FPC 3.2.2 + GDB

[paule32]

what is the error ?
- a range overflow ?

Code: Bash  [Select][+][-]

1. Thread 1 received signal SIGSEGV, Segmentation fault.
2. 0x00000001100041ee in STRCAT_ (DEST=0x11000a5e8 '1', SOURCE=0x110007a38 '') at SysUtils.pas:216
3. 216       D[i + J + 2] := #0;
4. (gdb)

This is the source code:

Code: Pascal  [Select][+][-]

1. function StrCat_(var Dest: PChar; Source: PChar): PChar; stdcall; export;
2. var
3.   D       : PChar;
4.   L, I, J : Integer;
5. begin
6.   L := StrLen(Dest) + StrLen(Source) + 1;
7.   D := StrAlloc(L);
8.  
9.   for i := 0 to StrLen(Dest) - 1 do
10.   D[i] := Dest[i];
11.  
12.   for J := 0 to StrLen(Source) - 1 do
13.   D[i + j + 1] := Source[J];
14.   D[i + J + 2] := #0;
15.  
16.   Dest := D;
17.   Exit(D);
18. end;

[440bx]

In a Pascal "for" loop the value of the index variable, in your case "i", is undefined when the loop ends.

Therefore, the lines 13 and 14 of your code are using a value which could be anything.  That's the reason you are getting a violation.

If you need to depend on the value of "i" when the loop ends, change it from a "for" to a "while" where you update the index yourself.

HTH.

[Khrys]

Like @440bx said, you cannot just use  i  and  j  after their respective loops have exited.
And if you want to be really pedantic, you shouldn't reuse the loop variable for anything else either; just for the loop itself. This has been discussed to death numerous times.

Also there's no reason to call  StrLen  twice for each parameter - you should cache the results.

[paule32]

Thanks for replies:

In my case, Source was empty/null, so I had add two conditionals that check the Dest and Source Arguments.
And I use repeat until instead while.
So, the result will work, now.

Thank you.

Code: Pascal  [Select][+][-]

1. function StrCat(var Dest: PChar; Source: PChar): PChar; stdcall; export;
2. var
3.   D       : PChar;
4.   L, I, J : Integer;
5. begin
6.   if Dest = nil then
7.   begin
8.     MessageBoxA(0,'Error: StrCat Dest not initialized.','Error',0);
9.     Exit;
10.   end;
11.  
12.   if Strlen(Source) < 1 then
13.   begin
14.     Dest := '';
15.     Exit(Dest);
16.   end;
17.  
18.   L := StrLen(Dest) + StrLen(Source) + 1;
19.   D := StrAlloc(L);
20.  
21.   L := StrLen(Dest);
22.   i := 0;
23.   repeat
24.     D[i] := Dest[i];
25.     inc(i);
26.   until i = L;
27.  
28.   L := StrLen(Source);
29.   j := 0;
30.   repeat
31.     D[i + j] := Source[j];
32.     inc(j);
33.   until j = L;
34.  
35.   D[i + j] := #0;
36.   Dest := D;
37.  
38.   Exit(D);
39. end;

[marcov]

And of course not caching strlen's.  Contrary to length() on Pascal strings, Strlen on zero terminated strings is an expensive operation.