msdn解释是：The atexit function is passed the address of a function (func) to be called when the program terminates normally. Successive calls to atexit create a register of functions that are executed in last-in, first-out (LIFO) order. The functions passed to atexit cannot take parameters. atexit and _onexit use the heap to hold the register of functions. Thus, the number of functions that can be registered is limited only by heap memory.
即atexit创建一个LIFO的函数寄存器。在程序正常结束之后，函数才被调用。且由于是LIFO，因此执行调用顺序与书写顺序相反。
程序如下：
#include <stdio.h>
#include <stdlib.h>
int a;
void exit_fn1(void)
{
a ++;
printf("Exit function #1 called: %d\n", a);
}
void exit_fn2(void)
{
a ++;
printf("Exit function #2 called: %d\n", a);
}
int main(void)
{
a = 0;
/* post exit function #1 */
atexit(exit_fn1);
/* post exit function #2 */
atexit(exit_fn2);
a = 5;
return 0;
}