P4060¶

Code: P4060
Message: VAR_IN_OUT argument cannot be proved writable

This error occurs when a function call passes a variable to a VAR_IN_OUT parameter and the compiler cannot prove the variable is writable.

A VAR_IN_OUT parameter is passed by reference, so the function can write to the argument. The argument therefore must not be a constant, and the compiler must be able to tell that from the calling POU’s own declarations. Where it cannot, the argument is rejected, even if the value never actually came from a constant.

An argument is rejected when its variable:

is declared CONSTANT, in any section, including VAR_EXTERNAL CONSTANT
is a VAR_INPUT of the calling POU: its value came from that POU’s caller, which may have passed a constant
is an output or local of a function block instance, such as fb.Q: only an instance’s inputs can be assigned from outside it
is reached through a dereference, such as p^: a reference can point at a variable whose value came from a constant
is a direct input address, such as %IX0.0: process inputs are written by the I/O image, not by the program

A VAR_IN_OUT parameter of the calling POU is writable, because the call that bound it proved its own argument writable.

Example¶

The following code will generate error P4060:

FUNCTION Increment : DINT
    VAR_IN_OUT
        counter : DINT;
    END_VAR
    counter := counter + 1;
    Increment := counter;
END_FUNCTION

FUNCTION_BLOCK Counter
    VAR_INPUT
        start : DINT;
    END_VAR
    VAR
        next : DINT;
    END_VAR
    next := Increment(start);   (* Error: start is a VAR_INPUT *)
END_FUNCTION_BLOCK

To fix this error, copy the value into a local variable and pass that:

FUNCTION_BLOCK Counter
    VAR_INPUT
        start : DINT;
    END_VAR
    VAR
        current : DINT;
        next : DINT;
    END_VAR
    current := start;
    next := Increment(current);
END_FUNCTION_BLOCK

P4060¶

Example¶

See Also¶