User`s guide

Subroutines
There are three methods of exchanging information between programs:
l global variables
l soft-signals
l program argument list
When using global variables, simply use the same variable names in the different programs.
Unless used carefully, this method can make program execution unpredictable and hard to
debug. It also makes it difficult to write generalized subroutines because the variable names
in the main program and subroutine must always be the same.
Soft-signals are internal program signals. These are digital software switches whose state can
be read and set by all tasks and programs (including across CPUs in multiple CPU systems).
See "Soft Signals" for details.
Exchanging information through the program argument list gives you better control over
changes made to variables. It also eliminates the requirement that the variable names in the
calling program be the same as the names in the subroutine. The following sections describe
exchanging data through the program parameter list.
Argument Passing
There are two important considerations when passing an argument list from a calling
program to a subroutine. The first is making sure the calling program passes arguments in
the way the subroutine expects to receive them (mapping). The second is determining how
you want the subroutine to be able to alter the variables (passing by value or reference).
Mapping the Argument List
An argument list is a list of variables or values separated by commas. The argument list
passed to a calling program must match the subroutine's argument list in number of
arguments and data type of each argument (see Undefined Arguments on page 45). The
variable names do not have to match.
When a calling program passes an argument list to a subroutine, the subroutine does not look
at the variable names in the list but the position of the arguments in the list. The argument
list in the CALL statement is mapped item for item to the argument list of the subroutine. It is
this mapping feature that allows you to write generalized subroutines that can be called by
any number of different programs, regardless of the actual values or variable names the
calling program uses.
The following figure shows the mapping of an argument list in a CALL statement to the
argument list in a subroutine. The arrows indicate that each item in the list must match in
position and data type but not necessarily in name. (The CALL statement argument list can
include values and expressions as well as variable names.)
Subroutines
V+Language User's Guide, v17.0
Page 42