12.3.9 Activation Records Whose Size Is Known at Unit Activation
There are language constructs where neither the size of the activation record nor the position of the information within the activation record is known until the unit begins execution.
One such construct is the dynamic array construct. Here, a unit can declare an array to have dimensions that are not fixed until run-time. Example 4 shows the program from Examples 2 and 3, with such a dynamic array declaration and its activation record structure.
In Example 4, the dimensions for P3 are known when procedure P is activated (called).
Clearly, if the values for array P3 are to kept in an activation record for P3, the size of the record cannot be fixed at translation time. If a is given a value within P, as well as within Main, it is possible that the activation record for P will be a different size for each invocation.
What can be created at compile-time is space in the activation record to store the size and bounds of the array. A place containing a pointer to the beginning of the array can also be created (this is necessary if there is more than one such dynamic structure). At execution time, the record can be fully expanded to the appropriate size to contain all the values or the values can be kept somewhere else, say on a heap (described below).
Variables, like the dynamic arrays just described, are called semidynamic variables. Space for them is allocated by reserving storage in the activation record for a descriptor of the semidynamic variable. This descriptor might be a pointer to the storage area as well as the upper and lower bounds of each dimension.
At run-time the storage required for semidynamic variables is allocated. The dimension entries are entered in the descriptor, the actual size of semidynamic variable is evaluated and the activation record is expanded to include space for the variable or a call is made to the operating system for space and the descriptor pointer is set to point to the area just allocated.