MASM32 : how to return a string?

I am writing a dll in masm32 for use with vb. how do i return a string? I am still learning masm32, please help me. The proc works fine in masm32 but returns nothing in vb or crashes.
Thanks.

Code:

GetNewTempFile proc strPrefix :LPCSTR, strNewFileName:LPSTR
local buf[256]:dword
local b[256]:dword
invoke GetTempPath,256,addr buf
invoke GetTempFileName,addr buf,strPrefix,0,addr b
invoke lstrlen,addr strNewFileName
ret
GetNewTempFile EndP

Just take a look at the default __stdcall specification - which is also used by VB - from MSDN.: here.

This is basic knowledge of how to write a function. But one information is missing in MSDN: Return values are given back to the caller via the EAX register. So put the pointer to your string into the EAX register. And beware: Never return a local variable pointer (which is allocated on the stack) back to the caller, since the stack get's cleaned up, the pointer will be invalid after the "ret" instruction. So use a dynamic allocator to allocate the string.

Thanks for your response NoHero. I wish to return the string in parameter 2. How do i go about it? I am still stuck, could you give a simple example please? Thank you.

Saching22,

You missing something in your code.
Where do you copy the buffer to the string?
Should be something like;

GetNewTempFile proc strPrefix :LPCSTR, strNewFileName:LPSTR
local buf[256]:dword
local b[256]:dword
invoke GetTempPath,256,addr buf
invoke GetTempFileName,addr buf,strPrefix,0,addr b
invoke lstrlen,addr buf
invoke strcpy,strNewFileName,addr buf,eax

invoke lstrlen,addr strNewFileName
ret
GetNewTempFile EndP

Something along those lines.
Cant remeber if that is that actual function name in masm.

Packetvb

You must use this function:

INVOKE SysAllocStringLen, offset lbBuffer, quantity
The offset lbuffer is used when you have a ready string loaded on a .data variable.
If you want to start a new string, pass 0 or NULL for lbBuffer

Ex: Testing a 4 CHARS unicode string , each char = 2 bytes or 1 word:

INVOKE SysAllocStringLen, 0, 4
.IF eax != 0
mov word ptr [eax], 'a' ;load a first char
mov word ptr [eax+2], 'b' ;second char
mov word ptr [eax+4], 'c' ;third char
mov word ptr [eax+6], 'd' ;third char
.ENDIF
ret ; eax has the pointer.

Ex: Testing a 4 CHARS ANSI string , each char = 1 bytes:

INVOKE SysAllocStringLen, 0, 2 ; <-- half as we use only 1 byte per char:
.IF eax != 0
mov BYTE ptr [eax], 'a' ;load a first char
mov BYTEptr [eax+1], 'b' ;second char
mov BYTEptr [eax+2], 'c' ;third char
mov BYTEptr [eax+3], 'd' ;third char
.ENDIF
ret
; eax has the pointer.

In the Vb code you declare the function normally, with String return type.
No need to pass a string reference nor size. SysAllocStringLen does all for you

You must use this function:

INVOKE SysAllocStringLen, offset lbBuffer, quantity
The offset lbuffer is used when you have a ready string loaded on a .data variable.
If you want to start a new string, pass 0 or NULL for lbBuffer

Ex: Testing a 4 CHARS unicode string , each char = 2 bytes or 1 word:

INVOKE SysAllocStringLen, 0, 4
.IF eax != 0
mov word ptr [eax], 'a' ;load a first char
mov word ptr [eax+2], 'b' ;second char
mov word ptr [eax+4], 'c' ;third char
mov word ptr [eax+6], 'd' ;third char
.ENDIF
ret ; eax has the pointer.

Ex: Testing a 4 CHARS ANSI string , each char = 1 bytes:

INVOKE SysAllocStringLen, 0, 2 ; <-- half as we use only 1 byte per char:
.IF eax != 0
mov BYTE ptr [eax], 'a' ;load a first char
mov BYTE ptr [eax+1], 'b' ;second char
mov BYTE ptr [eax+2], 'c' ;third char
mov BYTE ptr [eax+3], 'd' ;third char
.ENDIF
ret
; eax has the pointer.

In the Vb code you declare the function normally, with String return type.
To return a string, there is NO need to pass a string reference nor size.
Using SysAllocStringLen, it does all for you, I have DLL functions that return string working FINE in VB. They were made on MASM32

You're right that VB passes and returns strings as BSTR (unless they're passed as a ByVal parameter, IIRC). However, I hope that Saching22 already solved his problem sometime during the last seven years... ;)

And please use code tags when posting code.