Add a syslog() based sink for Linux.

This commit introduce a new sink: syslog_sink.
This sink is Linux only, and will write log entries to the system
logger, using the syslog() library call.

It is instanciable using spdlog::syslog_logger(name). Note that
the suffix _st or _mt is not present, as syslog() is thread-safe.

I also applied @gabime reviews and added license header.

example/example.cpp

@@ -64,6 +64,10 @@ int main(int, char* [])
         SPDLOG_TRACE(file_logger, "This is a trace message (only #ifdef _DEBUG)", 123);
 
 
+	#ifdef __linux__
+	auto syslog_logger = spd::syslog_logger("syslog");
+	syslog_logger->warn("This is warning that will end up in syslog. This is Linux only!");
+	#endif
     }
     catch (const spd::spdlog_ex& ex)
     {

include/spdlog/common.h

@@ -46,17 +46,17 @@ namespace level
 {
 typedef enum
 {
-    TRACE,
-    DEBUG,
-    INFO,
-    NOTICE,
-    WARN,
-    ERR,
-    CRITICAL,
-    ALERT,
-    EMERG,
-    ALWAYS,
-    OFF
+    TRACE    = 0,
+    DEBUG    = 1,
+    INFO     = 2,
+    NOTICE   = 3,
+    WARN     = 4, 
+    ERR      = 5,
+    CRITICAL = 6,
+    ALERT    = 7,
+    EMERG    = 8,
+    ALWAYS   = 9,
+    OFF      = 10
 } level_enum;
 
 static const char* level_names[] { "trace", "debug", "info", "notice", "warning", "error", "critical",

include/spdlog/details/fast_oss.h

@@ -126,7 +126,7 @@ public:
         swap(first._dev, second._dev);
     }
 
-    std::string str()
+    std::string str() const
     {
         auto& buffer = _dev.buf();
         const char*data = buffer.data();

include/spdlog/details/spdlog_impl.h

@@ -30,6 +30,7 @@
 #include "registry.h"
 #include "../sinks/file_sinks.h"
 #include "../sinks/stdout_sinks.h"
+#include "../sinks/syslog_sink.h"
 
 inline std::shared_ptr<spdlog::logger> spdlog::get(const std::string& name)
 {
@@ -81,6 +82,13 @@ inline std::shared_ptr<spdlog::logger> spdlog::stderr_logger_st(const std::strin
 }
 
 
+// Create syslog logger
+inline std::shared_ptr<spdlog::logger> spdlog::syslog_logger(const std::string& logger_name)
+{
+    return create<spdlog::sinks::syslog_sink>(logger_name);
+}
+
+
 inline std::shared_ptr<spdlog::logger> spdlog::create(const std::string& logger_name, spdlog::sinks_init_list sinks)
 {
     return details::registry::instance().create(logger_name, sinks);

include/spdlog/sinks/syslog_sink.h

+/*************************************************************************/
+/* spdlog - an extremely fast and easy to use c++11 logging library.     */
+/* Copyright (c) 2014 Gabi Melman.                                       */
+/*                                                                       */
+/* Permission is hereby granted, free of charge, to any person obtaining */
+/* a copy of this software and associated documentation files (the       */
+/* "Software"), to deal in the Software without restriction, including   */
+/* without limitation the rights to use, copy, modify, merge, publish,   */
+/* distribute, sublicense, and/or sell copies of the Software, and to    */
+/* permit persons to whom the Software is furnished to do so, subject to */
+/* the following conditions:                                             */
+/*                                                                       */
+/* The above copyright notice and this permission notice shall be        */
+/* included in all copies or substantial portions of the Software.       */
+/*                                                                       */
+/* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,       */
+/* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF    */
+/* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.*/
+/* IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY  */
+/* CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,  */
+/* TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE     */
+/* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
+/*************************************************************************/
+
+#pragma once
+
+#ifdef __linux__
+
+#include <cassert>
+#include <string>
+#include <mutex>
+#include <atomic>
+#include <syslog.h>
+#include "./sink.h"
+#include "../formatter.h"
+#include "../common.h"
+#include "../details/log_msg.h"
+
+
+namespace spdlog
+{
+  namespace sinks
+  {
+    /**
+     * Sink that write to syslog using the `syscall()` library call.
+     *
+     * Locking is not needed, as `syslog()` itself is thread-safe.
+     */
+    class syslog_sink : public sink
+    {
+    public:
+      syslog_sink()
+	{
+	  _priorities[static_cast<int>(level::TRACE)] = LOG_DEBUG;
+	  _priorities[static_cast<int>(level::DEBUG)] = LOG_DEBUG;
+	  _priorities[static_cast<int>(level::INFO)] = LOG_INFO;
+	  _priorities[static_cast<int>(level::NOTICE)] = LOG_NOTICE;
+	  _priorities[static_cast<int>(level::WARN)] = LOG_WARNING;
+	  _priorities[static_cast<int>(level::ERR)] = LOG_ERR;
+	  _priorities[static_cast<int>(level::CRITICAL)] = LOG_CRIT;
+	  _priorities[static_cast<int>(level::ALERT)] = LOG_ALERT;
+	  _priorities[static_cast<int>(level::EMERG)] = LOG_EMERG;
+
+	  _priorities[static_cast<int>(level::ALWAYS)] = LOG_INFO;
+	  _priorities[static_cast<int>(level::OFF)] = LOG_INFO;
+	}
+      virtual ~syslog_sink() = default;
+  
+      syslog_sink(const syslog_sink&) = delete;
+      syslog_sink& operator=(const syslog_sink&) = delete;
+  
+      void log(const details::log_msg &msg) override
+      {
+	syslog(syslog_prio_from_level(msg), "%s", msg.formatted.str().c_str());
+      };
+
+    protected:
+      /**
+       * Simply maps spdlog's log level to syslog priority level.
+       */
+      int syslog_prio_from_level(const details::log_msg &msg) const
+      {
+	return _priorities[static_cast<int>(msg.level)];
+      }
+
+    private:
+      std::array<int, 11> _priorities;
+    };
+  }
+}
+
+#endif

include/spdlog/spdlog.h

@@ -68,6 +68,11 @@ std::shared_ptr<logger> stderr_logger_mt(const std::string& logger_name);
 std::shared_ptr<logger> stderr_logger_st(const std::string& logger_name);
 
 
+// Create a syslog logger
+#ifdef __linux__
+ std::shared_ptr<logger> syslog_logger(const std::string& logger_name);
+#endif
+
 
 //
 // Create a logger with multiple sinks