Treat logging to /dev/stderr specially, log there by default

... mainly for integration with systemd.

Adapted from
NixOS/nixpkgs@83e1400

Author: aszlig

cfg.cpp

@@ -58,7 +58,7 @@ Cfg::Cfg() : currentSession(-1)
 	options.insert(option("auto_login", "no"));
 	options.insert(option("current_theme", "default"));
 	options.insert(option("lockfile", "/var/run/slim.lock"));
-	options.insert(option("logfile", "/var/log/slim.log"));
+	options.insert(option("logfile", "/dev/stderr"));
 	options.insert(option("authfile", "/var/run/slim.auth"));
 	options.insert(option("shutdown_msg", "The system is halting..."));
 	options.insert(option("reboot_msg", "The system is rebooting..."));

log.cpp

@@ -1,4 +1,5 @@
 #include "log.h"
+#include <cstring>
 #include <iostream>
 
 bool LogUnit::openLog(const char * filename)
@@ -9,8 +10,12 @@ bool LogUnit::openLog(const char * filename)
 			 << endl;
 		logFile.close();
 	}
-	logFile.open(filename, ios_base::app);
 
+	// cerr is the default
+	if (strcmp(filename, "/dev/stderr") == 0)
+		return true;
+
+	logFile.open(filename, ios_base::app);
 	return !(logFile.fail());
 }
 

log.h

@@ -9,12 +9,14 @@
 #endif
 #include "const.h"
 #include <fstream>
+#include <iostream>
 
 using namespace std;
 
 static class LogUnit
 {
 	ofstream logFile;
+	inline ostream & getStream() { return logFile.is_open() ? logFile : cerr; }
 
 public:
 	bool openLog(const char * filename);
@@ -24,22 +26,22 @@ static class LogUnit
 
 	template <typename Type> LogUnit & operator<<(const Type & text)
 	{
-		logFile << text;
-		logFile.flush();
+		getStream() << text;
+		getStream().flush();
 		return *this;
 	}
 
 	LogUnit & operator<<(ostream & (*fp)(ostream &))
 	{
-		logFile << fp;
-		logFile.flush();
+		getStream() << fp;
+		getStream().flush();
 		return *this;
 	}
 
 	LogUnit & operator<<(ios_base & (*fp)(ios_base &))
 	{
-		logFile << fp;
-		logFile.flush();
+		getStream() << fp;
+		getStream().flush();
 		return *this;
 	}
 } logStream;