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(g77-295.info)Alternate Entry Points

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Alternate Entry Points (ENTRY)
==============================

   The GBE does not understand the general concept of alternate entry
points as Fortran provides via the ENTRY statement.  `g77' gets around
this by using an approach to compiling procedures having at least one
`ENTRY' statement that is almost identical to the approach used by
`f2c'.  (An alternate approach could be used that would probably
generate faster, but larger, code that would also be a bit easier to
debug.)

   Information on how `g77' implements `ENTRY' is provided for those
trying to debug such code.  The choice of implementation seems unlikely
to affect code (compiled in other languages) that interfaces to such
code.

   `g77' compiles exactly one public procedure for the primary entry
point of a procedure plus each `ENTRY' point it specifies, as usual.
That is, in terms of the public interface, there is no difference
between

     SUBROUTINE X
     END
     SUBROUTINE Y
     END

and:

     SUBROUTINE X
     ENTRY Y
     END

   The difference between the above two cases lies in the code compiled
for the `X' and `Y' procedures themselves, plus the fact that, for the
second case, an extra internal procedure is compiled.

   For every Fortran procedure with at least one `ENTRY' statement,
`g77' compiles an extra procedure named `__g77_masterfun_X', where X is
the name of the primary entry point (which, in the above case, using
the standard compiler options, would be `x_' in C).

   This extra procedure is compiled as a private procedure--that is, a
procedure not accessible by name to separately compiled modules.  It
contains all the code in the program unit, including the code for the
primary entry point plus for every entry point.  (The code for each
public procedure is quite short, and explained later.)

   The extra procedure has some other interesting characteristics.

   The argument list for this procedure is invented by `g77'.  It
contains a single integer argument named `__g77_which_entrypoint',
passed by value (as in Fortran's `%VAL()' intrinsic), specifying the
entry point index--0 for the primary entry point, 1 for the first entry
point (the first `ENTRY' statement encountered), 2 for the second entry
point, and so on.

   It also contains, for functions returning `CHARACTER' and (when
`-ff2c' is in effect) `COMPLEX' functions, and for functions returning
different types among the `ENTRY' statements (e.g. `REAL FUNCTION R()'
containing `ENTRY I()'), an argument named `__g77_result' that is
expected at run time to contain a pointer to where to store the result
of the entry point.  For `CHARACTER' functions, this storage area is an
array of the appropriate number of characters; for `COMPLEX' functions,
it is the appropriate area for the return type; for
multiple-return-type functions, it is a union of all the supported
return types (which cannot include `CHARACTER', since combining
`CHARACTER' and non-`CHARACTER' return types via `ENTRY' in a single
function is not supported by `g77').

   For `CHARACTER' functions, the `__g77_result' argument is followed
by yet another argument named `__g77_length' that, at run time,
specifies the caller's expected length of the returned value.  Note
that only `CHARACTER*(*)' functions and entry points actually make use
of this argument, even though it is always passed by all callers of
public `CHARACTER' functions (since the caller does not generally know
whether such a function is `CHARACTER*(*)' or whether there are any
other callers that don't have that information).

   The rest of the argument list is the union of all the arguments
specified for all the entry points (in their usual forms, e.g.
`CHARACTER' arguments have extra length arguments, all appended at the
end of this list).  This is considered the "master list" of arguments.

   The code for this procedure has, before the code for the first
executable statement, code much like that for the following Fortran
statement:

            GOTO (100000,100001,100002), __g77_which_entrypoint
     100000 ...code for primary entry point...
     100001 ...code immediately following first ENTRY statement...
     100002 ...code immediately following second ENTRY statement...

(Note that invalid Fortran statement labels and variable names are used
in the above example to highlight the fact that it represents code
generated by the `g77' internals, not code to be written by the user.)

   It is this code that, when the procedure is called, picks which
entry point to start executing.

   Getting back to the public procedures (`x' and `Y' in the original
example), those procedures are fairly simple.  Their interfaces are
just like they would be if they were self-contained procedures (without
`ENTRY'), of course, since that is what the callers expect.  Their code
consists of simply calling the private procedure, described above, with
the appropriate extra arguments (the entry point index, and perhaps a
pointer to a multiple-type- return variable, local to the public
procedure, that contains all the supported returnable non-character
types).  For arguments that are not listed for a given entry point that
are listed for other entry points, and therefore that are in the
"master list" for the private procedure, null pointers (in C, the
`NULL' macro) are passed.  Also, for entry points that are part of a
multiple-type- returning function, code is compiled after the call of
the private procedure to extract from the multi-type union the
appropriate result, depending on the type of the entry point in
question, returning that result to the original caller.

   When debugging a procedure containing alternate entry points, you
can either set a break point on the public procedure itself (e.g.  a
break point on `X' or `Y') or on the private procedure that contains
most of the pertinent code (e.g. `__g77_masterfun_X').  If you do the
former, you should use the debugger's command to "step into" the called
procedure to get to the actual code; with the latter approach, the
break point leaves you right at the actual code, skipping over the
public entry point and its call to the private procedure (unless you
have set a break point there as well, of course).

   Further, the list of dummy arguments that is visible when the
private procedure is active is going to be the expanded version of the
list for whichever particular entry point is active, as explained
above, and the way in which return values are handled might well be
different from how they would be handled for an equivalent single-entry
function.