How do I convert a std::wstring to an LSA_UNICODE_STRING

Question

Today I was able to write a simple C++ program that granted a user the "Log on as a service" privilege. Part of this involved converting between a LPCWSTR and an LSA_UNICODE_STRING. The code to do that is here:

LSA_UNICODE_STRING StringToLsaUnicodeString(LPCWSTR string) {
    LSA_UNICODE_STRING lsaString;
    DWORD dwLen = 0;

    dwLen = wcslen(string);
    lsaString.Buffer = (LPWSTR) string;
    lsaString.Length = (USHORT)((dwLen) * sizeof(WCHAR));
    lsaString.MaximumLength = (USHORT)((dwLen + 1) * sizeof(WCHAR));
    return lsaString;
}

When I had some small errors in this function, my call to LsaLookupNames2() failed with a code 87(hex 0x57) "The parameter is incorrect." I am trying to make this call in a C++ app that uses std::wstring and it is failing. My current function there is as follows:

#if defined(_UNICODE)
    LSA_UNICODE_STRING toLsaUnicodeString (std::wstring str) {
        LSA_UNICODE_STRING lsaWStr;
        DWORD len = 0;

        LPWSTR cstr = (LPWSTR)str.c_str();
        len = wcslen(cstr);
        lsaWStr.Buffer = cstr;
        lsaWStr.Length = (USHORT)((len) * sizeof(WCHAR));
        lsaWStr.MaximumLength = (USHORT)((len + 1) * sizeof(WCHAR));
        return lsaWStr;
    } 
#endif

What am I doing wrong?

Answer 1

You're likely encountering a lifetime issue with the wchar_t* returned from str.c_str(). str.c_str() will return a pointer to an underlying string whose lifetime is governed by str. Since str is passed by-value, it will be destroyed at the end of the toLsaUnicodeString function, resulting in the returned LSA_UNICODE_STRING pointing at memory that has been deallocated. In order to avoid this, you'll need to make a copy of the underlying string in the toLsaUnicodeString function, and associate the copy with the returned LSA_UNICODE_STRING, something like:

LSA_UNICODE_STRING toLsaUnicodeString (const std::wstring& str) {
    LSA_UNICODE_STRING lsaWStr;
    DWORD len = 0;

    len = str.length(); 
    LPWSTR cstr = new WCHAR[len + 1];
    memcpy(cstr, str.c_str(), (len + 1) * sizeof(WCHAR));
    lsaWStr.Buffer = cstr;
    lsaWStr.Length = (USHORT)((len) * sizeof(WCHAR));
    lsaWStr.MaximumLength = (USHORT)((len + 1) * sizeof(WCHAR));
    return lsaWStr;
}

Since the memory is now allocated on the heap, you are responsible for making sure it is deallocated. You can use a function like the following to take care of this.

void freeLsaUnicodeString(LSA_UNICODE_STRING& str) {
    delete [] str.Buffer;
    str.Buffer = 0;
    str.Length = 0;
    str.MaximumLength = 0;
}

Even better would be to use RAII to manage the memory and guarantee that it is released when the variable is no longer in use. See Mr_C64's answer for details on this approach.

Answer 2

You can use the RtlInitUnicodeString function in order to initialize a unicode string. After using the UNICODE_STRING call RtlFreeUnicodeString.

The UNICODE_STRING and LSA_UNICODE_STRING are identical.

Answer 3

I think the correct way of doing this in C++ is to write a RAII wrapper class around the raw C structure LSA_UNICODE_STRING.

The constructor overloads of this class properly initialize it, the destructor releases allocated resources (helping writing exception-safe code), and you can provide some operator= overloads to do proper deep-copies.

Instead of using explicit new[] and delete[], the dynamically-allocated WCHAR buffer is managed by an instance of std::vector, which simplifies the code (e.g. std::vector's destructor will automatically release the allocated memory).

Something like this:

#include <windows.h>     // Win32 SDK header
#include <LsaLookup.h>   // LSA_UNICODE_STRING
#include <vector>        // std::vector
#include <string>        // std::wstring


//
// C++ RAII wrapper to LSA_UNICODE_STRING
//
class LsaUnicodeString
{
public:

    LsaUnicodeString()
    {
        SetEmpty();
    }


    LsaUnicodeString(const LsaUnicodeString & source)
    {
        CopyFrom(source);
    }


    explicit LsaUnicodeString(const std::wstring & source)
    {
        CopyFrom(source);
    }


    ~LsaUnicodeString()
    {
        // Nothing to do:
        // the string buffer is managed by std::vector data member
    }


    LsaUnicodeString & operator=(const LsaUnicodeString & source)
    {
        if (&source != this)
        {
            CopyFrom(source);
        }
        return *this;
    }


    LsaUnicodeString & operator=(const std::wstring & source)
    {
        CopyFrom(source);
        return *this;
    }


    const LSA_UNICODE_STRING & Get() const
    {
        return m_us;
    }


    //
    // Implementation
    //
private:
    LSA_UNICODE_STRING m_us;        // raw C structure
    std::vector<WCHAR> m_buffer;    // string content


    void SetEmpty()
    {
        m_buffer.resize(1);
        m_buffer[0] = L'\0'; // end-of-string

        m_us.Length = 0;
        m_us.MaximumLength = sizeof(WCHAR);
        m_us.Buffer = &m_buffer[0];
    }


    void CopyFrom(const std::wstring & source)
    {
        if ( source.empty() )
        {
            SetEmpty();
            return;
        }

        const int len = source.length();
        m_buffer.resize(len + 1);
        ::CopyMemory(&m_buffer[0], source.c_str(), (len+1)*sizeof(WCHAR));

        m_us.Length = len * sizeof(WCHAR);
        m_us.MaximumLength = m_us.Length + sizeof(WCHAR);
        m_us.Buffer = &m_buffer[0];
    }


    void CopyFrom(const LsaUnicodeString & source)
    {
        if (source.m_us.Length == 0)
        {
            SetEmpty();
            return;
        }

        m_buffer = source.m_buffer;
        m_us.Length = source.m_us.Length;
        m_us.MaximumLength = source.m_us.MaximumLength;
        m_us.Buffer = &m_buffer[0];
    }
};