/in/foo.c: In function 'zhishu':
/in/foo.c:5:17: warning: implicit declaration of function 'sqrt' [-Wimplicit-function-declaration]
  for(int i=2;i<=sqrt(a);i++)
                 ^~~~
/in/foo.c:5:17: warning: incompatible implicit declaration of built-in function 'sqrt'
/in/foo.c:5:17: note: include '<math.h>' or provide a declaration of 'sqrt'
/in/foo.c:4:6: warning: unused variable 'i' [-Wunused-variable]
  int i;
      ^