I am using a mac, and I wrote a simple code using Xcode

Code:
#include <stdio.h>
int main () {
   printf ("hello world\n");
   return 0;
}
I saved it as testing.c then I go to terminal and compile it
gcc testing.c

then it gives me the following error:
testing.c:1:19: error: stdio.h: No such file or directory
testing.c: In function ‘main’:
testing.c:5: warning: incompatible implicit declaration of built-in function ‘printf’

how come...?