How to avoid standard include files during compilation

Question

I am trying to use my own printf function so i don't want to include standard include files... so I am compiling my code with -nostdinc

I have created my program something like this:

extern int printf(const char*,...);
printf("Value:%d",1234);
//printf("\n");

It is working fine for this code, but when I use printf("\n") then it is showing undefined reference to 'putchar'.

If i comment printf("\n"); then nm command is showing

$ nm test1.o
         U exit
00000000 T main
         U printf
00000030 T _start

but if I use printf("\n"); then nm command is showing

$nm test1.o
         U exit
00000000 T main
         U printf
         U putchar
0000003c T _start

I am not getting how and from where putchar is getting included

gcc version 4.8.2 (GCC)

Answer 1

gcc optimizes printf in certain situations. You can look at the function fold_builtin_printf here for the complete details. IIRC, it optimizes calls with one argument followed by a newline to puts/putchar. You can turn it off by specifying -fno-builtin(gcc docs).