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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:
VARÂ Â VarPlaceholder:Â Â Â Â Â Â ARRAY[1..255]OFBYTE;Â Â ptrPlaceholder:Â Â Â Â Â Â POINTERTOARRAY[1..255]OFBYTE;Â Â VarFloatingPoint:Â Â Â Â REAL;Â Â ptrFloatingPoint:Â Â Â Â POINTERTOREAL;END_VARptrPlaceholder:=ADR(VarPlaceholder);Â Â //getapointertoavariableoftypeARRAY[1..255]OFBYTEptrFloatingPoint:=ptrPlaceholder;Â Â Â Â //passthepointertoadifferenttypeofpointer,POINTERTOREALVarFloatingPoint:=ptrFloatingPoint^;Â Â Â Â //dereferencethepointerasaREAL
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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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
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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
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
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:
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?
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.
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
You are right Yannick
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