Syntax conventions for the VFS callable services

A callable service is a programming interface that uses the CALL macro to access system services. To code a callable service, code the CALL macro followed by the name of the callable service and a parameter list. A syntax diagram for a callable service follows.

 CALL Service_name,(Parm_1,
                    Parm_2,
                    .
                    .
                    Return_value,
                    Return_code,
                    Reason_code)

This format does not show the assembler column dependence (columns 1, 10, 16, and 72) or parameter list options (VL and MF). The exact syntax is shown in the examples in Callable services examples.

When you code a callable service:

You must code all the parameters in the parameter list, because parameters are positional in a callable service interface. That is, the function of each parameter is determined by its position with respect to the other parameters in the list. Omitting a parameter, therefore, assigns the omitted parameter's function to the next parameter in the list.
You must place values explicitly into all supplied parameters, because callable services do not set defaults.