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Using and Porting GNU Fortran

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8.11.1 The %VAL() Construct

 
%VAL(arg)

The %VAL() construct specifies that an argument, arg, is to be passed by value, instead of by reference or descriptor.

%VAL() is restricted to actual arguments in invocations of external procedures.

Use of %VAL() is recommended only for code that is accessing facilities outside of GNU Fortran, such as operating system or windowing facilities. It is best to constrain such uses to isolated portions of a program--portions the deal specifically and exclusively with low-level, system-dependent facilities. Such portions might well provide a portable interface for use by the program as a whole, but are themselves not portable, and should be thoroughly tested each time they are rebuilt using a new compiler or version of a compiler.

Implementation Note: Currently, g77 passes all arguments either by reference or by descriptor.

Thus, use of %VAL() tends to be restricted to cases where the called procedure is written in a language other than Fortran that supports call-by-value semantics. (C is an example of such a language.)

See section Procedures (SUBROUTINE and FUNCTION), for detailed information on how this particular version of g77 passes arguments to procedures.


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