Strings

1. Identifier for size and length specifications [++]
2. Recommended default size [+]
3. Transfer of large strings [+]
4. Processing of large strings [+]

Identifier for size and length specifications

Refer to the size of a string variable in bytes as "Size".

Refer the current length of a string in characters as "Len".

Positive sample:

VAR
    sText     : T_MaxString;
    nTextSize : UDINT;
    nTextLen  : UDINT;
END_VAR
-------------------------------------------
nTextSize := SIZEOF(sText);
nTextLen  := LEN(sText);

Recommended default size

The recommended default size of a string variable is T_MaxString (alias of STRING(255)). This applies to the local declaration of the string as well as to the declaration of the parameter in a function/method.

If another size is fixed for the use case, this should be used as the size limit (example: T_AmsNetId).

The size of T_MaxString is defined as maximum for functions like LEN() and CONCAT() and leads to acceptable performance of regular parameter assignments as "call by value".

Static Analysis:

Thematically recommended Static Analysis rule:

• SA0026: Possible truncated strings

Positive sample:

FUNCTION F_Sample : BOOL
VAR_INPUT
    sParam1  : T_MaxString;
    sParam2  : T_AmsNetId;
END_VAR

PROGRAM MAIN
VAR
    sLocal1  : T_MaxString;
    sLocal2  : T_AmsNetId;
    bReturn  : BOOL;
END_VAR

bReturn := F_Sample(sParam1 := sLocal1,
                    sParam2 := sLocal2);

Transfer of large strings

If a larger string than T_MaxString is required, the transfer should be used as "call by reference". This can be achieved using either VAR_IN_OUT CONSTANT for read-only access or POINTER TO STRING.

An input parameter as REFERENCE TO STRING(1023) is only suitable to a limited extent, because this would always require a fixed size of the assigned variable. If this restriction is acceptable or desired, prefer this variant.

Positive sample:

FUNCTION F_Sample : BOOL
VAR_IN_OUT CONSTANT
    sLonger1     : STRING;
END_VAR
VAR_INPUT
    pLonger2     : POINTER TO STRING;
    nLonger2Size : UDINT;
END_VAR

PROGRAM MAIN
VAR
    sLocal       : STRING(1023);
    bReturn      : BOOL;
END_VAR

bReturn := F_Sample(sLonger1     := sLocal,
                    pLonger2     := ADR(sLocal),
                    nLonger2Size := SIZEOF(sLocal));

Processing of large strings

When using T_MaxString or smaller string variables, you can use string functions like CONCAT() or LEN() (Tc2_Standard).

For larger string variables you have to use functions like CONCAT2() or LEN2() (Tc2_Utilities).