#include <stdio.h>
int printf(const char * restrict format, ...);
The printf function is equivalent to fprintf with the argument stdout interposed
before the arguments to printf.
并且,从 §7.21.6.1 开始,“fprintf 函数”(强调我的)
The fprintf function writes output to the stream pointed to by stream, under control
of the string pointed to by format that specifies how subsequent arguments are
converted for output. If there are insufficient arguments for the format, the behavior is
undefined.