string to char* marshaling

Question

I wrote a managed C++ class that has the following function:

void EndPointsMappingWrapper::GetLastError(char* strErrorMessage)
{
    strErrorMessage = (char*) Marshal::StringToHGlobalAnsi(_managedObject->GetLastError()).ToPointer();
}

As you can see, this is a simple method to copy the managed string of the last error to the unmanaged world (char*).

From my unmanaged class I call the method like this:

char err[1000];
ofer->GetLastError(err);

Putting a breakpoint at the managed C++ method shows that the string is successfully translated into the char*. However, once I return to the unmanaged class, the content of err[1000] is lost and it's empty again.

Answer 1

You are assigning the value of the passed parameter (strErrorMessage) instead of copying to that address the content of the buffer returned by Marshal::StringToHGlobalAnsi.

A correct implementation should be:

void EndPointsMappingWrapper::GetLastError(char* strErrorMessage, int len) 
{ char *str = (char*) Marshal::StringToHGlobalAnsi(_managedObject->GetLastError()).ToPointer(); 
 strncpy(strErrorMessage,str,len);
 strErrorMessage[len-1] = '\0';
 Marshal::FreeHGlobal(IntPtr(str));
}

The length is the size of the buffer passed.

strncpy() will copy at the most len bytes. If there is no null byte among the first n bytes of the str, the destination string won't be null terminated. For that reason we force the '\0' in the last byte of the buffer.

Answer 2

We use the following C++ Class to do the conversions for us and it works fine. You should be able to modify your method to use it.

H File

public ref class  ManagedStringConverter
{
public:
  ManagedStringConverter( System::String^ pString );
  ~ManagedStringConverter();

  property char* PrimitiveString
  {
     char* get() { return m_pString; }
  }

  /// <summary>
  /// Converts a System::String to a char * string.  You must release this with FreeString.
  /// </summary>
  static const char* StringToChar( System::String^ str );

  /// <summary>
  /// Converts a System::String to a __wchar_t * string.  You must release this with FreeString.
  /// </summary>
  static const __wchar_t * StringToWChar( System::String^ str );

  /// <summary>
  /// Frees memory allocated in StringToChar()
  /// </summary>
  static void FreeString( const char * pszStr );

private:
  char* m_pString;
};

CPP File

ManagedStringConverter::ManagedStringConverter( System::String^ pString )
{
  m_pString = const_cast<char*>( ManagedStringConverter::StringToChar( pString ) );
}

ManagedStringConverter::~ManagedStringConverter()
{
  ManagedStringConverter::FreeString( m_pString );
}

// static
const char * ManagedStringConverter::StringToChar( System::String^ str )
{
  IntPtr^ ip = Marshal::StringToHGlobalAnsi( str );
  if ( ip != IntPtr::Zero )
  {
     return reinterpret_cast<const char *>( ip->ToPointer() );
  }
  else
  {
     return nullptr;
  }
}

// static
const __wchar_t * ManagedStringConverter::StringToWChar( System::String^ str )
{
  IntPtr^ ip = Marshal::StringToHGlobalUni( str );
  if ( ip != IntPtr::Zero )
  {
     return reinterpret_cast<const __wchar_t *>( ip->ToPointer() );
  }
  else
  {
     return nullptr;
  }
}

// static
void ManagedStringConverter::FreeString( const char * pszStr )
{
  IntPtr ip = IntPtr( (void *)pszStr );
  Marshal::FreeHGlobal( ip );
}

Answer 3

The problem is that StringToHGlobalAnsi creates a new unmanged memory and does not copy into the memory you intended to use which you assigned into strErrorMessage.
To resolve this you should do some thing like:

void EndPointsMappingWrapper::GetLastError(char** strErrorMessage) 
{ 
  *strErrorMessage = (char*) Marshal::StringToHGlobalAnsi(_managedObject->GetLastError()).ToPointer(); 
}

And the usage should look like:

char* err;
GetLastError(&err);

//and here you need to free the error string memory

for more information check out this msdn article