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我在C写"hello world"程序.

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

该程序编译时带有警告

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:~$

这怎么可能？操作系统如何在不包含任何标题的情况下链接库？