/in/foo.c:2:1: error: unknown type name 'bool'
 bool isPrime(int x)
 ^~~~
/in/foo.c: In function 'isPrime':
/in/foo.c:5:10: error: 'false' undeclared (first use in this function)
   return false;
          ^~~~~
/in/foo.c:5:10: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:7:3: warning: this 'if' clause does not guard... [-Wmisleading-indentation]
   if(x%i==0)
   ^~
/in/foo.c:9:4: note: ...this statement, but the latter is misleadingly indented as if it is guarded by the 'if'
    return true;
    ^~~~~~
/in/foo.c:9:11: error: 'true' undeclared (first use in this function)
    return true;
           ^~~~
/in/foo.c: In function 'main':
/in/foo.c:15:2: error: expected ';' before 'for'
  for(int x1=3;x1<=x/2;x1=x1+2)
  ^~~
/in/foo.c: In function 'isPrime':
/in/foo.c:10:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^