/in/foo.c:2:19: error: expected ';', ',' or ')' before 'int'
 int  gcd  (int x1 int x2)
                   ^~~
/in/foo.c: In function 'main':
/in/foo.c:16:5: error: 'x' undeclared (first use in this function)
     x=  gcd(x1,x2);
     ^
/in/foo.c:16:5: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:16:9: warning: implicit declaration of function 'gcd' [-Wimplicit-function-declaration]
     x=  gcd(x1,x2);
         ^~~