foo.c: In function 'main':
foo.c:7:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
    7 |  scanf("%d %d %d", &n1, &n2, &n3);
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
foo.c:10:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   10 |   scanf("%d", &a1[i]);
      |   ^~~~~~~~~~~~~~~~~~~
foo.c:14:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   14 |   scanf("%d", &a2[i]);
      |   ^~~~~~~~~~~~~~~~~~~
foo.c:18:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   18 |   scanf("%d", &a3[i]);
      |   ^~~~~~~~~~~~~~~~~~~
foo.c: In function 'merge':
foo.c:54:1: warning: control reaches end of non-void function [-Wreturn-type]
   54 | }
      | ^

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