foo.c: In function 'main':
foo.c:21:5: warning: implicit declaration of function 'scanf_s'; did you mean 'scanf'? [-Wimplicit-function-declaration]
   21 |     scanf_s("%d %d",&a,&b);
      |     ^~~~~~~
      |     scanf
foo.c: In function 'gcd':
foo.c:17:1: warning: control reaches end of non-void function [-Wreturn-type]
   17 | }
      | ^
/nix/store/j2gmfdd29cp3rcvghmn1a5kgcnmf8myp-binutils-2.40/bin/ld: /tmp/ccVVoSaB.o: in function `main':
foo.c:(.text.startup+0x16): undefined reference to `scanf_s'
collect2: error: ld returned 1 exit status

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