linux - C program without header

Question

I write "hello world" program in C.

void main()
{ printf("Hello World"); }
// note that I haven't included any header file

The program compiles with warning as

vikram@vikram-Studio-XPS-1645:~$ gcc hello.c 
hello.c: In function ‘main’:
hello.c:2:2: warning: incompatible implicit declaration of built-in function ‘printf’
vikram@vikram-Studio-XPS-1645:~$ ./a.out 
Hello Worldvikram@vikram-Studio-XPS-1645:~$

How is this possible? How does the OS link a library without including any header?

Answer 1

The printf function is in the C library (libc in your case) which is linked implicitly (actually gcc has a printf builtin but it's outside the point).

Including the header doesn't bring in any functions for the linker, it simply informs the compiler about their declarations (i.e. "what they look like").

Obviously you should always include headers otherwise you force the compiler into making assumptions about what the functions look like.