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Using and Porting GNU Fortran

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13.9 Adjustable Arrays (DIMENSION)

Adjustable and automatic arrays in Fortran require the implementation (in this case, the g77 compiler) to "memorize" the expressions that dimension the arrays each time the procedure is invoked. This is so that subsequent changes to variables used in those expressions, made during execution of the procedure, do not have any effect on the dimensions of those arrays.

For example:

 
REAL ARRAY(5)
DATA ARRAY/5*2/
CALL X(ARRAY, 5)
END
SUBROUTINE X(A, N)
DIMENSION A(N)
N = 20
PRINT *, N, A
END

Here, the implementation should, when running the program, print something like:

 
20   2.  2.  2.  2.  2.

Note that this shows that while the value of `N' was successfully changed, the size of the `A' array remained at 5 elements.

To support this, g77 generates code that executes before any user code (and before the internally generated computed GOTO to handle alternate entry points, as described below) that evaluates each (nonconstant) expression in the list of subscripts for an array, and saves the result of each such evaluation to be used when determining the size of the array (instead of re-evaluating the expressions).

So, in the above example, when `X' is first invoked, code is executed that copies the value of `N' to a temporary. And that same temporary serves as the actual high bound for the single dimension of the `A' array (the low bound being the constant 1). Since the user program cannot (legitimately) change the value of the temporary during execution of the procedure, the size of the array remains constant during each invocation.

For alternate entry points, the code g77 generates takes into account the possibility that a dummy adjustable array is not actually passed to the actual entry point being invoked at that time. In that case, the public procedure implementing the entry point passes to the master private procedure implementing all the code for the entry points a NULL pointer where a pointer to that adjustable array would be expected. The g77-generated code doesn't attempt to evaluate any of the expressions in the subscripts for an array if the pointer to that array is NULL at run time in such cases. (Don't depend on this particular implementation by writing code that purposely passes NULL pointers where the callee expects adjustable arrays, even if you know the callee won't reference the arrays--nor should you pass NULL pointers for any dummy arguments used in calculating the bounds of such arrays or leave undefined any values used for that purpose in COMMON--because the way g77 implements these things might change in the future!)


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