foo.c: In function 'main':
foo.c:19:3: warning: implicit declaration of function 'scanf_s'; did you mean 'scanf'? [-Wimplicit-function-declaration]
   19 |   scanf_s("%d", &a[i]);
      |   ^~~~~~~
      |   scanf
foo.c:15:22: warning: unused variable 'j' [-Wunused-variable]
   15 |  int a[10000], n, i, j;
      |                      ^
foo.c:16:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   16 |  scanf("%d", &n);
      |  ^~~~~~~~~~~~~~~
/usr/bin/ld: /tmp/cctRtKdy.o: in function `main':
foo.c:(.text.startup+0x7c): undefined reference to `scanf_s'
collect2: error: ld returned 1 exit status

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