Solving tcsncpy_s.inl assertion (line 24)

Question

I've a fairly simple program which needs user input in the form of a text string. I've a CLR form with an edit box and I need to take that input and pass it into my class which just copies it to a member variable.

In the Form.h code, handling the TextChanged event is...

int textLength = m_userDest->TextLength;

if (textLength > 2 && textLength < 5)
{
     // Could be an ICAO code in here
     char dest[5];
     String^ text = m_userDest->Text->ToUpper();
     sprintf_s(dest, 5, "%s", text);
     airTraffic.SetUserDest(dest);
}

My class (airTraffic) SetUserDest function is just

void CAirTraffic::SetUserDest(char* dest)
{
    strncpy_s(m_userDest, 5, dest, 5);
}

When this is run I get this debug assertion, it doesn't stay on the screen and automatically clears after a few seconds.

Debug Assertion Failed!

Program: ...sual Studio 2010\Projects\FSAirTraffic\Debug\FSAirTraffic.exe

File: f:\dd\vctools\crt_bld\self_x86\crt\tcsncpy_s.inl

Line: 24

Expression: ((_Dst)) != NULL && ((_SizeInBytes)) > 0

I don't have an f:\ drive so I'm guessing this is some internal Microsoft(?) code so I can't see the context of the assertion and exactly what it's problem is. I don't have a file called tcsncpy_s.inl on my machine.

If I don't call my class function then there's no assertion so I assumed that was the problem.

Curiously though, when stepping through the debugger the assertion occurs as I step out of the TextChanged event, with the rest of the functions operating as intended (as far as I can see).

Does anyone know what the problem is and how I can go about solving it?

Answer 1

I don't understand how your code works. You use m_userDest twice, first it appears to be a pointer to a structure of some sort, maybe a handle to a TextBox control:

int textLength = m_userDest->TextLength;

Later you pass it to strncpy_s, which needs a char*, not a pointer to some structure.

void CAirTraffic::SetUserDest(char* dest)
{
    strncpy_s(m_userDest, 5, dest, 5);
}

While it's possible for a structure to implicitly convert to a char*, it's not possible for a structure pointer to do so. Perhaps there's a smart pointer involved? Or you are using the same member variable name for completely different purposes in different classes¹?

In any case, strncpy_s is inspecting the value of its first argument and not liking it.

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¹ Note that the new "wisdom" saying not to use Hungarian notation has destroyed the ability to understand this code in textual form. We don't have an IDE providing mouseover information about the data type of variables. Applications Hungarian is still a good idea in the real world, despite how many "best practices" documents decry it. Amazing how many code style documents are written from a purely theoretical basis.