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<H2><A NAME="SECTION00051000000000000000"></A><A NAME="xref_arguments"></A><A NAME="args"></A>
<BR>
Arguments
</H2>
<P>
This is the main method for passing data between a calling program and the
called subprogram. The calling program takes the actual arguments of the call
to the subprogram, constructs an argument list and then passes execution to
that routine. The subprogram then uses the values in the argument list to
access the actual arguments. It may pass data back to the calling program by
modifying the data in some or all of the arguments. As far as passing arguments
between a program and a subprogram is concerned, the principal difference
between FORTRAN and C is the method used for passing the arguments.
<P>
Note that the above paragraph refers to modifying arguments.
On all the machines we support, the actual contents of the argument list is
never modified.
What may be modified is the contents of the location pointed to by an element
of the argument list. This is not to say that other computers would not modify
the argument list itself.
<P>
There are three commonly used methods for passing subprogram arguments: call by
value, call by reference and call by descriptor. Call by value passes the
actual value of the argument to the called routine, call by reference passes a
pointer to the value of the argument (<I>i.e.</I> the address of the
argument) and call by descriptor passes a pointer to a structure describing
the argument.
<P>
Although these are the basic methods of passing arguments, a particular type of
argument may be passed by a combination of these. For example, some compilers
use a combination of call by reference and call by value to pass character
arguments.
What is common is that all arguments are passed by exchanging data values.
It is how those values are to be interpreted that gives rise to the different
mechanisms.
<P>
The FORTRAN standard does not specify how arguments should be passed to
subprograms and indeed different compilers for different machines do use
different methods. It is most usual for numeric data types to be passed by
reference since the subprogram may modify the value of the argument. This is
most easily achieved by passing a pointer to the storage location containing
the data value, rather than a copy of the value itself. On the other hand, the
C standard explicitly states that values cannot be returned to the calling
routine directly through arguments and so call by value is most commonly used.
It is worth recalling that the argument list of a routine is simply a sequence
of computer words. If these are a list of addresses of data values then
everything is simple. However, suppose that an array was passed by value. This
would mean that the compiler would have to arrange for a copy of the entire
array to be placed in the argument list that was passed to the called
subprogram. Consequently, arrays are invariably passed by reference or by
descriptor, never by value.
<P>
It may seem tedious to have to think about the actual mechanisms that a
compiler uses to pass data between routines when all you want to do is to get
on with your programming. However, understanding this is the key to mixed
language programming. Fortunately the facilities described in
<A HREF="node18.html#f77_cfromf">More on Calling C from FORTRAN</A>
and
<A HREF="node36.html#f77_ffromc">More on Calling FORTRAN from C</A>
hide much of this from the programmer.
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<ADDRESS>
<I>CNF and F77 Mixed Language Programming -- FORTRAN and C<BR>Starlink User Note 209<BR>P.M. Allan<BR>A.J. Chipperfield<BR>R.F. Warren-Smith<BR>19 January 2000<BR>E-mail:<A HREF="mailto:ussc@star.rl.ac.uk">ussc@star.rl.ac.uk</A></I>
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