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Using and Porting GNU Fortran

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8.11.3 The %DESCR() Construct

 
%DESCR(arg)

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

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

Use of %DESCR() 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 %DESCR() supplying a pointer and/or a length passed by value to the procedure being invoked. While that is a likely implementation choice, other implementation choices are available that preserve the pass-by-reference semantics without passing a pointer to the argument, arg. (For example, a copy-in/copy-out implementation.) And, future versions of g77 might change the way descriptors are implemented, such as passing a single argument pointing to a record containing the pointer/length information instead of passing that same information via two arguments as it currently does.

Implementation Note: Currently, g77 passes all variables and arrays of type CHARACTER by descriptor. Future versions of, or dialects supported by, g77 might pass CHARACTER functions by descriptor as well.

Thus, use of %DESCR() tends to be restricted to cases where arg is not type CHARACTER but the called procedure accesses it via a means similar to 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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