String appending - how does it work

[expert_vision]

I want to append a string to a buffer received from a DllCalll containing a null terminated string, but instead of being appended it's simply placed at the start of the string.

The code snippet below depicts the issue.

Code: Select all

;~ ANSI only
VarSetCapacity(string, 8, 0)
DllCall("RtlFillMemory", "Ptr", &string, "UInt", 3, "UChar", Asc("a")&0xff) ; fill in the first 3 bytes with ASCII value of "a"
MsgBox % string ; all good; result is "aaa"
Loop % 8
	bin1 .= *(&string+A_Index-1) . "(" . Chr(*(&string+A_Index-1)) . ")" . "_"
MsgBox % bin1 ; all good; 97 97 97 0 => a NULL terminated string
string .= "b" ; append "b" to "aaa"
MsgBox % string ; result is "b" .. why?
Loop % 8
	bin2 .= *(&string+A_Index-1) . "(" . Chr(*(&string+A_Index-1)) . ")" . "_"
MsgBox % bin2 ; 98 0 97 0 . "b" was not appended, was simply assigend to string. Why?

[LinearSpoon]

Add VarSetCapacity(string,-1) after the DllCall. The DllCall modifies the string, AHK's internal string length counter now has the wrong value. Append is probably optimized to jump to the end of the string based on the length counter rather than searching for the NULL. VarSetCapacity(string,-1) asks AHK to update the length of the string explicitly.

[expert_vision]

I missed that in docs. Thanks!

[lexikos]

If you use "str", string instead of "ptr", &string, DllCall will do it for you automatically.