, a GDD cannot be created on the +stack as a local variable. The gdd class forbids the user from deleting +the gdd. In order for referencing counting to work correctly the user must +"unreference" the gdd instance instead of delete it. The gdd class does +take over the memory management routines for itself and all it's subclass. +This means that you are not using the malloc()/free() memory management when +GDDs are created and destroyed. It is important to remember that since +reference counting is used, a GDD passed into a function must be referenced +before the function returns if the GDD is to be kept for longer then the +running of that function. In other words, if you are creating a library +function "add" that records process variable names in a linked list, and the +process variable names are passed to you as GDDs, then you must reference +the GDD since the linked list exists after the return of the "add" function. +If you are creating a GDD, you must unreference it when you are finished +with it, even it you have passed it into other library functions. +Generalizing on this, it is the responsibility of the GDD creator or +GDD referencer to unreference the GDD instance when they are finished +with it. +
+
+ aitInt8 8 bit character + aitUint8 8 bit unsigned character + aitInt16 16 bit short + aitUint16 16 bit unsigned short + aitEnum16 16 enumerated value + aitInt32 32 bit integer + aitUint32 32 bit unsigned integer + aitFloat32 32 bit floating point number + aitFloat64 64 bit floating point number + aitPointer Standard pointer + aitIndex 32 bit index value + aitStatus 32 bit unsigned integer for status value + aitFixedString 40 byte string of characters + aitString Variable length string data type + aitTimeStamp Two 32 bit integers describing time (seconds/nanoseconds) ++These data types should be used whenever possible to prevent problems when +compiling programs for different architectures. Most of they data types +are enumerated as descrived above for use as a primitive type code. The +enumerated names are just the above type names with the word "Enum" +inserted after "ait". It should be noted that aitTimeStamp is not a +standard primitive type. +
+typedef enum {
+ aitEnumInvalid=0,
+ aitEnumInt8,
+ aitEnumUint8,
+ aitEnumInt16,
+ aitEnumUint16,
+ aitEnumEnum16,
+ aitEnumInt32,
+ aitEnumUint32,
+ aitEnumFloat32,
+ aitEnumFloat64,
+ aitEnumFixedString,
+ aitEnumString,
+ aitEnumContainer
+} aitEnum;
+
+The enumerated type code allows a user to dynamically convert from one
+type to another. The AIT portion of the GDD library contains a large
+primitive type conversion matrix. The conversion matrix is indexed by
+the source and destination enumeration type codes. The matrix is a
+
++Several important issues related to where the actual data is stored +must be remembered when using GDDs: +