added SPDLOG_DISABLE_TID_CACHING macro to prevent invalid thread ids after fork

include/spdlog/details/os.h

@@ -316,7 +316,7 @@ inline int utc_minutes_offset(const std::tm& tm = details::os::localtime())
 }
 
 //Return current thread id as size_t
-//It exists because the std::this_thread::get_id() is much slower(espcially under VS 2013)
+//It exists because the std::this_thread::get_id() is much slower(especially under VS 2013)
 inline size_t _thread_id()
 {
 #ifdef _WIN32
@@ -342,15 +342,15 @@ inline size_t _thread_id()
 //Return current thread id as size_t (from thread local storage)
 inline size_t thread_id()
 {
-#if defined(_MSC_VER) && (_MSC_VER < 1900) || defined(__clang__) && !__has_feature(cxx_thread_local)
+#if defined(SPDLOG_DISABLE_TID_CACHING) || (defined(_MSC_VER) && (_MSC_VER < 1900)) || (defined(__clang__) && !__has_feature(cxx_thread_local))
     return _thread_id();
-#else
+#else // cache thread id in tls
     static thread_local const size_t tid = _thread_id();
     return tid;
 #endif
-}
 
 
+}
 
 
 // wchar support for windows file names (SPDLOG_WCHAR_FILENAMES must be defined)
@@ -424,7 +424,7 @@ inline int pid()
 }
 
 
-// Detrmine if the terminal supports colors
+// Determine if the terminal supports colors
 // Source: https://github.com/agauniyal/rang/
 inline bool is_color_terminal()
 {

include/spdlog/tweakme.h

@@ -23,7 +23,7 @@
 
 ///////////////////////////////////////////////////////////////////////////////
 // Uncomment if date/time logging is not needed and never appear in the log pattern.
-// This will prevent spdlog from quering the clock on each log call.
+// This will prevent spdlog from querying the clock on each log call.
 //
 // WARNING: If the log pattern contains any date/time while this flag is on, the result is undefined.
 //          You must set new pattern(spdlog::set_pattern(..") without any date/time in it
@@ -34,7 +34,7 @@
 
 ///////////////////////////////////////////////////////////////////////////////
 // Uncomment if thread id logging is not needed (i.e. no %t in the log pattern).
-// This will prevent spdlog from quering the thread id on each log call.
+// This will prevent spdlog from querying the thread id on each log call.
 //
 // WARNING: If the log pattern contains thread id (i.e, %t) while this flag is on, the result is undefined.
 //
@@ -42,6 +42,16 @@
 ///////////////////////////////////////////////////////////////////////////////
 
 
+///////////////////////////////////////////////////////////////////////////////
+// Uncomment to prevent spdlog from caching thread ids in thread local storage.
+// By default spdlog saves thread ids in tls to gain a few micros for each call.
+//
+// WARNING: if your program forks, UNCOMMENT this flag to prevent undefined thread ids in the children logs.
+//
+// #define SPDLOG_DISABLE_TID_CACHING
+///////////////////////////////////////////////////////////////////////////////
+
+
 ///////////////////////////////////////////////////////////////////////////////
 // Uncomment if logger name logging is not needed.
 // This will prevent spdlog from copying the logger name  on each log call.
@@ -59,7 +69,7 @@
 
 ///////////////////////////////////////////////////////////////////////////////
 // Uncomment to avoid locking in the registry operations (spdlog::get(), spdlog::drop() spdlog::register()).
-// Use only if your code never modifes concurrently the registry.
+// Use only if your code never modifies concurrently the registry.
 // Note that upon creating a logger the registry is modified by spdlog..
 //
 // #define SPDLOG_NO_REGISTRY_MUTEX