Re: pass character string to COM - COM Discussion Boards

29-Jun-08 21:34

I want to make a function in COM dll which receive one character string and returns one character string. Since scripting languages dont use char* please suggest how to make this function in COM and how to use it. Please give any sample.

Manoj Kumar Chauhan

Vi229-Jun-08 21:54

Vi2

29-Jun-08 21:54

[id(...)] HRESULT SomeMethod([in] BSTR inString, [out,retval] BSTR * outString);

In script this method can be used as

string2 = obj.SomeMethod(string1)
string2 = obj.SomeMethod("text1")

With best wishes,
Vita

MKUser29-Jun-08 22:50

29-Jun-08 22:50

Thanks it's working
Now if i use CHAR* instear of BSTR* then will it work for scripting language?
Please give some samples where to use BSTR* and CHAR* for scripting & non scripting language. If function inside COM dll takes CHAR* then i should pass BSTR if i'm using dll in scripting language
please give some samples for this

Manoj Kumar Chauhan

Vi229-Jun-08 23:14

Vi2

29-Jun-08 23:14

Now if i use CHAR* instead of BSTR* then will it work for scripting language?

No. At the most, [in] parameter might be considered as WCHAR*.

Please give some samples where to use BSTR* and CHAR* for scripting & non scripting language. If function inside COM dll takes CHAR* then i should pass BSTR if i'm using dll in scripting language

COM object doesn't use "CHAR*" in specification of functions, but can convert "CHAR*" to "BSTR" and viceversa.

With best wishes,
Vita

MKUser29-Jun-08 23:45

29-Jun-08 23:45

Thanks for explanation
So you mean it's always preferable to use BSTR for COM not any char*
And if i make any function in COM which takes char* as input then i need to pass char* when i call this function. Is it right?
Because ATL gives the option to make char* for any function argument so i can give it. My question is how i'll handle this char* in scripting language

Manoj Kumar Chauhan

Vi230-Jun-08 21:04

Vi2

30-Jun-08 21:04

When I spoke "COM object doesn't use "CHAR*" in specification of functions", I meant that for string parameter, not for CHAR parameter, which is valid in COM. So you can write

f(..., [in] CHAR* c, ...)
f(..., [in,out] CHAR* c, ...)
f(..., [out] CHAR* c, ...)
f(..., [out,retval] CHAR* c)

but you will be able to pass only one symbol (or one byte), not the whole string.

With best wishes,
Vita

MKUser1-Jul-08 3:20

1-Jul-08 3:20

Thanks for your explanation

Is it possible if we pass any iostream object to COM function and COM function write some text in the file attached to iostream object and return.

Manoj Kumar Chauhan

Vi21-Jul-08 17:40

Vi2

1-Jul-08 17:40

"iostream" object is C++ object, not accessible directly from scripts and non-C++ clients and unallowed in COM accordingly.

With best wishes,
Vita

MKUser1-Jul-08 18:42

1-Jul-08 18:42

Thanks. There is one more doubt please clear this also.
If i want to return any variable length string from COM then i can allocate memory in COM and return it to exe (return BSTR*). Finall exe which's using the COM will free the memory. Is this OK? How to make same parameter as input/output in COM function?
How to return more than one string through COM function. suppose if i have function
int func(BSTR inputstring, BSTR* outstring1, BSTR* outstring 2);
Here i want to return 2 strings from this function.
COM dont give the option to pass output argument when we call the function

Manoj Kumar Chauhan

MKUser1-Jul-08 21:22

1-Jul-08 21:22

I implement one function in COM which allocate some memory to array and return this.
int AllocateData(int length, BSTR* OutArray)
{
// there is some data in a ansi array, i return this data from this COM function

wchar_t* wstr = (wchar_t*)calloc(sizeof(WCHAR),length+1);
MultiByteToWideChar(CP_ACP, 0, ansiarray, length, wstr, length) ; free(ansiarray);
*OutArray = (BSTR)wstr;
}

When i use this dll i call this function like
BSTR st;
st = cominterface->AllocateData(500);
When this function is called i get some error for memory leak. Please suggest how to fix it.

Manoj Kumar Chauhan

CPallini1-Jul-08 22:05

1-Jul-08 22:05

MKUser wrote:
COM dont give the option to pass output argument when we call the function

That's not true (see the [OUT] IDL attribute).
Smile | :)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong.
-- Iain Clarke

[My articles]

MKUser1-Jul-08 23:19

1-Jul-08 23:19

Thanks. As per your comments i have solved the error when use [out] in idl file. But if i use [out,retval] then i get error.

I create a sample. Please check the below code.

main()
{
BSTR ab;
ab = allocmem();//i get run time memory error when return from this function
}

_bstr_t allocmem()
{
BSTR x;
allocmem2(&x);
return _bstr_t(x, false);
}

void allocmem2(BSTR *x)
{
char ansi[27];
memset(ansi,0,27);
for(int i = 0; i < 26; i++)
ansi[i] = 48 + i + 1;
wchar_t* wstr = (wchar_t*)calloc(sizeof(WCHAR),26);
MultiByteToWideChar(CP_ACP, 0, ansi, 26, wstr, 26) ;
*x = (BSTR)wstr;
}

Please suggest how to solve this error.

Manoj Kumar Chauhan

CPallini1-Jul-08 23:33

1-Jul-08 23:33

BSTR are strange beasts, you need to use SysAllocString [^](or similar) to allocate a new BSTR object.
Smile | :)

MKUser2-Jul-08 19:53

2-Jul-08 19:53

Ok Let me explain my problem in detail.
In COM i create one function which takes one input and another output argument
idl file contains
HRESULT functionname([in] int length,[out,retval] BSTR* OutArray)

In function definition in COM class

HRESULT functionname(int length,BSTR* OutArray)
{
WSTR warray;
suppose warray contains some string and i want to return that string so i'll write

*OutArray = (BSTR) warray;

or if i try with following code then i think it's not correct
*OutArray = SysAllocString(wstr);
}

i use this dll and call it like
BSTR str;
str = cominterface->functionname(pass input length here);

When i build exe it generate tlh and tli files.
The tli file contains following code for that function
inline _bstr_t IExportComDll::functionname( int length ) {
BSTR _result = 0;
HRESULT _hr = raw_functionname(length, &result);
if (FAILED(_hr)) _com_issue_errorex(_hr, this, __uuidof(this));
return _bstr_t(_result, false);
}

Now please tell me where i can change the code. Where to use SysAllocString. I cant change the code inside tli file. This function returns a string. So ATL write its wrapper code in tli file. When i call this function in some exe,After returning from wrapper code of tli file i get memory error because it try to free BSTR string.

Manoj Kumar Chauhan

CPallini2-Jul-08 21:18

2-Jul-08 21:18

Why do you need to pass the length?
For instance the following should work fine:

HRESULT functionname(BSTR * OutArray)
{
  *OutArray = SysAllocString( L"Foo");
}

in the client application you may do

BSTR str;
str = cominterface->functionname();

You should also remember to call SysFreeString(str); there.
Smile | :)

MKUser2-Jul-08 21:56

2-Jul-08 21:56

I'm doing following

HRESULT functionname(BSTR * OutArray)
{
// suppose there is some ansi array which contains some data
// length is int type which keeps size of this ansi array
wchar_t* wstr = (wchar_t*)calloc(sizeof(WCHAR),length+1);
MultiByteToWideChar(CP_ACP, 0, ansiarray, length, wstr, length) ;
free(ansiarray);
ansiarray= NULL;
*OutArray = SysAllocString( wstr);//(BSTR)wstr;
memset(wstr,65,sizeof(WCHAR) * length+1);
free(wstr);
wstr = NULL;
}

When i use this dll
BSTR st;
st = functionname();
After this statement st contains some garbage data.

While when i debug the code in tli file, it returns valid data

inline _bstr_t IExportComDll::functionname( )
{
BSTR _result = 0;
HRESULT _hr = raw_functionname(&_result);
if (FAILED(_hr)) _com_issue_errorex(_hr, this, __uuidof(this));
return _bstr_t(_result, false);
//here _result contains valid data which is returned by COM function
//HRESULT functionname(BSTR * OutArray)

}

Now please tell me where i'm wrong. Please correct my mistake

Manoj Kumar Chauhan

CPallini2-Jul-08 22:08

2-Jul-08 22:08

MKUser wrote:
inline _bstr_t IExportComDll::functionname( )

The above function returns a _bstr_t instance. Possibly it cannot assigned as it stands to as BSTR variable, hence you need either to
define st as _bstr_t, for instance

_bstr_t st;
st = YourCOMInterface->functionname();

or

use the Detach method of _bstr_t (see [^]), for instance

BSTR st;
st = (YourCOMInterface->functionname()).Detach();

MKUser2-Jul-08 22:58

2-Jul-08 22:58

Thanks
it's working now

Manoj Kumar Chauhan

Vi22-Jul-08 22:08

Vi2

2-Jul-08 22:08

Imported "cominterface->functionname" returns C++ class "_bstr_t", which has a destructor and misleading "operator wchar_t*".

Valid variant is:

str = cominterface->functionname(pass input length here).copy();

With best wishes,
Vita

MKUser2-Jul-08 23:02

2-Jul-08 23:02

Thanks it's also working

Manoj Kumar Chauhan

Vi22-Jul-08 17:38

Vi2

2-Jul-08 17:38

If i want to return any variable length string from COM then i can allocate memory in COM and return it to exe (return BSTR*). Finall exe which's using the COM will free the memory. Is this OK?

Yes.

String Manipulation Functions Descriptions 
------------------------------------------
SysAllocString        Creates and initializes a string. 
SysAllocStringByteLen Creates a zero-terminated string of a specified length (32-bit only). 
SysAllocStringLen     Creates a string of a specified length. 
SysFreeString         Frees a previously created string. 
SysReAllocString      Changes the size and value of a string. 
SysReAllocStringLen   Changes the size of an existing string. 
SysStringByteLen      Returns the length of a string in bytes (32-bit only). 
SysStringLen          Returns the length of a string.

There are some C++ classes working with BSTR: CComBSTR, _bstr_t.

How to make same parameter as input/output in COM function?

You can specify [in] for input parameter and [out] for output. I have shown this recently.

How to return more than one string through COM function. suppose if i have function
int func(BSTR inputstring, BSTR* outstring1, BSTR* outstring 2);
Here i want to return 2 strings from this function.

Simple answer is:

int func([in] BSTR inputstring, [out] BSTR* outstring1, [out] BSTR* outstring 2)

But this code will not be working with some scripts, because the basic data for script is VARIANT and not all IDispatch handler can process these data. Moreover, for example, Jscript cannot receive any [out] data at all, but only with [out, retval].

With best wishes,
Vita

CPallini1-Jul-08 22:08

1-Jul-08 22:08

Vi2 wrote:
"iostream" object is C++ object, not accessible directly from scripts

True.

Vi2 wrote:
and unallowed in COM accordingly

False. It's not allowed in COM automation only.

CPallini1-Jul-08 3:25