Attribute: no_assign, no_assign_warning

The pragma 'no_assign' results in compiler errors being displayed if an instance of the function block is assigned to another instance of the same function block. Such assignments are often to be avoided if the function block contains pointers and pointers lead to problems, because they are copied as well during the value assignment.

The pragma 'no_assign_warning' results in the same as for the pragma 'no_assign' with compiler warnings instead of compiler errors.

Syntax:

{attribute 'no_assign'}

Insert location: Top line in the declaration part of a function block.

Example 281. Example

Assignment of function block instances containing pointers.

In this example the value assignment of the function block instances will lead to problems during the execution of fb_exit:

VAR_GLOBAL
    inst1 : TestFB;
    awsBufferLogFile : ARRAY [0..9] OF WSTRING(66);(* Area: 0, Offset: 0x1304 (4868)*)
    LogFile : SEDL.LogRecord := (sFileName := 'LogFile.log', pBuffer := ADR(awsBufferLogFile), udiMaxEntriesFile := UDINT#10000, udiMaxBuffered := UDINT#10, uiLineSize := UINT#64, wsSep := " ", xCircular := TRUE, siDateFormat := SINT#0, siTimeFormat := SINT#0);
END_VAR

PROGRAM PLC_PRG
VAR
    inst2 : TestFB := inst1;
    LogFileNew
END_VAR

In this case LogRecord manages a list of pointers, for which various actions are executed in the case of fb_exit. Problems result due to the assignment, because fb_exit will be executed twice. You should prevent this by adding the attribute 'no_assign' in the declaration of the function block TestFB:

{attribute 'no_assign'}
FUNCTION_BLOCK TestFB
VAR_INPUT
...

The following compiler errors are then displayed:

C0328: Assignment not allowed for type TestFB
C0328: Assignment not allowed for type LogRecord

If the pragma no_assign_warning is used instead of the pragma no_assign for the function block TestFB, then the C0328 message is issued as compiler warning, not as a compiler error.