foo.c: In function 'main':
foo.c:11:13: error: expected identifier or '(' before ',' token
     int a,b,,dc;
             ^
foo.c:14:17: error: 'd' undeclared (first use in this function)
     scanf("%d",&d);
                 ^
foo.c:14:17: note: each undeclared identifier is reported only once for each function it appears in
foo.c:15:5: error: 'c' undeclared (first use in this function)
     c=gcd(a,b); 
     ^