/in/foo.c:7:51: error: expected ';', ',' or ')' before '&' token
 long long exgcd(long long a,long long b,long long &x,long long &y)
                                                   ^
/in/foo.c: In function 'main':
/in/foo.c:28:21: warning: implicit declaration of function 'exgcd' [-Wimplicit-function-declaration]
         long long d=exgcd(a,b,x,y);
                     ^~~~~