CODEWITHSUNDEEP
c++variadic-functions
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SPlatten

Reputation: 5762

C function with variable arguments crashing

I have written a function to help me debug a project:

    static void outputDebug(unsigned uintLineNo, const char* cpszFormat, ...) {
        char szPrefix[80], szOut[256];
        va_list ap;
        va_start(ap, cpszFormat);
        sprintf_s(szPrefix, sizeof(szPrefix), "%s %s %05ld %s\n"
                          , __DATE__, __TIME__, uintLineNo, cpszFormat);
        vsprintf_s(szOut, sizeof(szOut), "%s", szPrefix, ap);   
        va_end(ap);
        OutputDebugString(szOut);
    }

An example of this function:

    outputDebug(__LINE__, "AdapterInit(%s)", "Test");

Everything after the format string AdapterInit(%s) is optional, I've single stepped the function and it constructs the szPrefix without problems, as an example this would contain something like; Apr 18 2018 07:33:07 01492 Adapter(%s)

The next line vsprintf results in a exception:

    Unhandled exception at 0x0781e9ee (msvcr90d.dll) in ....

I can't see what I've done wrong, how can I resolve this?

[Edit] The problem was in the line starting vsprintf, removing the unnecessary %s resolved the problem.

Working solution:

    static void outputDebug(unsigned uintLineNo, const char* cpszFormat, ...) {
        char szPrefix[80], szOut[256];
        va_list ap;
        va_start(ap, cpszFormat);
        sprintf_s(szPrefix, sizeof(szPrefix), "%s %s %05ld %s\n"
                          , __DATE__, __TIME__, uintLineNo, cpszFormat);
        vsprintf_s(szOut, sizeof(szOut), szPrefix, ap); 
        va_end(ap);
        OutputDebugString(szOut);
    }

Upvotes: 0

Views: 192

Answers (1)

StoryTeller - Unslander Monica
StoryTeller - Unslander Monica

Reputation: 170203

If I gather correctly, you construct szPrefix as the new format string (by interpolating cpszFormat into it, adding a prefix). So the call to vsprintf_s should use that, instead of %s. I.e.

vsprintf_s(szOut, sizeof(szOut), szPrefix, ap);

Your chosen size for szPrefix (that 80) is also fairly optimistic. May be worth increasing it a notch or two.

Upvotes: 1

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