how to use SIZEOF() operator in ST (TC3)

#PLC #TwinCAT3 #Beckhoff

Learn more at Part 4 - Data types and arrays (IEC 61131-3)

1) Example

PROGRAM MAIN
VAR
	fOneFloat : REAL;
	sThisIsAString : STRING;
	nAnInteger : INT;

	nReturnedNumberOfBytesForFloat : UDINT;
	nReturnedNumberOfBytesForString : UDINT;
	nReturnedNumberOfBytesForInteger : UDINT;
	
END_VAR

------------------------------------------------------------------------

nReturnedNumberOfBytesForFloat := SIZEOF(fOneFloat);
nReturnedNumberOfBytesForString := SIZEOF(sThisIsAString);
nReturnedNumberOfBytesForInteger := SIZEOF(nAnInteger);

2) Documentation

This operator is used for is to determine the required size in bytes for a specific variable.

Note: the SIZEOF() operator is very useful when you work with buffers

You can learn more here

3) Pointers and References: using the SIZEOF() operator

You can do a SIZEOF() on the reference, and get the size of the referenced object.

So here we have an integer, that is 2 bytes, and the SIZEOF() the reference is 2.

If you would do a SIZEOF() on a pointer, you would get the size of the pointer, not the SIZEOF the object the pointer is pointing to.

So in this example we have an integer, and a pointer to an integer. And the size isn't the size of the integer, but the size of the pointer. So again if we have a 64-bit system then the size of the pointer is 8 bytes.