POU ‘CheckBounds’

Functions for Bound Checks: CheckBounds

The task of this monitoring function is to handle bound violations appropriately. Examples of reactions to violations include setting error flags and changing the value of the array index. The check is performed only for one variable array index. An incorrect constant array index causes a compiler error. CODESYS calls the function implicitly when values are assigned to an ARRAY variable.

Caution

To obtain the feature for monitoring functions, do not edit the declaration section. However, you are permitted to add local variables.

Example for using the CheckBounds function; throws an exception when array bounds are violated.

Add the following libraries in the library manager of the application:

  • CmpApp.library and SysExcept.library as placeholder libraries
  • SysTypes2_Itfs.librar with Always newest version

Add a CheckBounds object below the application and modify the implementation code as shown below.

Program a GVL object and a MAIN_PRG object below the application with the contents shown below.

When you load and start this application, an exception will be thrown when array bounds are violated. Processing stops in CheckBounds so that the type of error can be detected.

Declaration Part

// Automatically generated code : DO NOT EDIT
FUNCTION CheckBounds : DINT
VAR_INPUT
\ index, lower, upper: DINT;
END_VAR
VAR
\ _pApp : POINTER TO CmpApp.APPLICATION;
\ _Result   : SysTypes.RTS_IEC_RESULT;
END_VAR

Implementation

// This automatically generated code is a suggested implementation.
_pApp := AppGetCurrent(pResult := _Result);
IF  index < lower THEN
\ CheckBounds := lower;
\ IF _pApp <> 0 THEN
\ \ AppGenerateException(pApp := _pApp, ulException := RtsExceptions.RTSEXCPT_ARRAYBOUNDS);
\ END_IF
ELSIF  index > upper THEN
\ CheckBounds := upper;
\ IF _pApp <> 0 THEN
\ \ AppGenerateException(pApp := _pApp, ulException := RtsExceptions.RTSEXCPT_ARRAYBOUNDS);
\ END_IF
ELSE
\ CheckBounds := index;
END_IF

PROGRAM MAIN_PRG
VAR
\ xInit    : BOOL;
\ arData   : ARRAY[0..7] OF BYTE;
\ i        : INT;
\ dwAdr\    : DWORD;
END_VAR

IF NOT xInit THEN
\ // Required for CheckBounds
\ xInit  := TRUE;
END_IF

// Set i to a value > 7 or < 0
// Generates an exception in CheckBounds, user-defined
arData[i] := 11;

When the CheckBounds function is called, it receives the following input parameters:

  • index: Index of the array element
  • lower: Lower limit of the array range
  • upper: Upper limit of the array range

The return value is the index of the array element, as long as it is within a valid range. If not, then the CODESYS returns either the upper or lower limit, depending on which threshold was violated.

Correction of the access to an array outside the defined array bounds

In the sample program below, the index exceeds the defined upper limit of the a array.

PROGRAM PLC_PRG
VAR
 a: ARRAY[0..7] OF BOOL;
\ b: INT:=10;
END_VAR

a[b]:=TRUE;

In this example, the CheckBounds function causes a to change the upper limit of the array range index to 10. The value TRUE is assigned then to the element a[7]. In this way, the function corrects array access outside of the valid array range.

See also