/in/foo.c:9:1: error: unknown type name 'bool'
 bool flag[21];
 ^~~~
/in/foo.c: In function 'f':
/in/foo.c:22:24: error: 'true' undeclared (first use in this function)
 data[i] = j, flag[j] = true;
                        ^~~~
/in/foo.c:22:24: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c: In function 'main':
/in/foo.c:32:12: error: 'true' undeclared (first use in this function)
 int flag = true;
            ^~~~
/in/foo.c:36:6: error: 'false' undeclared (first use in this function)
 flag=false;
      ^~~~~
/in/foo.c: In function 'f':
/in/foo.c:25:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^