How come lock_guard doesn't work?

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Below is my code snippet for mutual exclusion of execution by multi-threads:

C#

void logMsg (char *msg)
{
    FILE * pFile;
    try {
        lock_guard<mutex> lock(mutexLog);
        pFile = openLogFile();
        if (pFile !=NULL)
        {
            fprintf (pFile, "%s", msg);
            fclose (pFile);
        }
        else
            printf ("failed to log %s in logMsg()\n", msg);
    } catch (std::exception& e) {
        std::cerr << "logMsg(): " << e.what() << "\n";
    }
}

I called logMsg() everywhere by more than one thread. Lock_guard seems not working because message of "failed to log ...." keeps being displayed. How come? Any wrong with my codes?

Posted 3-Aug-15 17:05pm

Stan Huang

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Sergey Alexandrovich Kryukov 4-Aug-15 0:50am

Where is the declaration and initialization of mutexLog. What do you mean by "not working"? How "keeps being displayed" related to the lock?
—SA

Ashish Tyagi 40 4-Aug-15 19:24pm

openLogFile() call is returning NULL, fix that first.

Stan Huang 4-Aug-15 19:38pm

I missed it and what you said is right.

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Sergey Alexandrovich Kryukov · Accepted Answer · 2015-08-03T18:57:00

Please see my comment to the question. If does not make any sense, because the "problem" 1) is not a problem at all, 2) is not related to locking in any way.
Your pFile value is null, that's all. See what openLogFile() does, use the debugger. That's the solution.

As to the logging, it is used when some resource is shared between two or more threads. If you don't do it, you don't need locking. I have an impression that you don't have a clue what is it used for. You should not write a single line you don't perfectly understand. So, please read:
https://en.wikipedia.org/wiki/Mutual_exclusion[^],
http://en.cppreference.com/w/cpp/thread/lock_guard[^].

—SA