/in/foo.c: In function 'main':
/in/foo.c:17:9: warning: implicit declaration of function 'isValidTriangle' [-Wimplicit-function-declaration]
   17 |     if (isValidTriangle(a, b, c)) {
      |         ^~~~~~~~~~~~~~~
/in/foo.c:19:28: warning: implicit declaration of function 'calculatePerimeter' [-Wimplicit-function-declaration]
   19 |         double perimeter = calculatePerimeter(a, b, c);
      |                            ^~~~~~~~~~~~~~~~~~
/usr/bin/ld: /tmp/ccpescXO.o: in function `main':
foo.c:(.text.startup+0x53): undefined reference to `isValidTriangle'
/usr/bin/ld: foo.c:(.text.startup+0x86): undefined reference to `calculatePerimeter'
collect2: error: ld returned 1 exit status