foo.c:4:6: warning: return type of 'main' is not 'int' [-Wmain]
 void main()
      ^~~~
foo.c: In function 'insert':
foo.c:41:2: warning: statement with no effect [-Wunused-value]
  for(i;i<n;i++)
  ^~~
foo.c: In function 'output':
foo.c:60:2: warning: statement with no effect [-Wunused-value]
  for(i;i<n;i++)
  ^~~
foo.c: In function 'main':
foo.c:7:10: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  int n1; scanf("%d",&n1);
          ^~~~~~~~~~~~~~~
foo.c:8:10: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  int n2; scanf("%d",&n2);
          ^~~~~~~~~~~~~~~
foo.c:9:10: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  int n3; scanf("%d",&n3);
          ^~~~~~~~~~~~~~~
foo.c:12:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   scanf("%d",&a1[i]);
   ^~~~~~~~~~~~~~~~~~
foo.c:16:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   scanf("%d",&a2[i]);
   ^~~~~~~~~~~~~~~~~~
foo.c:20:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   scanf("%d",&a3[i]);
   ^~~~~~~~~~~~~~~~~~
foo.c: In function 'insert':
foo.c:39:6: warning: 'j' may be used uninitialized in this function [-Wmaybe-uninitialized]
  int j;
      ^

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