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Tech Tip: Problems at Runtime When Calling a NAG Routine

What can be wrong when your Fortran program, calling a NAG routine, 'blows up' during execution?

NAG optimization, quadrature or ordinary differential equation routines require the user to specify a function or subroutine in their argument list. Such routines should be declared as EXTERNAL in the calling subprogram. If they are not, and the compiler is unable to determine otherwise, perhaps by a separate call of the routine elsewhere in the subprogram, then the compiler will assume that the argument is a scalar. This will of course result in disastrous consequences at runtime.

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