foo.c: In function 'main':
foo.c:9:14: warning: implicit declaration of function 'gcd' [-Wimplicit-function-declaration]
    9 |     int y1 = gcd(x1, x2);
      |              ^~~
/nix/store/j2gmfdd29cp3rcvghmn1a5kgcnmf8myp-binutils-2.40/bin/ld: /tmp/ccKoGcq2.o: in function `main':
foo.c:(.text.startup+0x26): undefined reference to `gcd'
collect2: error: ld returned 1 exit status

[Hydro](https://hydro.ac)提供评测服务