/in/foo.c:1:1: error: unknown type name 'def'
    1 | def count_characters():
      | ^~~
/in/foo.c: In function 'count_characters':
/in/foo.c:1:23: error: expected declaration specifiers before ':' token
    1 | def count_characters():
      |                       ^
/in/foo.c:25: warning: control reaches end of non-void function [-Wreturn-type]