thili@M2-Pro Misc % cat deprecated.c 
void foo() {}

int main(int argc, char **argv) {
  foo();
  return 0;
}
thili@M2-Pro Misc % gcc deprecated.c -Wall -Wextra -Wpedantic
deprecated.c:1:9: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
void foo() {}
        ^
         void
deprecated.c:3:14: warning: unused parameter 'argc' [-Wunused-parameter]
int main(int argc, char **argv) {
             ^
deprecated.c:3:27: warning: unused parameter 'argv' [-Wunused-parameter]
int main(int argc, char **argv) {
                          ^
3 warnings generated.