No Bill, i do not include stdio.h on purpose! My program is exactly as follows: main(){ printf("kkkkkkkk=\n"); } and I compile it as: gcc -o prog1 prog1.c prog1.c: In function ‘main’: prog1.c:2: warning: incompatible implicit declaration of built-in function ‘printf’ I could not get my answer by google. I guess that printf is a built-in function and has a function prototype and probably the built-in version is being used here. So why implicit declaration? and how it is incompatible? Is there a complete list of GCC error and warning messages (and their causes)? Regards On Sun, Dec 26, 2010 at 5:33 PM, Bill McEnaney <bill@xxxxxxxxxxxx> wrote: > Did you include stdio.h? >