The numbers following the filename are flags:
1: This indicates the start of a new file.
2: This indicates returning to a file (after having included another file).
3: This indicates that the following text comes from a system header file, so certain warnings should be suppressed.
4: This indicates that the following text should be treated as being wrapped in an implicit extern "C" block.
Source: https://gcc.gnu.org/onlinedocs/cpp/Preprocessor-Output.html