foo.c: In function 'main':
foo.c:32:13: warning: variable 'daynum' set but not used [-Wunused-but-set-variable]
   32 |         int daynum;
      |             ^~~~~~
foo.c: In function 'DayNum':
foo.c:23:16: warning: 'daynum' may be used uninitialized [-Wmaybe-uninitialized]
   23 |         return daynum;
      |                ^~~~~~
foo.c:4:13: note: 'daynum' was declared here
    4 |         int daynum;
      |             ^~~~~~
foo.c: In function 'main':
foo.c:37:19: warning: 'daynum' may be used uninitialized [-Wmaybe-uninitialized]
   37 |                 a += DayNum(i);
      |                   ^~
foo.c:4:13: note: 'daynum' was declared here
    4 |         int daynum;
      |             ^~~~~~

[Hydro](https://hydro.ac)提供评测服务