Question

Please don’t crucify me for this one. I decided it might be good to use a char* because the string I intended to build was of a known size. I am also aware that if timeinfo->tm_hour returns something other than 2 digits, things are going to go badly wrong. That said, when this function returns VIsual Studio goes ape at me about HEAP CORRUPTION. What’s going wrong? (Also, should I just use a stringbuilder?)

void cLogger::_writelogmessage(std::string Message)
{
    time_t rawtime;
    struct tm* timeinfo = 0;

    time(&rawtime);
    timeinfo = localtime(&rawtime);

    char* MessageBuffer = new char[Message.length()+11];
    char* msgptr = MessageBuffer;

    _itoa(timeinfo->tm_hour, msgptr, 10);
    msgptr+=2;

    strcpy(msgptr, "::");
    msgptr+=2;

    _itoa(timeinfo->tm_min, msgptr, 10);
    msgptr+=2;

    strcpy(msgptr, "::");
    msgptr+=2;

    _itoa(timeinfo->tm_sec, msgptr, 10);
    msgptr+=2;

    strcpy(msgptr, " ");
    msgptr+=1;

    strcpy(msgptr, Message.c_str());

    _file << MessageBuffer;

    delete[] MessageBuffer;
}

Answer 1

You need to allocate one more byte, since .length of a string returns its length without the terminating NUL, for which you also need space in the char*.

I.e. suppose Message.length() returns 10. You allocate 21 bytes. Copy 11 bytes into the buffer, then copy the message, which needs 10 bytes + one for NUL. Total: 22 bytes, and you only have 21 allocated.