g77.info: %DESCR()

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.

   *Note Procedures (SUBROUTINE and FUNCTION): Procedures, for detailed
information on how this particular version of `g77' passes arguments to
procedures.