Storage for an automatic variable is allocated with each activation of the procedure or the BEGIN block in which the variable is declared. Storage is allocated within the stack frame that represents that block activation. If a block is activated recursively, each of its stack frames contains a distinct instance of all automatic variables declared in that block.
When a block activation ends, its stack frame is popped from the stack, thereby freeing the storage of all automatic variables allocated within it. This phenomenon normally occurs when a procedure returns to its caller or when a BEGIN block executes its END statement; however, it also occurs when a GOTO statement transfers control back to a previous block activation. In the latter case, all block activations between the one to which control is transferred and the one in which the GOTO was executed are ended. Their stack frames are popped from the stack, releasing the storage for all of their automatic variables. The INITIAL attribute can be specified in the declaration of an automatic variable to initialize the variable.
Extents of automatic character string and array variables may be specified as integer valued expressions, provided these do not contain references to other non-static variables declared in the same block. Extent expressions for these automatic variables are evaluated each time the containing block is activated. The extent expression values are saved in the stack frame and effectively fix the size of the automatic variable for that block activation. Subsequent assignment to a variable used as an extent does not affect the size of the associated automatic variable. For example:
DECLARE STRING_SIZE FIXED; . . . COPY: BEGIN; DECLARE TEXT CHARACTER(STRING_SIZE);
When the BEGIN block in this example is activated, the extent of TEXT is evaluated. The variable is allocated storage depending on the value of STRING_SIZE, which must be a valid integer value. For example:
DECLARE A(N) FIXED; . . . N = 10; /* WILL NOT AFFECT THE SIZE OF ARRAY A */
In this example, the size of A is determined upon entry to the containing block by evaluating N and storing its value in the stack frame. The assignment to N does not change the size of A. Upon entry to the block, N must have a correct value and must not be a non-static variable declared in this block. Use of the HBOUND built-in function within this block activation would return the original value of N that was saved in the stack frame.