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.

Are you debugging an optimised build? That will probably not bother to preserve `pDst` across the final function call, since it's not needed after the assignment to `lDest`. What happens if you step out of the function, and look at the result from the caller's viewpoint? — Mike Seymour, Jan 17 '12 at 11:25
Absolutely not, this is a debug build (/MDd), and once returned, the value in my buffer is also ÌÌÌÌÌÌÌÌÌ... — moala, Jan 17 '12 at 11:28

score 5 · Accepted Answer · answered Jan 17 '12 at 11:27

5

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.

answered Jan 17 '12 at 11:27

Some programmer dude

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score 1 · Answer 2 · answered Jan 17 '12 at 11:27

1

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.

answered Jan 17 '12 at 11:27

spraff

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I should have read the pointer value, at least I would have found that it was changing in the UuidToStringA call. – moala Jan 17 '12 at 11:40

score 0 · Answer 3 · edited May 23 '17 at 11:55

0

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);
}

edited May 23 '17 at 11:55

Community

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answered Jan 17 '12 at 12:24

moala

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reinterpret_cast on pointers pitfall between char and unsigned char?

3 Answers3