c - How does preprocessor make that expansion (define macro found in included header file ) -

i have header file contain constant f_cpu , use macro guard

header.h

#ifndef f_cpu #define f_cpu  1000000ul #endif 

and source.c file

#define f_cpu 16000000ul #include "header.h" 

how first macro(in c file ) expand value not included yet?

use -e option of preprocessor follow happens. define f_cpu macro in first line of source.c , after when header file included, macro definition in header not takes place because of #ifndef guard. note there no macro expansion in code. expansion takes place when use macro.