Language: C

double gaiseki(double x0,double y0,double x1,double y1,double x2,double y2){
  return(x1-x0)*(y2-y0)-(x2-x0)*(y1-y0);
}
int main(){
  double x0,y0,x1,y1,x2,y2,x3,y3;
  while (scanf("%lf,%lf,%lf,%lf,%lf,%lf,%lf,%lf",&x0,&y0,&x1,&y1,&x2,&y2,&x3,&y3)!=EOF){
    if((x1-x0)*(y3-y0)-(x3-x0)*(y1-y0)>0){
      if((x1-x0)*(y3-y0)-(x3-x0)*(y1-y0)<0||(x3-x2)*(y1-y2)-(x1-x2)*(y3-y2)<0||(x0-x3)*(y2-y3)-(x2-x3)*(y0-y3)<=0){
	printf("NO\n");
      }else{
	printf("YES\n");
      }
    }else  if((x1-x0)*(y3-y0)-(x3-x0)*(y1-y0)<0){
      if((x1-x0)*(y3-y0)-(x3-x0)*(y1-y0)<0||(x3-x2)*(y1-y2)-(x1-x2)*(y3-y2)<0||(x0-x3)*(y2-y3)-(x2-x3)*(y0-y3)>=0){
	printf("NO\n");
      }else{
	printf("YES\n");
      }
    }else{
      printf("NO\n");
    }
  }
  return 0;
}

Compiler output:
main.c: In function 'main':
main.c:6:10: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
    6 |   while (scanf("%lf,%lf,%lf,%lf,%lf,%lf,%lf,%lf",&x0,&y0,&x1,&y1,&x2,&y2,&x3,&y3)!=EOF){
      |          ^~~~~
main.c:1:1: note: include '<stdio.h>' or provide a declaration of 'scanf'
  +++ |+#include <stdio.h>
    1 | double gaiseki(double x0,double y0,double x1,double y1,double x2,double y2){
main.c:6:10: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
    6 |   while (scanf("%lf,%lf,%lf,%lf,%lf,%lf,%lf,%lf",&x0,&y0,&x1,&y1,&x2,&y2,&x3,&y3)!=EOF){
      |          ^~~~~
main.c:6:10: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:6:84: error: 'EOF' undeclared (first use in this function)
    6 |   while (scanf("%lf,%lf,%lf,%lf,%lf,%lf,%lf,%lf",&x0,&y0,&x1,&y1,&x2,&y2,&x3,&y3)!=EOF){
      |                                                                                    ^~~
main.c:6:84: note: 'EOF' is defined in header '<stdio.h>'; this is probably fixable by adding '#include <stdio.h>'
main.c:6:84: note: each undeclared identifier is reported only once for each function it appears in
main.c:9:9: error: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
    9 |         printf("NO\n");
      |         ^~~~~~
main.c:9:9: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:9:9: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
main.c:9:9: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:11:9: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   11 |         printf("YES\n");
      |         ^~~~~~
main.c:11:9: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:15:9: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   15 |         printf("NO\n");
      |         ^~~~~~
main.c:15:9: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:17:9: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   17 |         printf("YES\n");
      |         ^~~~~~
main.c:17:9: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:20:7: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   20 |       printf("NO\n");
      |       ^~~~~~
main.c:20:7: note: include '<stdio.h>' or provide a declaration of 'printf'