All libraries shipped with the C++ compiler are multithreading safe. If you want to build a multithreaded application, or if you want to link your application to a multithreaded library, you must compile and link your program with the –mt option. This option passes –D_REENTRANT to the preprocessor and passes –lthread in the correct order to ld. By default, the -mt option ensures that libthread is linked before libCrun. Use of —mt is recommended as a simpler and less error-prone alternative to specifying the macro and library.