dereferencing a pointer in a function block

2020-02-27T14:00:30.411000Z

Thank you for the replies. Yes, it was an alignment problem. If I get a pointer to BYTE, it can only be dereferenced as a WORD type if the address is divisible by 2, and as a DWORD type if the address is divisible by 4. So depending on where the pointer landed, it would work or not work.
Tim

dereferencing a pointer in a function block

2020-02-27T14:00:30.165000Z

You are right Yannick

dereferencing a pointer in a function block

2020-02-27T14:00:29.915000Z

Instead of using a POINTER TO ARRAY[1..255] OF BYTE; just use a pointer to BYTE and pass it the array.
You will still be able to iterate through the array in your FB.

This is what I do in TwinCAT and it works fine.
I tried using variable arrays but found some bugs. See this post: l viewtopic.php?t=7558#p17267 l

dereferencing a pointer in a function block

2020-02-27T14:00:29.656000Z

Hi.

It could be an alignment trouble.

Try to create an array of 64 reals. Point to him an pointer to array of 254 bytes.

Buffer: array [0..63] of real.

Bytes:pointer to array[0..255] of byte....

Bytes:=adr(buffer).

Then the array of bytes will be aligned as an real.

dereferencing a pointer in a function block

2020-02-27T14:00:28.853000Z

I'm having some issues with dereferencing a pointer in a function block. I've tried to simplify the issue down to the following code:

VAR
   VarPlaceholder:         ARRAY[1..255] OF BYTE;
   ptrPlaceholder:         POINTER TO ARRAY[1..255] OF BYTE;
   VarFloatingPoint:      REAL;
   ptrFloatingPoint:      POINTER TO REAL;
END_VAR
ptrPlaceholder:= ADR(VarPlaceholder);   //get a pointer to a variable of type ARRAY[1..255] OF BYTE
ptrFloatingPoint:= ptrPlaceholder;      //pass the pointer to a different type of pointer, POINTER TO REAL
VarFloatingPoint:= ptrFloatingPoint^;      //dereference the pointer as a REAL

This code works as expected if I put it in a test program. If I change the values of the array, I get different REAL numbers in VarFloatingPoint.

However, if I put the exact same code in a function block and call it from the same test program, I will get an access violation exception on the third line, where it dereferences the pointer. Line 1 does get a valid pointer, and Line 2 does copy that pointer over to ptrFloatingPoint (see attached image).

Is there something different about the way a pointer is handled in a function block?

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