在 Unix 上将 C++ std::clog 重定向到 syslog [英] Redirect C++ std::clog to syslog on Unix

问题描述

我在 Unix 上开发一个 C++ 程序，该程序将消息发送到系统日志.

I work on Unix on a C++ program that send messages to syslog.

当前代码使用与 printf 类似的 syslog 系统调用.

The current code uses the syslog system call that works like printf.

现在我更愿意为此目的使用流，通常是内置的std::clog.但是 clog 只是将输出重定向到 stderr，而不是 syslog，这对我来说毫无用处，因为我也将 stderr 和 stdout 用于其他目的.

Now I would prefer to use a stream for that purpose instead, typically the built-in std::clog. But clog merely redirect output to stderr, not to syslog and that is useless for me as I also use stderr and stdout for other purposes.

我在另一个答案中看到很容易将其重定向到一个使用 rdbuf() 的文件，但我认为无法应用该方法来调用 syslog，因为 openlog 不返回我可以用来在其上绑定流的文件处理程序.

I've seen in another answer that it's quite easy to redirect it to a file using rdbuf() but I see no way to apply that method to call syslog as openlog does not return a file handler I could use to tie a stream on it.

还有其他方法可以做到这一点吗?(对于 Unix 编程看起来非常基础)?

Is there another method to do that ? (looks pretty basic for unix programming) ?

我正在寻找不使用外部库的解决方案.@Chris 提出的建议可能是一个好的开始，但要成为公认的答案仍然有点含糊.

I'm looking for a solution that does not use external library. What @Chris is proposing could be a good start but is still a bit vague to become the accepted answer.

编辑:使用 Boost.IOStreams 没问题，因为我的项目无论如何都已经使用了 Boost.

Edit: using Boost.IOStreams is OK as my project already use Boost anyway.

与外部库链接是可能的，但也是一个问题，因为它是 GPL 代码.依赖也是一种负担，因为它们可能与其他组件冲突，在我的 Linux 发行版上不可用，引入第三方错误等.如果这是唯一的解决方案，我可能会考虑完全避免流......(可惜).

Linking with external library is possible but is also a concern as it's GPL code. Dependencies are also a burden as they may conflict with other components, not be available on my Linux distribution, introduce third-party bugs, etc. If this is the only solution I may consider completely avoiding streams... (a pity).

推荐答案

我也需要像这样简单的东西，所以我把它放在一起:

I needed something simple like this too, so I just put this together:

log.h:

#include <streambuf>
#include <syslog.h>
enum LogPriority {
    kLogEmerg   = LOG_EMERG,   // system is unusable
    kLogAlert   = LOG_ALERT,   // action must be taken immediately
    kLogCrit    = LOG_CRIT,    // critical conditions
    kLogErr     = LOG_ERR,     // error conditions
    kLogWarning = LOG_WARNING, // warning conditions
    kLogNotice  = LOG_NOTICE,  // normal, but significant, condition
    kLogInfo    = LOG_INFO,    // informational message
    kLogDebug   = LOG_DEBUG    // debug-level message
};

std::ostream& operator<< (std::ostream& os, const LogPriority& log_priority);

class Log : public std::basic_streambuf<char, std::char_traits<char> > {
public:
    explicit Log(std::string ident, int facility);

protected:
    int sync();
    int overflow(int c);

private:
    friend std::ostream& operator<< (std::ostream& os, const LogPriority& log_priority);
    std::string buffer_;
    int facility_;
    int priority_;
    char ident_[50];
};

log.cc:

#include <cstring>
#include <ostream>
#include "log.h"

Log::Log(std::string ident, int facility) {
    facility_ = facility;
    priority_ = LOG_DEBUG;
    strncpy(ident_, ident.c_str(), sizeof(ident_));
    ident_[sizeof(ident_)-1] = '';

    openlog(ident_, LOG_PID, facility_);
}

int Log::sync() {
    if (buffer_.length()) {
        syslog(priority_, "%s", buffer_.c_str());
        buffer_.erase();
        priority_ = LOG_DEBUG; // default to debug for each message
    }
    return 0;
}

int Log::overflow(int c) {
    if (c != EOF) {
        buffer_ += static_cast<char>(c);
    } else {
        sync();
    }
    return c;
}

std::ostream& operator<< (std::ostream& os, const LogPriority& log_priority) {
    static_cast<Log *>(os.rdbuf())->priority_ = (int)log_priority;
    return os;
}

在 main() 中，我初始化了堵塞:

In main() I initialize clog:

std::clog.rdbuf(new Log("foo", LOG_LOCAL0));

然后每当我想登录时，这很容易:

Then whenever I want to log, it's easy:

std::clog << kLogNotice << "test log message" << std::endl;

std::clog << "the default is debug level" << std::endl;

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