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Using and Porting GNU Fortran

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8.11.2 The %REF() Construct

 
%REF(arg)

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

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

Use of %REF() 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.

Do not depend on %REF() supplying a pointer to the procedure being invoked. While that is a likely implementation choice, other implementation choices are available that preserve Fortran pass-by-reference semantics without passing a pointer to the argument, arg. (For example, a copy-in/copy-out implementation.)

Implementation Note: Currently, g77 passes all arguments (other than variables and arrays of type CHARACTER) by reference. Future versions of, or dialects supported by, g77 might not pass CHARACTER functions by reference.

Thus, use of %REF() tends to be restricted to cases where arg is type CHARACTER but the called procedure accesses it via a means other than the method used for Fortran CHARACTER arguments.

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


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