Fprintf奇怪的调试输出

Question

I'm trying to write a function that prints the local time plus the given string, and appends a newline character at the end for readability.

Thats the current code:

void errLog(const std::string errorString)
{
    FILE* errorLog;
    fopen_s(&errorLog, "ErrorLog.txt", "ab+");
    time_t rawTime;
    time (&rawTime);
    struct tm errorTime;
    localtime_s(&errorTime, &rawTime);
    char timeString[20];
    strftime(timeString, 20, "%d-%m-%Y %H:%M:%S", &errorTime);
    fprintf(errorLog, "%s Error: %s\n", timeString, errorString);
    fclose(errorLog);
}

The output is the one I was expecting on Release mode. On the working program ErrorLog.txt has this new line, as intended:

16-09-2014 10:58:45 Error: devFont.ttf load failed

But on debug mode, the output seems to have taken a random string. These are two consecutive outputs, random without apparent explanation to me:

16-09-2014 11:13:57 Error: ΘυB
16-09-2014 11:14:09 Error: Μϊ;

The function is given a hard-coded string on call, so the input of errLog is exactly the same.

Answer 1

You need to use c_str() to access the character pointer to pass to fprintf:

fprintf(errorLog, "%s Error: %s\n", timeString, errorString.c_str());

It's also generally more efficient in most cases to accept your parameter as a const reference const std::string& errorString than to force construction of a new string for the parameter when it may not be required.