reinterpret_cast on pointers pitfall between char and unsigned char?

Question

I'm using the following function in order to create an UUID and write it in a human-readable form into a pre-allocated buffer. Something goes wrong.

void createUUID(char* pDst)
{
    UUID    lUUIDObj;
    UuidCreate(&lUUIDObj);
    unsigned char*  lDest = reinterpret_cast<unsigned char*>(pDst);
    UuidToStringA(&lUUIDObj, &lDest)
}

At the end of the method, the debugger says:

• lDest = 0x01fe4fd8 "df4a5ed8-c0d2-495a-84d7-ce0e07cf2113"
• pDst = 0x0012ec7c "ÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌ"

I thought that both would have the same content, however it is not the case.

What happened? Thanks.

Answer 1

Looking at the documentation for UuidToStringA, it says:

The RPC run-time library allocates memory for the string returned in the StringUuid parameter.

This means that after the call lDest does no longer point to pDst.

Answer 2

void createUUID(char* pDst)
{
    UUID    lUUIDObj;
    UuidCreate(&lUUIDObj);
    unsigned char*  lDest = reinterpret_cast<unsigned char*>(pDst);
    //UuidToStringA(&lUUIDObj, &lDest);
    UuidToStringA(&lUUIDObj, lDest);
}

Looks like you clobbered the value of lDest without changing the values it originally pointed to.

Answer 3

In order to complete Joachim Pileborg's answer, here is the corrected function:

void createUUID(char* pDst)
{
    UUID    lUUIDObj;
    UuidCreate(&lUUIDObj);
    unsigned char*  lTmpStr;
    UuidToStringA(&lUUIDObj, &lTmpStr);
    sprintf(pDst, reinterpret_cast<char*>(lTmpStr));
    RpcStringFreeA(&lTmpStr);
}