call Expression

The call expression calls an external function in a library by using the Standard calling convention.

Syntax

The syntax of the call expression is:

call external-function([actual-parameters]);

The external-function identifier can begin with an uppercase or a lowercase character.

Description

The parameters of the call expression can be passed by value or by reference. (For details, see "Defining External Functions", in Chapter 8 of the JADE Development Environment User’s Guide. See also "Calling External Functions from JADE Thin Clients", in Chapter 1 of the JADE External Interface Developer’s Reference.)

A call expression can:

• Be used as an instruction in its own right

• Appear as the right-hand side of an assignment

• Be part of a more-complex expression (for example, a call could be used as one of the parameters to a method call)

The JADE external function facility passes parameters using the following conventions.

• Structured parameter (String and Binary) types

  • Always passed by reference; that is, the address of the String or Binary variable is taken and passed

  • When usage is output or io (which signifies the calling function may update the value), length must be specified in the parameter definition

  • A function of String primitive type is assumed to return a zero-terminated string

  • A function of Binary primitive type must have the length of the Binary specified in the return type

• Simple parameter (Integer, Character, Real, Boolean, Time, and Date) types

  • Parameter is passed by value when the usage is defined as constant or input

  • Parameter is passed by reference when the usage is defined as output or io

Example

The following example shows the use of the call expression.

getIniParameter(key: String; default: String): String;
vars
    value : String[100];
begin
    call GetProfileString("Widgets", key, default,
                          value, value.maxLength);
    return value;
end;

In this example, a fixed-length string is passed to the value parameter. The length of the value parameter string is passed in the length parameter. (This could also have been hard-coded to the length of the value parameter defined in the function signature.)