tc-datatypes-memory

name	tc-datatypes-memory
description	Use this skill when the user asks about data types, arrays, structures (STRUCT), enumerations (ENUM), pointers (POINTER), references (REFERENCE), or dynamic memory allocation (NEW/DELETE). It also contains critical rules for memory alignment (pack_mode) to minimize padding bytes.
disallowedTools	["edit","write","replace_string_in_file","multi_replace_string_in_file","create_file","grep_search","file_search","list_dir"]

TwinCAT Data Types & Memory (Cap. 2, 13)

Standard Data Types

Category	Type	Size	Range / Notes
Boolean	BOOL	8 bit	TRUE (1), FALSE (0).
Bit String	BYTE	8 bit	0 to 255
	WORD	16 bit	0 to 65535
	DWORD	32 bit	0 to 4294967295
	LWORD	64 bit	0 to 2^64-1
Signed Integer	SINT	8 bit	-128 to 127
	INT	16 bit	-32768 to 32767
	DINT	32 bit	-2147483648 to 2147483647
	LINT	64 bit	64-bit signed integer
Unsigned Integer	USINT	8 bit	0 to 255
	UINT	16 bit	0 to 65535
	UDINT	32 bit	0 to 4294967295
	ULINT	64 bit	64-bit unsigned integer
Floating Point	REAL	32 bit	IEEE 754 single precision
	LREAL	64 bit	IEEE 754 double precision
String	STRING(n)	n+1 bytes	Default 80 chars, max 255.
	WSTRING	2*(n+1) bytes	Unicode string.
Time	TIME	32 bit	T#0ms to T#49d17h2m47s295ms
	LTIME	64 bit	High-resolution time.
Date	DATE	32 bit	D#1970-01-01 format.
	TIME_OF_DAY (TOD)	32 bit	TOD#00:00:00 format.
	DATE_AND_TIME (DT)	32 bit	DT#1970-01-01-00:00:00 format.

User-Defined Data Types

Type	Syntax	Notes
ARRAY	ARRAY [lo..hi] OF <type>	1D, 2D, 3D supported. Max 9 dimensions via nesting.
STRUCT	TYPE ST_Name : STRUCT ... END_STRUCT END_TYPE	8-byte alignment (TwinCAT 3). Order by descending size to minimize padding. Unpacked structs inside packed structs are not allowed (error 3850).
ENUM	TYPE E_Name : ( val1, val2, ... );	Always use {attribute 'strict'} + {attribute 'qualified_only'}. Access: E_Name.eValue.
ALIAS	TYPE T_Name : <base_type>; END_TYPE	Supports default init and subrange combination.
SUBRANGE	TYPE T_Name : INT (lo..hi); END_TYPE	Only on integer types.
POINTER	POINTER TO <type>	Check <> 0 before dereferencing with ^. Use ADR() to get address.
REFERENCE	REFERENCE TO <type>	Assign with REF=. Check with __ISVALIDREF().
INTERFACE	I_Name	Check <> 0 before use. Convert with __QUERYINTERFACE().

Type Literals / Constants

nHex    := 16#FF;         (* Hexadecimal *)
nOct    := 8#77;          (* Octal *)
nBin    := 2#1010_1010;   (* Binary, underscores for readability *)
nVal    := SINT#127;      (* Explicit type prefix *)
fVal    := REAL#3.14;
tDelay  := T#100ms;
tLong   := T#2h30m;
dToday  := D#2026-02-20;
todNow  := TOD#18:30:00;
dtStamp := DT#2026-02-20-18:00:00;
sText   := 'Hello World';
sEscape := '$N';          (* Newline escape *)

Reference Assignment Operators

Operator	Effect
refA REF= value	refA now points to value (address assignment)
refA := value	Writes value to location referenced by refA (value copy)

refInt REF= nMyInt;
refInt := 42;
bValid := __ISVALIDREF(refInt);

ipSample := fbSample;
IF ipSample <> 0 THEN
    nResult := ipSample.Add(3, 6);
END_IF;
bOk := __QUERYINTERFACE(iBase, iDerived);

Enumeration with Strict & Qualified Access

{attribute 'qualified_only'}
{attribute 'strict'}
TYPE E_Color :
(
    eRed,
    eYellow,
    eGreen := 10,
    eBlue
) UINT := E_Color.eRed;
END_TYPE

eMyColor := E_Color.eBlue;   (* Always qualified *)

Structure Alignment (Cap. 13)

TwinCAT 3 uses 8-byte alignment (vs. 1-byte in TC2 x86).

• Variable aligned to multiple of MIN(data_type_size, pack_mode).
• Structure total size is multiple of MIN(largest_member_size, pack_mode).
• Implicit padding bytes are inserted (NOT initialized — relevant for MEMCMP).
• Each FB implicitly contains a pointer to virtual method table (8 bytes on 64-bit).

TYPE ST_Optimal :      (* 16 bytes total — minimal padding *)
STRUCT
    fValue : LREAL;    (* 8 bytes at offset 0 *)
    nCount : DINT;     (* 4 bytes at offset 8 *)
    bFlag  : BOOL;     (* 1 byte at offset 12 + 3 padding *)
END_STRUCT
END_TYPE

TYPE ST_Wasteful :     (* 24 bytes total — bad ordering *)
STRUCT
    bFlag  : BOOL;     (* 1 byte + 7 padding *)
    fValue : LREAL;    (* 8 bytes *)
    nCount : DINT;     (* 4 bytes + 4 padding *)
END_STRUCT
END_TYPE
(* Tip: order members by descending size to minimize padding *)

Controlling Alignment with pack_mode

{attribute 'pack_mode' := '1'}   (* 1-byte — no padding *)
TYPE ST_Packed :
STRUCT
    bFlag  : BOOL;
    nValue : DINT;     (* immediately follows bFlag, no padding *)
END_STRUCT
END_TYPE

pack_mode value	Alignment
'0'	8-byte (default TwinCAT 3)
'1'	1-byte (no padding)
'2'	2-byte
'4'	4-byte

Dynamic Memory — __NEW / __DELETE

FBs/DUTs require {attribute 'enable_dynamic_creation'}.

VAR
    pFB : POINTER TO FB_Dynamic;
END_VAR

pFB := __NEW(FB_Dynamic);
IF pFB <> 0 THEN
    nResult := pFB^.DoWork(nParam := 42);
END_IF;
__DELETE(pFB);   (* releases memory, sets pFB to 0, calls FB_exit *)

• __NEW allocates from router memory pool. Must pair with __DELETE to avoid leaks.
• __DELETE releases memory and sets pointer to 0.
• For inherited FBs, use derived type pointer to ensure correct FB_exit call.

Exception Handling

VAR
    exc : __SYSTEM.ExceptionCode;
END_VAR
__TRY
    nResult := nA / nB;
__CATCH(exc)
    IF exc = __SYSTEM.ExceptionCode.RTSEXCPT_DIVIDEBYZERO THEN
        bError := TRUE;
    END_IF;
__ENDTRY

Warning: F_RaiseException() outside a __TRY block stops the controller.

UNION Type

TYPE U_Convert :
UNION
    nValue : DWORD;
    aBytes : ARRAY [0..3] OF BYTE;
END_UNION
END_TYPE

• All members share the same memory location (size = largest member).
• Useful for type-punning (e.g., reading individual bytes of a DWORD).
• Combine with {attribute 'pack_mode' := '1'} to remove padding when overlaying I/O data.

Platform-Independent Types

Type	Description
PVOID	Untyped pointer; size matches platform pointer width (4 bytes on 32-bit, 8 bytes on 64-bit). Recommended return type of ADR().
XINT / UXINT	Platform-native signed/unsigned integer (32-bit on x86, 64-bit on x64).
XWORD	Platform-native bit-string.
BIT	Single bit (only valid as struct member, packed into bytes). Cannot use ADR — use BITADR.

Use these to write code that compiles unchanged on both 32-bit and 64-bit TwinCAT runtimes.

Variable-Length Arrays (VAR_IN_OUT only)

FUNCTION F_Sum : DINT
VAR_IN_OUT
    aData : ARRAY [*] OF DINT;   (* Bounds resolved at call site *)
END_VAR
VAR
    i, nLow, nHigh : DINT;
END_VAR
nLow  := LOWER_BOUND(aData, 1);
nHigh := UPPER_BOUND(aData, 1);
FOR i := nLow TO nHigh DO
    F_Sum := F_Sum + aData[i];
END_FOR;

References (Beckhoff InfoSys)

• Data Types
• Alignment
• Global Data Types