search path of header files

[ranjith kumar]

hi,

1) when we  write
                         #include<stdio.h>
whrer will gcc search for header file stdio.h. ???
How to know the list of all directories it searches for header files?

2) I have two .c (1.c and 2.c) files. Both include a(same) header file(1.h).
     1.h defines a variable say "int a"
    When we compile 1.c and 2.c to produce a single executable file
why gcc will not  generate error " int a is decleared two times"?
3) At waht intermediate representation the contents of a header file
will be copied to contents of .c file?

4) If GCC is written in C language who will compile it to produce the
the the executable (compiler itself)?
I mean ..when GCC is written in C for the first time, how did they
compile it?? At that time no C compiler was  there.