foo.c: In function 'prime':
foo.c:7:9: warning: unused variable 'n' [-Wunused-variable]
    7 |   int i,n;
      |         ^
foo.c: At top level:
foo.c:14:11: warning: return type of 'main' is not 'int' [-Wmain]
   14 |      void main()
      |           ^~~~
foo.c: In function 'main':
foo.c:24:4: warning: 'j' may be used uninitialized in this function [-Wmaybe-uninitialized]
   24 |   j++;
      |   ~^~

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