std：：cerr 不会等待 std：：cout(运行 CTest 时)

上下文

我编写了一个记录器，为用户打印消息。级别为"debug"、"info"或"warning"的消息在std::cout中打印，级别为"error"或"system_error"的消息在std::cerr中打印。我的程序不是多线程的。我在Linux openSUSE 12.3下工作，gcc 4.7.2和CMake 3.1.0。

我发现，有时，当一条错误消息(在std::cerr中打印)跟在一条长信息消息(在std::cout中打印)之后，当CTest将输出重定向到文件LastTest.log时，错误消息出现在信息消息中(参见下面的示例)。我不太理解这种行为，但我假设为std::cout启动了一个写线程，然后代码继续，为std::cerr启动了另一个写线程，而不等待第一个线程终止。

如果不使用std::cout，是否可以避免这种情况?

我的终端机没有问题。只有当CTest将输出重定向到LastTest.log文件时才会发生这种情况。

注意，我的缓冲区被刷新了。这不是std::endl在调用std::cerr之后出现的问题!

期望行为:

[ 12:06:51.497   TRACE ] Start test
[ 12:06:52.837 WARNING ] This
                         is
                         a
                         very
                         long
                         warning
                         message...
[ 12:06:52.837   ERROR ] AT LINE 49 : 7
[ 12:06:52.841   ERROR ] AT LINE 71 : 506
[ 12:06:52.841   TRACE ] End of test

发生了什么:

[ 12:06:51.497   TRACE ] Start test
[ 12:06:52.837 WARNING ] This
                         is
                         a
                         very
                         long
[ 12:06:52.837   ERROR ] AT LINE 49 : 7
                         warning
                         message...
[ 12:06:52.841   ERROR ] AT LINE 71 : 506
[ 12:06:52.841   TRACE ] End of test

如何调用日志

下面是我如何用记录器调用std::cout或std::cerr的示例。我用这样的宏调用记录器:

#define LOG_DEBUG(X) {if(Log::debug_is_active()){std::ostringstream o;o<<X;Log::debug(o.str());}}
#define LOG_ERROR(X) {if(Log::error_is_active()){std::ostringstream o;o<<X;Log::error(o.str());}}
//...
LOG_DEBUG("This" << std::endl << "is" << std::endl << "a message");
LOG_ERROR("at line " << __LINE__ << " : " << err_id);

void Log::debug(const std::string& msg)
{
    Log::write_if_active(Log::DEBUG, msg);
}
void Log::error(const std::string& msg)
{
    Log::write_if_active(Log::ERROR, msg);
}
//...
void Log::write_if_active(unsigned short int state, const std::string& msg)
{
    Instant now;
    now.setCurrentTime();
    std::vector<std::string> lines;
    for(std::size_t k = 0; k < msg.size();)
    {
        std::size_t next_endl = msg.find('n', k);
        if(next_endl == std::string::npos)
            next_endl = msg.size();
        lines.push_back(msg.substr(k, next_endl - k));
        k = next_endl + 1;
    }
    boost::mutex::scoped_lock lock(Log::mutex);
    for(unsigned long int i = 0; i < Log::chanels.size(); ++i)
        if(Log::chanels[i])
            if(Log::chanels[i]->flags & state)
                Log::chanels[i]->write(state, now, lines);
}

这里，日志通道是专用于终端输出的对象，写入函数为:

void Log::StdOut::write(unsigned short int state, const Instant& t, const std::vector<std::string>& lines)
{
    assert(lines.size() > 0 && "PRE: empty lines");
    std::string prefix =  "[ ";
    if(this->withDate || this->withTime)
    {
        std::string pattern = "";
        if(this->withDate)
            pattern += "%Y-%m-%d ";
        if(this->withTime)
            pattern += "%H:%M:%S.%Z ";
        prefix += t.toString(pattern);
    }
    std::ostream* out = 0;
    if(state == Log::TRACE)
    {
        prefix += "  TRACE";
        out = &std::cout;
    }
    else if(state == Log::DEBUG)
    {
        prefix += "  DEBUG";
        out = &std::cout;
    }
    else if(state == Log::INFO)
    {
        prefix += "   INFO";
        out = &std::cout;
    }
    else if(state == Log::WARNING)
    {
        prefix += "WARNING";
        out = &std::cout;
    }
    else if(state == Log::ERROR)
    {
        prefix += "  ERROR";
        out = &std::cerr;
    }
    else if(state == Log::SYS_ERROR)
    {
        prefix += "SYERROR";
        out = &std::cerr;
    }
    else
        assert(false && "PRE: Invalid Log state");
    prefix += " ] ";
    (*out) << prefix << lines[0] << "n";
    prefix = std::string(prefix.size(), ' ');
    for(unsigned long int i = 1; i < lines.size(); ++i)
        (*out) << prefix << lines[i] << "n";
    out->flush();
}

您可以看到，当执行日志指令时，我的缓冲区被刷新。