/in/foo.c: In function 'isPrime':
/in/foo.c:5:16: warning: implicit declaration of function 'sqrt' [-Wimplicit-function-declaration]
  for(int i=2;i<sqrt(x);i++)
                ^~~~
/in/foo.c:5:16: warning: incompatible implicit declaration of built-in function 'sqrt'
/in/foo.c:5:16: note: include '<math.h>' or provide a declaration of 'sqrt'