How to assign value to record using pointer (SIGSEGV runtime error)

[dculp]

In the following code I want to assign a value to KeyCode. However, I get a runtime error "raised exception class 'External: SIGSEGV' ".
Note that the TKey record can't be changed since it's actually used as part of a much larger project.
Thanks,
Don C.

Code: Pascal  [Select][+][-]

1. program exPointers3;
2. // https://www.tutorialspoint.com/pascal/pascal_pointers.htm
3.  
4. type
5.   PKey = ^TKey;
6.   TKey = record // ***** CAN'T CHANGE *****
7.     KeyCode: Smallint;
8.     {other items}
9.   end;
10.  
11. var
12.    ZPkey: PKey;
13.    ZTkey: ^TKey;
14. //   ZTkey: PKey;
15.  
16.  
17. var
18.    number: integer;
19.    iptr: ^integer;
20.    y: ^word;
21.  
22. begin
23.    iptr := @number;
24.       {If above isn't used then the next line gives runtime error -
25.       "raised exception class 'External: SIGSEGV'}
26.    iptr^ := 200;
27.    writeln('Number is: ', number);
28.    writeln('iptr points to a value: ', iptr^);
29.  
30.    y := addr(iptr);
31.    writeln(y^);
32.    writeln;
33.    readln;
34.    // Above demo from https://www.tutorialspoint.com/pascal/pascal_pointers.htm
35.    // Runs OK.
36.  
37.    Number:= 10;
38.    with ZTkey^ do
39.       KeyCode:= Number;  // runtime error - "raised exception class 'External: SIGSEGV'
40. //   ZTkey^.KeyCode:= 10; // runtime error - "raised exception class 'External: SIGSEGV'
41.  
42.    writeln('ZTkey^.KeyCode = ', ZTkey^.KeyCode); // runtime error
43.    readln;
44. end.
45.  

[440bx]

ZTKey has not been assigned the address of a TKey variable therefore its value is nil (because it is a global variable) which when dereferenced produces an access violation.

Solution: assign an appropriate address to ZTKey before the "with ZTKey^ do" statement.

HTH.

[d4eva]

You need an actual record for ZTkey to point to, e.g.

Code: Pascal  [Select][+][-]

1. program exPointers3;
2. type
3.   PKey = ^TKey;
4.   TKey = record
5.     KeyCode: smallint;
6.   end;
7.  
8. var
9.   ZTkey: PKey;
10.   key: TKey;
11.  
12. var
13.   number: smallint;
14.  
15. begin
16.   ZTkey := @key; // now ZTkey points to key
17.   Number := 10;
18.   with ZTkey^ do begin
19.     KeyCode := Number;
20.     writeln('ZTkey^.KeyCode = ', ZTkey^.KeyCode);
21.     ZTkey^.KeyCode := 20;
22.     writeln('ZTkey^.KeyCode = ', ZTkey^.KeyCode);
23.   end;
24.   readln;
25. end.

[MarkMLl]

ZTKey has not been assigned the address of a TKey variable therefore its value is nil (because it is a global variable) which when dereferenced produces an access violation.

Solution: assign an appropriate address to ZTKey before the "with ZTKey^ do" statement.

In slightly more detail, ZTKey is nil because globals are (normally) zeroed. If it were a local variable (i.e. on the stack) it would simply contain rubbisch, which might or might not be a valid address and would likely be far more difficult to faultfind.

I'd add that it's unusual and refreshing for somebody asking a question to post a complete and succint compilable example (although for completeness adding the FPC version etc. is always useful). Keep up the good work!

MarkMLl