c++中多个文件都定义同个变量，为什么可以这样，

1 _srs_log - Global Variable in Srs_utest.cpp (utest) at line 33
定义有 ISrsLog* _srs_log = new MockEmptyLog(SrsLogLevel::Disabled);
2 _srs_log - Global Variable in Srs_main_server.cpp (main) at line 52
定义有 ISrsLog* _srs_log = new SrsFastLog();
3 _srs_log - Global Variable in Srs_librtmp.cpp (libs) at line 53
定义有 ISrsLog* _srs_log = new ISrsLog();
4 _srs_log - Global Variable in Srs_main_ingest_hls.cpp (main) at line 58
定义有 ISrsLog* _srs_log = new SrsFastLog();