Redirect C++ std::clog to syslog on Unix

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] = '\0';

    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;
}

In main() I initialize clog:

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

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;

You could define an streambuf that calls syslog. For example:

// Pseudo-code
class syslog_streambuf : public streambuf { 
private: 
    void internal_log(string& log) { 
        syslog(..., log, ...); 
    }
public: 
    int sputc ( char c ) { 
        internal_log(...); 
    }
    streamsize sputn ( const char * s, streamsize n ) { 
        internal_log(...); 
    } 
}

then you would simply write the following to redirect clog:

clog.rdbuf( new syslog_streambuf ); 

There are a few more functions you would probably have to override, here's a good reference to the streambuf api.

Another version in part inspired by eater. It doesn't redirect std::clog per se, but uses familiar stream syntax.

#ifndef SYSLOG_HPP
#define SYSLOG_HPP

#include <ostream>
#include <streambuf>
#include <string>

#include <syslog.h>

namespace log
{

enum level
{
    emergency = LOG_EMERG,
    alert     = LOG_ALERT,
    critical  = LOG_CRIT,
    error     = LOG_ERR,
    warning   = LOG_WARNING,
    notice    = LOG_NOTICE,
    info      = LOG_INFO,
    debug     = LOG_DEBUG,
};

enum type
{
    auth   = LOG_AUTH,
    cron   = LOG_CRON,
    daemon = LOG_DAEMON,
    local0 = LOG_LOCAL0,
    local1 = LOG_LOCAL1,
    local2 = LOG_LOCAL2,
    local3 = LOG_LOCAL3,
    local4 = LOG_LOCAL4,
    local5 = LOG_LOCAL5,
    local6 = LOG_LOCAL6,
    local7 = LOG_LOCAL7,
    print  = LOG_LPR,
    mail   = LOG_MAIL,
    news   = LOG_NEWS,
    user   = LOG_USER,
    uucp   = LOG_UUCP,
};

}

class syslog_stream;

class syslog_streambuf: public std::basic_streambuf<char>
{
public:
    explicit syslog_streambuf(const std::string& name, log::type type):
        std::basic_streambuf<char>()
    {
        openlog(name.size() ? name.data() : nullptr, LOG_PID, type);
    }
    ~syslog_streambuf() override { closelog(); }

protected:
    int_type overflow(int_type c = traits_type::eof()) override
    {
        if(traits_type::eq_int_type(c, traits_type::eof()))
            sync();
        else buffer += traits_type::to_char_type(c);

        return c;
    }

    int sync() override
    {
        if(buffer.size())
        {
            syslog(level, "%s", buffer.data());

            buffer.clear();
            level = ini_level;
        }
        return 0;
    }

    friend class syslog_stream;
    void set_level(log::level new_level) noexcept { level = new_level; }

private:
    static constexpr log::level ini_level = log::info;
    log::level level = ini_level;

    std::string buffer;
};

class syslog_stream: public std::basic_ostream<char>
{
public:
    explicit syslog_stream(const std::string& name = std::string(), log::type type = log::user):
        std::basic_ostream<char>(&streambuf),
        streambuf(name, type)
    { }

    syslog_stream& operator<<(log::level level) noexcept
    {
        streambuf.set_level(level);
        return (*this);
    }

private:
    syslog_streambuf streambuf;
};

#endif // SYSLOG_HPP

To use it, you can do something like:

syslog_stream clog;

clog << "Hello, world!" << std::endl;
clog << log::emergency << "foo" << "bar" << "baz" << 42 << std::endl;