Core: Add compile time option to use native OS logging facility

* This change makes it easier to debug issues in UI applications
  which don't necessarily have a console connected to stderr.
* Outputting to the debugger shouldn't occur in normal situations so
  this change has to be explicitly enabled by a build-time config flag.
* Uses OutputDebugString() on Windows platforms or the syslog facility
  on other OSes, if available.
* Also align the report of configure defaults to autotool's.

4 files changed, 56 insertions, 11 deletions

diff --git a/configure.ac b/configure.ac
index 669fcf8..72ed647 100644
--- a/configure.ac
+++ b/configure.ac
@@ -89,7 +89,7 @@ linux)
 	AC_SUBST(OS_LINUX)
 	AC_SEARCH_LIBS(clock_gettime, rt, [], [], -pthread)
 	AC_ARG_ENABLE([udev],
-		[AC_HELP_STRING([--enable-udev], [use udev for device enumeration and hotplug support (recommended, default: yes)])],
+		[AC_HELP_STRING([--enable-udev], [use udev for device enumeration and hotplug support (recommended) [default=yes]])],
 		[], [enable_udev="yes"])
 		if test "x$enable_udev" = "xyes" ; then
 			# system has udev. use it or fail!
@@ -158,7 +158,7 @@ fi
 AC_CHECK_HEADER([sys/timerfd.h], [timerfd_h=1], [timerfd_h=0])
 AC_ARG_ENABLE([timerfd],
 	[AS_HELP_STRING([--enable-timerfd],
-		[use timerfd for timing (default auto)])],
+		[use timerfd for timing [default=auto]])],
 	[use_timerfd=$enableval], [use_timerfd='auto'])
 
 if test "x$use_timerfd" = "xyes" -a "x$timerfd_h" = "x0"; then
@@ -193,23 +193,38 @@ if test "x$log_enabled" != "xno"; then
 fi
 
 AC_ARG_ENABLE([debug-log], [AS_HELP_STRING([--enable-debug-log],
-	[start with debug message logging enabled (default n)])],
+	[start with debug message logging enabled [default=no]])],
 	[debug_log_enabled=$enableval],
 	[debug_log_enabled='no'])
 if test "x$debug_log_enabled" != "xno"; then
 	AC_DEFINE([ENABLE_DEBUG_LOGGING], 1, [Start with debug message logging enabled])
 fi
 
+AC_ARG_ENABLE([system-log], [AS_HELP_STRING([--enable-system-log],
+	[output logging messages to system wide log, if supported by the OS [default=no]])],
+	[system_log_enabled=$enableval],
+	[system_log_enabled='no'])
+if test "x$system_log_enabled" != "xno"; then
+	AC_DEFINE([USE_SYSTEM_LOGGING_FACILITY], 1, [Enable output to system log])
+fi
+
+# Check if syslog is available in standard C library
+AC_CHECK_HEADERS(syslog.h)
+AC_CHECK_FUNC([syslog], [have_syslog=yes], [have_syslog=no])
+if test "x$have_syslog" != "xno"; then
+	AC_DEFINE([HAVE_SYSLOG_FUNC], 1, [syslog() function available])
+fi
+
 # Examples build
 AC_ARG_ENABLE([examples-build], [AS_HELP_STRING([--enable-examples-build],
-	[build example applications (default n)])],
+	[build example applications [default=no]])],
 	[build_examples=$enableval],
 	[build_examples='no'])
 AM_CONDITIONAL([BUILD_EXAMPLES], [test "x$build_examples" != "xno"])
 
 # Tests build
 AC_ARG_ENABLE([tests-build], [AS_HELP_STRING([--enable-tests-build],
-	[build test applications (default n)])],
+	[build test applications [default=no]])],
 	[build_tests=$enableval],
 	[build_tests='no'])
 AM_CONDITIONAL([BUILD_TESTS], [test "x$build_tests" != "xno"])
diff --git a/libusb/core.c b/libusb/core.c
index 4f2f366..1418415 100644
--- a/libusb/core.c
+++ b/libusb/core.c
@@ -33,6 +33,9 @@
 #ifdef HAVE_SYS_TIME_H
 #include <sys/time.h>
 #endif
+#ifdef HAVE_SYSLOG_H
+#include <syslog.h>
+#endif
 
 #ifdef __ANDROID__
 #include <android/log.h>
@@ -2026,10 +2029,34 @@ int usbi_gettimeofday(struct timeval *tp, void *tzp)
 }
 #endif
 
-static void usbi_log_str(struct libusb_context *ctx, const char * str)
+static void usbi_log_str(struct libusb_context *ctx,
+	enum libusb_log_level level, const char * str)
 {
-	UNUSED(ctx);
+#if defined(USE_SYSTEM_LOGGING_FACILITY)
+#if defined(OS_WINDOWS) || defined(OS_WINCE)
+	/* Windows CE only supports the Unicode version of OutputDebugString. */
+	WCHAR wbuf[USBI_MAX_LOG_LEN];
+	MultiByteToWideChar(CP_UTF8, 0, str, -1, wbuf, sizeof(wbuf));
+	OutputDebugStringW(wbuf);
+#elif defined(HAVE_SYSLOG_FUNC)
+	int syslog_level = LOG_INFO;
+	switch (level) {
+	case LIBUSB_LOG_LEVEL_INFO: syslog_level = LOG_INFO; break;
+	case LIBUSB_LOG_LEVEL_WARNING: syslog_level = LOG_WARNING; break;
+	case LIBUSB_LOG_LEVEL_ERROR: syslog_level = LOG_ERR; break;
+	case LIBUSB_LOG_LEVEL_DEBUG: syslog_level = LOG_DEBUG; break;
+	case LIBUSB_LOG_LEVEL_NONE: break;
+	}
+	syslog(syslog_level, "%s", str);
+#else /* All of gcc, Clang, XCode seem to use #warning */
+#warning System logging is not supported on this platform. Logging to stderr will be used instead.
 	fputs(str, stderr);
+#endif
+#else
+	fputs(str, stderr);
+#endif /* USE_SYSTEM_LOGGING_FACILITY */
+	UNUSED(ctx);
+	UNUSED(level);
 }
 
 void usbi_log_v(struct libusb_context *ctx, enum libusb_log_level level,
@@ -2084,8 +2111,8 @@ void usbi_log_v(struct libusb_context *ctx, enum libusb_log_level level,
 	usbi_gettimeofday(&now, NULL);
 	if ((global_debug) && (!has_debug_header_been_displayed)) {
 		has_debug_header_been_displayed = 1;
-		usbi_log_str(ctx, "[timestamp] [threadID] facility level [function call] <message>\n");
-		usbi_log_str(ctx, "--------------------------------------------------------------------------------\n");
+		usbi_log_str(ctx, LIBUSB_LOG_LEVEL_DEBUG, "[timestamp] [threadID] facility level [function call] <message>\n");
+		usbi_log_str(ctx, LIBUSB_LOG_LEVEL_DEBUG, "--------------------------------------------------------------------------------\n");
 	}
 	if (now.tv_usec < timestamp_origin.tv_usec) {
 		now.tv_sec--;
@@ -2143,7 +2170,7 @@ void usbi_log_v(struct libusb_context *ctx, enum libusb_log_level level,
 	}
 	strcpy(buf + header_len + text_len, USBI_LOG_LINE_END);
 
-	usbi_log_str(ctx, buf);
+	usbi_log_str(ctx, level, buf);
 #endif
 }
 
diff --git a/libusb/version_nano.h b/libusb/version_nano.h
index 7ee3d41..ab03695 100644
--- a/libusb/version_nano.h
+++ b/libusb/version_nano.h
@@ -1 +1 @@
-#define LIBUSB_NANO 10805
+#define LIBUSB_NANO 10806
diff --git a/msvc/config.h b/msvc/config.h
index bb542c5..4b418db 100644
--- a/msvc/config.h
+++ b/msvc/config.h
@@ -25,6 +25,9 @@
 /* Uncomment to start with debug message logging enabled */
 // #define ENABLE_DEBUG_LOGGING 1
 
+/* Uncomment to enabling logging to system log */
+// #define USE_SYSTEM_LOGGING_FACILITY
+
 /* type of second poll() argument */
 #define POLL_NFDS_TYPE unsigned int